virtio — archive

[Rusty Russell]

OK, this is a rework of the previously posted text spec into core vs. per-transport parts. Most stuff was cut & paste, some words were added for uniformity (eg mmio now doesn't refer to PCI, but to the core) and sentences reworked where required. ISTR we are supposed to use OASIS hosting for OASIS work, so I am waiting for the SVN repository. Meanwhile, I will start adapting my changes to the git repo into changes to this basis document. (ie. don't need feedback on this right now, probably better to wait until it's updated). Cheers, Rusty. 1. INTRODUCTION =============== This document describes the specifications of the “virtio” family of devices. These are devices are found in virtual environments, yet by design they are not all that different from physical devices, and this document treats them as such. This allows the guest to use standard drivers and discovery mechanisms. The purpose of virtio and this specification is that virtual environments and guests should have a straightforward, efficient, standard and extensible mechanism for virtual devices, rather than boutique per-environment or per-OS mechanisms. Straightforward: Virtio devices use normal bus mechanisms of interrupts and DMA which should be familiar to any device driver author. There is no exotic page-flipping or COW mechanism: it's just a normal device.[1] Efficient: Virtio devices consist of rings of descriptors for input and output, which are neatly separated to avoid cache effects from both guest and device writing to the same cache lines. Standard: Virtio makes no assumptions about the environment in which it operates, beyond supporting the bus attaching the device. Virtio devices are implemented over PCI and other buses, and earlier drafts been implemented on other buses not included in this spec.[2] Extensible: Virtio PCI devices contain feature bits which are acknowledged by the guest operating system during device setup. This allows forwards and backwards compatibility: the device offers all the features it knows about, and the driver acknowledges those it understands and wishes to use. 1.1.1. Key words ----------------- The key words must, must not, required, shall, shall not, should, should not, recommended, may, and optional are to be interpreted as described in [RFC 2119]. Note that for reasons of style, these words are not capitalized in this document. 1.1.2. Definitions ------------------- term Definition 1.1.3. Key concepts -------------------- Guest Definition... Host Definition Device Definition Driver Definition 1.2. Normative References ========================= [RFC 2119] S. Bradner, Key words for use in RFCs to Indicate Requirement Levels, http://www.ietf.org/rfc/rfc2119.txt IETF (Internet Engineering Task Force) RFC 2119, March 1997. 1.3. Non-Normative References ========================= 2 The Virtio Standard ========================= 2.1 Basic Facilities of a Virtio Device ======================================= A virtio device is discovered and identified by a bus-specific method (see the bus specific sections *XREF*). Each device consists of the following parts: o Device Status field o Feature bits o Configuration space o One or more virtqueues 2.1.1 Device Status Field ------------------------- The Device Status field is updated by the guest to indicate its progress. This provides a simple low-level diagnostic: it's most useful to imagine them hooked up to traffic lights on the console indicating the status of each device. This field is 0 upon reset, otherwise at least one bit should be set: ACKNOWLEDGE (1) Indicates that the guest OS has found the device and recognized it as a valid virtio device. DRIVER (2) Indicates that the guest OS knows how to drive the device. Under Linux, drivers can be loadable modules so there may be a significant (or infinite) delay before setting this bit. DRIVER_OK (4) Indicates that the driver is set up and ready to drive the device. FAILED (128) Indicates that something went wrong in the guest, and it has given up on the device. This could be an internal error, or the driver didn't like the device for some reason, or even a fatal error during device operation. The device must be reset before attempting to re-initialize. 2.1.2 Feature Bits ------------------ Each virtio device lists all the features it understands. During device initialization, the guest reads this and tells the device the subset that it understands. The only way to renegotiate is to reset the device. This allows for forwards and backwards compatibility: if the device is enhanced with a new feature bit, older guests will not write that feature bit back to the device and it can go into backwards compatibility mode. Similarly, if a guest is enhanced with a feature that the device doesn't support, it see the new feature is not offered and can go into backwards compatibility mode (or, for poor implementations, set the FAILED Device Status bit). Feature bits are allocated as follows: 0 to 23: Feature bits for the specific device type 24 to 32: Feature bits reserved for extensions to the queue and feature negotiation mechanisms For example, feature bit 0 for a network device (i.e. Subsystem Device ID 1) indicates that the device supports checksumming of packets. In particular, new fields in the device configuration space are indicated by offering a feature bit, so the guest can check before accessing that part of the configuration space. 2.1.3 Configuration Space ------------------------- Configuration space is generally used for rarely-changing or initialization-time parameters. Note that this space is generally the guest's native endian, rather than PCI's little-endian. 2.1.4 Virtqueues ---------------- The mechanism for bulk data transport on virtio devices is pretentiously called a virtqueue. Each device can have zero or more virtqueues: for example, the simplest network device has one for transmit and one for receive. Each queue has a 16-bit queue size parameter, which sets the number of entries and implies the total size of the queue. Each virtqueue occupies two or more physically-contiguous pages (usually defined as 4096 bytes, but depending on the transport) and consists of three parts: + ------------------- + ----------------------------------- + ----------- + Descriptor Table Available Ring (padding) Used Ring + ------------------- + ----------------------------------- + ----------- + The bus-specific Queue Size field controls the total number of bytes required for the virtqueue according to the following formula: #define ALIGN(x) (((x) + PAGE_SIZE) & ~PAGE_SIZE) static inline unsigned vring_size(unsigned int qsz) { return ALIGN(sizeof(struct vring_desc)*qsz + sizeof(u16)*(2 + qsz)) + ALIGN(sizeof(struct vring_used_elem)*qsz); } This currently wastes some space with padding, but also allows future extensions. The virtqueue layout structure looks like this: struct vring { // The actual descriptors (16 bytes each) struct vring_desc desc[ Queue Size ]; // A ring of available descriptor heads with free-running index. struct vring_avail avail; // Padding to the next PAGE_SIZE boundary. char pad[ Padding ]; // A ring of used descriptor heads with free-running index. struct vring_used used; }; When the driver wants to send a buffer to the device, it fills in a slot in the descriptor table (or chains several together), and writes the descriptor index into the available ring. It then notifies the device. When the device has finished a buffer, it writes the descriptor into the used ring, and sends an interrupt. 2.1.4.1 A Note on Virtqueue Endianness -------------------------------------- Note that the endian of fields and in the virtqueue is the native endian of the guest, not little-endian as PCI normally is. This makes for simpler guest code, and it is assumed that the host already has to be deeply aware of the guest endian so such an “endian-aware” device is not a significant issue. 2.1.4.2 Message Framing ----------------------- The descriptors used for a buffer should not effect the semantics of the message, except for the total length of the buffer. For example, a network buffer consists of a 10 byte header followed by the network packet. Whether this is presented in the ring descriptor chain as (say) a 10 byte buffer and a 1514 byte buffer, or a single 1524 byte buffer, or even three buffers, should have no effect. In particular, no implementation should use the descriptor boundaries to determine the size of any header in a request.[10] 2.1.4.3 The Virtqueue Descriptor Table -------------------------------------- The descriptor table refers to the buffers the guest is using for the device. The addresses are physical addresses, and the buffers can be chained via the next field. Each descriptor describes a buffer which is read-only or write-only, but a chain of descriptors can contain both read-only and write-only buffers. No descriptor chain may be more than 2^32 bytes long in total. struct vring_desc { /* Address (guest-physical). */ u64 addr; /* Length. */ u32 len; /* This marks a buffer as continuing via the next field. */ #define VRING_DESC_F_NEXT 1 /* This marks a buffer as write-only (otherwise read-only). */ #define VRING_DESC_F_WRITE 2 /* This means the buffer contains a list of buffer descriptors. */ #define VRING_DESC_F_INDIRECT 4 /* The flags as indicated above. */ u16 flags; /* Next field if flags & NEXT */ u16 next; }; The number of descriptors in the table is defined by the queue size for this virtqueue. 2.1.4.3.1 Indirect Descriptors ------------------------------ Some devices benefit by concurrently dispatching a large number of large requests. The VIRTIO_RING_F_INDIRECT_DESC feature can be used to allow this (see FIXME: Reserved Feature Bits). To increase ring capacity it is possible to store a table of indirect descriptors anywhere in memory, and insert a descriptor in main virtqueue (with flags&VRING_DESC_F_INDIRECT on) that refers to memory buffer containing this indirect descriptor table; fields addr and len refer to the indirect table address and length in bytes, respectively. The indirect table layout structure looks like this (len is the length of the descriptor that refers to this table, which is a variable, so this code won't compile): struct indirect_descriptor_table { /* The actual descriptors (16 bytes each) */ struct vring_desc desc[len / 16]; }; The first indirect descriptor is located at start of the indirect descriptor table (index 0), additional indirect descriptors are chained by next field. An indirect descriptor without next field (with flags&VRING_DESC_F_NEXT off) signals the end of the indirect descriptor table, and transfers control back to the main virtqueue. An indirect descriptor can not refer to another indirect descriptor table (flags&VRING_DESC_F_INDIRECT must be off). A single indirect descriptor table can include both read-only and write-only descriptors; write-only flag (flags&VRING_DESC_F_WRITE) in the descriptor that refers to it is ignored. 2.1.4.4 The Virtqueue Available Ring ------------------------------------ The available ring refers to what descriptors we are offering the device: it refers to the head of a descriptor chain. The “flags” field is currently 0 or 1: 1 indicating that we do not need an interrupt when the device consumes a descriptor from the available ring. Alternatively, the guest can ask the device to delay interrupts until an entry with an index specified by the “used_event” field is written in the used ring (equivalently, until the idx field in the used ring will reach the value used_event + 1). The method employed by the device is controlled by the VIRTIO_RING_F_EVENT_IDX feature bit (see FIXME: Reserved Feature Bits). This interrupt suppression is merely an optimization; it may not suppress interrupts entirely. The “idx” field indicates where we would put the next descriptor entry (modulo the queue size). This starts at 0, and increases. struct vring_avail { #define VRING_AVAIL_F_NO_INTERRUPT 1 u16 flags; u16 idx; u16 ring[ /* Queue Size */ ]; u16 used_event; }; 2.1.4.5 The Virtqueue Used Ring ------------------------------- The used ring is where the device returns buffers once it is done with them. The flags field can be used by the device to hint that no notification is necessary when the guest adds to the available ring. Alternatively, the “avail_event” field can be used by the device to hint that no notification is necessary until an entry with an index specified by the “avail_event” is written in the available ring (equivalently, until the idx field in the available ring will reach the value avail_event + 1). The method employed by the device is controlled by the guest through the VIRTIO_RING_F_EVENT_IDX feature bit (see FIXME: Reserved Feature Bits).[7] Each entry in the ring is a pair: the head entry of the descriptor chain describing the buffer (this matches an entry placed in the available ring by the guest earlier), and the total of bytes written into the buffer. The latter is extremely useful for guests using untrusted buffers: if you do not know exactly how much has been written by the device, you usually have to zero the buffer to ensure no data leakage occurs. /* u32 is used here for ids for padding reasons. */ struct vring_used_elem { /* Index of start of used descriptor chain. */ u32 id; /* Total length of the descriptor chain which was used (written to) */ u32 len; }; struct vring_used { #define VRING_USED_F_NO_NOTIFY 1 u16 flags; u16 idx; struct vring_used_elem ring[ /* Queue Size */]; u16 avail_event; }; 2.1.4.6 Helpers for Operating Virtqueues ---------------------------------------- The Linux Kernel Source code contains the definitions above and helper routines in a more usable form, in include/linux/virtio_ring.h. This was explicitly licensed by IBM and Red Hat under the (3-clause) BSD license so that it can be freely used by all other projects, and is reproduced (with slight variation to remove Linux assumptions) in *XREF*. 2.2 General Initialization And Device Operation =============================================== We start with an overview of device initialization, then expand on the details of the device and how each step is preformed. This section should be read along with the bus-specific section which describes how to communicate with the specific device. 2.2.1 Device Initialization --------------------------- 1. Reset the device. This is not required on initial start up. 2. The ACKNOWLEDGE status bit is set: we have noticed the device. 3. The DRIVER status bit is set: we know how to drive the device. 4. Device-specific setup, including reading the device feature bits, discovery of virtqueues for the device, optional per-bus setup, and reading and possibly writing the device's virtio configuration space. 5. The subset of device feature bits understood by the driver is written to the device. 6. The DRIVER_OK status bit is set. 7. The device can now be used (ie. buffers added to the virtqueues)[4] If any of these steps go irrecoverably wrong, the guest should set the FAILED status bit to indicate that it has given up on the device (it can reset the device later to restart if desired). 2.2.2 Device Operation ---------------------- There are two parts to device operation: supplying new buffers to the device, and processing used buffers from the device. As an example, the simplest virtio network device has two virtqueues: the transmit virtqueue and the receive virtqueue. The driver adds outgoing (read-only) packets to the transmit virtqueue, and then frees them after they are used. Similarly, incoming (write-only) buffers are added to the receive virtqueue, and processed after they are used. 2.2.2.1 Supplying Buffers to The Device --------------------------------------- Actual transfer of buffers from the guest OS to the device operates as follows: 1. Place the buffer(s) into free descriptor(s). (a) If there are no free descriptors, the guest may choose to notify the device even if notifications are suppressed (to reduce latency).[8] 2. Place the id of the buffer in the next ring entry of the available ring. 3. The steps (1) and (2) may be performed repeatedly if batching is possible. 4. A memory barrier should be executed to ensure the device sees the updated descriptor table and available ring before the next step. 5. The available “idx” field should be increased by the number of entries added to the available ring. 6. A memory barrier should be executed to ensure that we update the idx field before checking for notification suppression. 7. If notifications are not suppressed, the device should be notified of the new buffers. Note that the above code does not take precautions against the available ring buffer wrapping around: this is not possible since the ring buffer is the same size as the descriptor table, so step (1) will prevent such a condition. In addition, the maximum queue size is 32768 (it must be a power of 2 which fits in 16 bits), so the 16-bit “idx” value can always distinguish between a full and empty buffer. Here is a description of each stage in more detail. 2.2.2.1.1 Placing Buffers Into The Descriptor Table --------------------------------------------------- A buffer consists of zero or more read-only physically-contiguous elements followed by zero or more physically-contiguous write-only elements (it must have at least one element). This algorithm maps it into the descriptor table: for each buffer element, b: (a) Get the next free descriptor table entry, d (b) Set d.addr to the physical address of the start of b (c) Set d.len to the length of b. (d) If b is write-only, set d.flags to VRING_DESC_F_WRITE, otherwise 0. (e) If there is a buffer element after this: i. Set d.next to the index of the next free descriptor element. ii. Set the VRING_DESC_F_NEXT bit in d.flags. In practice, the d.next fields are usually used to chain free descriptors, and a separate count kept to check there are enough free descriptors before beginning the mappings. 2.2.2.1.2 Updating The Available Ring ------------------------------------- The head of the buffer we mapped is the first d in the algorithm above. A naive implementation would do the following: avail->ring[avail->idx % qsz] = head; However, in general we can add many descriptors before we update the “idx” field (at which point they become visible to the device), so we keep a counter of how many we've added: avail->ring[(avail->idx + added++) % qsz] = head; 2.2.2.1.3 Updating The Index Field ---------------------------------- Once the index field of the virtqueue is updated, the device will be able to access the descriptor entries we've created and the memory they refer to. This is why a memory barrier is generally used before the index update, to ensure it sees the most up-to-date copy. The index field always increments, and we let it wrap naturally at 65536: avail->idx += added; 2.2.2.1.4 Notifying The Device ------------------------------ The actual method of device notification is bus-specific, but generally it can be expensive. So the device can suppress such notifications if it doesn't need them. We have to be careful to expose the new index value before checking if notifications are suppressed: it's OK to notify gratuitously, but not to omit a required notification. So again, we use a memory barrier here before reading the flags or the avail_event field. If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated, and if the VRING_USED_F_NOTIFY flag is not set, we go ahead and notify the device. If the VIRTIO_F_RING_EVENT_IDX feature is negotiated, we read the avail_event field in the available ring structure. If the available index crossed_the avail_event field value since the last notification, we go ahead and write to the PCI configuration space. The avail_event field wraps naturally at 65536 as well, iving the following algorithm for calculating whether a device needs notification: (u16)(new_idx - avail_event - 1) < (u16)(new_idx - old_idx) 2.2.2.2 Receiving Used Buffers From The Device ---------------------------------------------- Once the device has used a buffer (read from or written to it, or parts of both, depending on the nature of the virtqueue and the device), it sends an interrupt, following an algorithm very similar to the algorithm used for the driver to send the device a buffer: 1. Write the head descriptor number to the next field in the used ring. 2. Update the used ring index. 3. Deliver an interrupt if necessary: (a) If the VIRTIO_F_RING_EVENT_IDX feature is not negotiated: check if the VRING_AVAIL_F_NO_INTERRUPT flag is not set in avail->flags. (b) If the VIRTIO_F_RING_EVENT_IDX feature is negotiated: check whether the used index crossed the used_event field value since the last update. The used_event field wraps naturally at 65536 as well: (u16)(new_idx - used_event - 1) < (u16)(new_idx - old_idx) For each ring, guest should then disable interrupts by writing VRING_AVAIL_F_NO_INTERRUPT flag in avail structure, if required. It can then process used ring entries finally enabling interrupts by clearing the VRING_AVAIL_F_NO_INTERRUPT flag or updating the EVENT_IDX field in the available structure. The guest should then execute a memory barrier, and then recheck the ring empty condition. This is necessary to handle the case where after the last check and before enabling interrupts, an interrupt has been suppressed by the device: vring_disable_interrupts(vq); for (;;) { if (vq->last_seen_used != vring->used.idx) { vring_enable_interrupts(vq); mb(); if (vq->last_seen_used != vring->used.idx) break; } struct vring_used_elem *e = vring.used->ring[vq->last_seen_used%vsz]; process_buffer(e); vq->last_seen_used++; } 2.2.2.3 Notification of Device Configuration Changes ---------------------------------------------------- For devices where the configuration information can be changed, an interrupt is delivered when a configuration change occurs. 2.4 Virtio Transport Options ============================ Virtio can use various different busses, thus the standard is split into virtio general and bus-specific sections. 2.4.1 Virtio Over PCI Bus ------------------------- Virtio devices are commonly implemented as PCI devices. 2.4.1.1 PCI Device Discovery ---------------------------- Any PCI device with Vendor ID 0x1AF4, and Device ID 0x1000 through 0x103F inclusive is a virtio device[3]. The device must also have a Revision ID of 0 to match this specification. The Subsystem Device ID indicates which virtio device is supported by the device. The Subsystem Vendor ID should reflect the PCI Vendor ID of the environment (it's currently only used for informational purposes by the guest). 2.4.1.2 PCI Device Layout ------------------------- To configure the device, we use the first I/O region of the PCI device. This contains a virtio header followed by a device-specific region. There may be different widths of accesses to the I/O region; the “natural” access method for each field in the virtio header must be used (i.e. 32-bit accesses for 32-bit fields, etc), but the device-specific region can be accessed using any width accesses, and should obtain the same results. Note that this is possible because while the virtio header is PCI (i.e. little) endian, the device-specific region is encoded in the native endian of the guest (where such distinction is applicable). 2.4.1.2.1 PCI Device Virtio Header ---------------------------------- The virtio header looks as follows: + ------------ ++ --------------------- + --------------------- + ---------- + -------- + --------- + --------- + --------- + -------- + Bits 32 32 32 16 16 16 8 8 + ------------ ++ --------------------- + --------------------- + ---------- + -------- + --------- + --------- + --------- + -------- + Read/Write R R+W R+W R R+W R+W R+W R + ------------ ++ --------------------- + --------------------- + ---------- + -------- + --------- + --------- + --------- + -------- + Purpose Device Guest Queue Queue Queue Queue Device ISR Features bits 0:31 Features bits 0:31 Address Size Select Notify Status Status + ------------ ++ --------------------- + --------------------- + ---------- + -------- + --------- + --------- + --------- + -------- + If MSI-X is enabled for the device, two additional fields immediately follow this header:[5] + ------------ ++ ---------------- + -------- + Bits 16 16 + ---------------- + -------- + + ------------ ++ ---------------- + -------- + Read/Write R+W R+W + ------------ ++ ---------------- + -------- + Purpose Configuration Queue (MSI-X) Vector Vector + ------------ ++ ---------------- + -------- + Immediately following these general headers, there may be device-specific headers: + ------------ ++ -------------------- + Bits Device Specific + -------------------- + + ------------ ++ -------------------- + Read/Write Device Specific + ------------ ++ -------------------- + Purpose Device Specific... + ------------ ++ -------------------- + 2.4.1.3 PCI-specific Initialization And Device Operation -------------------------------------------------------- The page size for a virtqueue on a PCI virtio device is defined as 4096 bytes. 2.4.1.3.1 Device Initialization ------------------------------- 2.4.1.3.1.1 Queue Vector Configuration -------------------------------------- When MSI-X capability is present and enabled in the device (through standard PCI configuration space) 4 bytes at byte offset 20 are used to map configuration change and queue interrupts to MSI-X vectors. In this case, the ISR Status field is unused, and device specific configuration starts at byte offset 24 in virtio header structure. When MSI-X capability is not enabled, device specific configuration starts at byte offset 20 in virtio header. Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of Configuration/Queue Vector registers, maps interrupts triggered by the configuration change/selected queue events respectively to the corresponding MSI-X vector. To disable interrupts for a specific event type, unmap it by writing a special NO_VECTOR value: /* Vector value used to disable MSI for queue */ #define VIRTIO_MSI_NO_VECTOR 0xffff Reading these registers returns vector mapped to a given event, or NO_VECTOR if unmapped. All queue and configuration change events are unmapped by default. Note that mapping an event to vector might require allocating internal device resources, and might fail. Devices report such failures by returning the NO_VECTOR value when the relevant Vector field is read. After mapping an event to vector, the driver must verify success by reading the Vector field value: on success, the previously written value is returned, and on failure, NO_VECTOR is returned. If a mapping failure is detected, the driver can retry mapping with fewervectors, or disable MSI-X. 2.4.1.3.1.2 Virtqueue Configuration ----------------------------------- As a device can have zero or more virtqueues for bulk data transport (for example, the simplest network device has two), the driver needs to configure them as part of the device-specific configuration. This is done as follows, for each virtqueue a device has: 1. Write the virtqueue index (first queue is 0) to the Queue Select field. 2. Read the virtqueue size from the Queue Size field, which is always a power of 2. This controls how big the virtqueue is (see 2.1.4 Virtqueues). If this field is 0, the virtqueue does not exist. 3. Allocate and zero virtqueue in contiguous physical memory, on a 4096 byte alignment. Write the physical address, divided by 4096 to the Queue Address field.[6] 4. Optionally, if MSI-X capability is present and enabled on the device, select a vector to use to request interrupts triggered by virtqueue events. Write the MSI-X Table entry number corresponding to this vector in Queue Vector field. Read the Queue Vector field: on success, previously written value is returned; on failure, NO_VECTOR value is returned. 2.4.1.3.2 Notifying The Device ------------------------------ Device notification occurs by writing the 16-bit virtqueue index of this virtqueue to the Queue Notify field of the virtio header in the first I/O region of the PCI device. 2.4.1.3.3 Receiving Used Buffers From The Device If an interrupt is necessary: (a) If MSI-X capability is disabled: i. Set the lower bit of the ISR Status field for the device. ii. Send the appropriate PCI interrupt for the device. (b) If MSI-X capability is enabled: i. Request the appropriate MSI-X interrupt message for the device, Queue Vector field sets the MSI-X Table entry number. ii. If Queue Vector field value is NO_VECTOR, no interrupt message is requested for this event. The guest interrupt handler should: 1. If MSI-X capability is disabled: read the ISR Status field, which will reset it to zero. If the lower bit is zero, the interrupt was not for this device. Otherwise, the guest driver should look through the used rings of each virtqueue for the device, to see if any progress has been made by the device which requires servicing. 2. If MSI-X capability is enabled: look through the used rings of each virtqueue mapped to the specific MSI-X vector for the device, to see if any progress has been made by the device which requires servicing. 2.4.1.3.4 Notification of Device Configuration Changes ------------------------------------------------------ Some virtio PCI devices can change the device configuration state, as reflected in the virtio header in the PCI configuration space. In this case: 1. If MSI-X capability is disabled: an interrupt is delivered and the second highest bit is set in the ISR Status field to indicate that the driver should re-examine the configuration space. Note that a single interrupt can indicate both that one or more virtqueue has been used and that the configuration space has changed: even if the config bit is set, virtqueues must be scanned. 2. If MSI-X capability is enabled: an interrupt message is requested. The Configuration Vector field sets the MSI-X Table entry number to use. If Configuration Vector field value is NO_VECTOR, no interrupt message is requested for this event. 2.4.2 Virtio Over MMIO ---------------------- Virtual environments without PCI support (a common situation in embedded devices models) might use simple memory mapped device (“ virtio-mmio”) instead of the PCI device. The memory mapped virtio device behaviour is based on the PCI device specification. Therefore most of operations like device initialization, queues configuration and buffer transfers are nearly identical. Existing differences are described in the following sections. 2.4.2.1 MMIO Device Discovery ----------------------------- Unlike PCI, MMIO provides no generic device discovery. For systems using a device-tree such as Linux's dtc or Open Firmware, the suggested format is: virtio_block@1e000 { compatible = "virtio,mmio"; reg = <0x1e000 0x100>; interrupts = <42>; } 2.4.2.2 MMIO Device Layout -------------------------- MMIO virtio devices provides a set of memory mapped control registers, all 32 bits wide, followed by device-specific configuration space. The following list presents their layout: • Offset from the device base address Direction Name Description • 0x000 R MagicValue “virt” string. • 0x004 R Version Device version number. Currently must be 1. • 0x008 R DeviceID Virtio Subsystem Device ID (ie. 1 for network card). • 0x00c R VendorID Virtio Subsystem Vendor ID. • 0x010 R HostFeatures Flags representing features the device supports. Reading from this register returns 32 consecutive flag bits, first bit depending on the last value written to HostFeaturesSel register. Access to this register returns bits HostFeaturesSel*32 to (HostFeaturesSel*32)+31, eg. feature bits 0 to 31 if HostFeaturesSel is set to 0 and features bits 32 to 63 if HostFeaturesSel is set to 1. Also see [sub:Feature-Bits] • 0x014 W HostFeaturesSel Device (Host) features word selection. Writing to this register selects a set of 32 device feature bits accessible by reading from HostFeatures register. Device driver must write a value to the HostFeaturesSel register before reading from the HostFeatures register. • 0x020 W GuestFeatures Flags representing device features understood and activated by the driver. Writing to this register sets 32 consecutive flag bits, first bit depending on the last value written to GuestFeaturesSel register. Access to this register sets bits GuestFeaturesSel*32 to (GuestFeaturesSel*32)+31, eg. feature bits 0 to 31 if GuestFeaturesSel is set to 0 and features bits 32 to 63 if GuestFeaturesSel is set to 1. Also see [sub:Feature-Bits] • 0x024 W GuestFeaturesSel Activated (Guest) features word selection. Writing to this register selects a set of 32 activated feature bits accessible by writing to the GuestFeatures register. Device driver must write a value to the GuestFeaturesSel register before writing to the GuestFeatures register. • 0x028 W GuestPageSize Guest page size. Device driver must write the guest page size in bytes to the register during initialization, before any queues are used. This value must be a power of 2 and is used by the Host to calculate Guest address of the first queue page (see QueuePFN). • 0x030 W QueueSel Virtual queue index (first queue is 0). Writing to this register selects the virtual queue that the following operations on QueueNum, QueueAlign and QueuePFN apply to. • 0x034 R QueueNumMax Maximum virtual queue size. Reading from the register returns the maximum size of the queue the Host is ready to process or zero (0x0) if the queue is not available. This applies to the queue selected by writing to QueueSel and is allowed only when QueuePFN is set to zero (0x0), so when the queue is not actively used. • 0x038 W QueueNum Virtual queue size. Queue size is a number of elements in the queue, therefore size of the descriptor table and both available and used rings. Writing to this register notifies the Host what size of the queue the Guest will use. This applies to the queue selected by writing to QueueSel. • 0x03c W QueueAlign Used Ring alignment in the virtual queue. Writing to this register notifies the Host about alignment boundary of the Used Ring in bytes. This value must be a power of 2 and applies to the queue selected by writing to QueueSel. • 0x040 RW QueuePFN Guest physical page number of the virtual queue. Writing to this register notifies the host about location of the virtual queue in the Guest's physical address space. This value is the index number of a page starting with the queue Descriptor Table. Value zero (0x0) means physical address zero (0x00000000) and is illegal. When the Guest stops using the queue it must write zero (0x0) to this register. Reading from this register returns the currently used page number of the queue, therefore a value other than zero (0x0) means that the queue is in use. Both read and write accesses apply to the queue selected by writing to QueueSel. • 0x050 W QueueNotify Queue notifier. Writing a queue index to this register notifies the Host that there are new buffers to process in the queue. • 0x60 R InterruptStatus Interrupt status. Reading from this register returns a bit mask of interrupts asserted by the device. An interrupt is asserted if the corresponding bit is set, ie. equals one (1). – Bit 0 Used Ring Update This interrupt is asserted when the Host has updated the Used Ring in at least one of the active virtual queues. – Bit 1 Configuration change This interrupt is asserted when configuration of the device has changed. • 0x064 W InterruptACK Interrupt acknowledge. Writing to this register notifies the Host that the Guest finished handling interrupts. Set bits in the value clear the corresponding bits of the InterruptStatus register. • 0x070 RW Status Device status. Reading from this register returns the current device status flags. Writing non-zero values to this register sets the status flags, indicating the Guest progress. Writing zero (0x0) to this register triggers a device reset. Also see [sub:Device-Initialization-Sequence] • 0x100+ RW Config Device-specific configuration space starts at an offset 0x100 and is accessed with byte alignment. Its meaning and size depends on the device and the driver. Virtual queue size is a number of elements in the queue, therefore size of the descriptor table and both available and used rings. The endianness of the registers follows the native endianness of the Guest. Writing to registers described as “R” and reading from registers described as “W” is not permitted and can cause undefined behavior. 2.4.2.3 MMIO-specific Initialization And Device Operation --------------------------------------------------------- 2.4.2.3.1 Device Initialization ------------------------------- Unlike the fixed page size for PCI, the virtqueue page size is defined by the GuestPageSize field, as written by the guest. This must be done before the virtqueues are configured. 2.4.2.3.1.1 Virtqueue Configuration ----------------------------------- 1. Select the queue writing its index (first queue is 0) to the QueueSel register. 2. Check if the queue is not already in use: read QueuePFN register, returned value should be zero (0x0). 3. Read maximum queue size (number of elements) from the QueueNumMax register. If the returned value is zero (0x0) the queue is not available. 4. Allocate and zero the queue pages in contiguous virtual memory, aligning the Used Ring to an optimal boundary (usually page size). Size of the allocated queue may be smaller than or equal to the maximum size returned by the Host. 5. Notify the Host about the queue size by writing the size to QueueNum register. 6. Notify the Host about the used alignment by writing its value in bytes to QueueAlign register. 7. Write the physical number of the first page of the queue to the QueuePFN register. 2.4.2.3.2 Notifying The Device ------------------------------ The device is notified about new buffers available in a queue by writing the queue index to register QueueNum. 2.4.2.3.3 Receiving Used Buffers From The Device ------------------------------------------------ The memory mapped virtio device is using single, dedicated interrupt signal, which is raised when at least one of the interrupts described in the InterruptStatus register description is asserted. After receiving an interrupt, the driver must read the InterruptStatus register to check what caused the interrupt (see the register description). After the interrupt is handled, the driver must acknowledge it by writing a bit mask corresponding to the serviced interrupt to the InterruptACK register. 2.4.2.4.4 Notification of Device Configuration Changes ------------------------------------------------------ This is indicated by bit 1 in the InterruptStatus register, as documented in the register description. 2.5 Device Types ================ On top of the queues, config space and feature negotiation facilities built into virtio, several specific devices are defined. The following device IDs are used to identify different types of virtio devices. Some device IDs are reserved for devices which are not currently defined in this standard. Discovering what devices are available and their type is bus-dependent. + ------------ + -------------------- + Device ID Virtio Device + ------------ + -------------------- + + ------------ + -------------------- + 1 network card + ------------ + -------------------- + 2 block device + ------------ + -------------------- + 3 console + ------------ + -------------------- + 4 entropy source + ------------ + -------------------- + 5 memory ballooning + ------------ + -------------------- + 6 ioMemory + ------------ + -------------------- + 7 rpmsg + ------------ + -------------------- + 8 SCSI host + ------------ + -------------------- + 9 9P transport + ------------ + -------------------- + 10 mac80211 wlan + ------------ + -------------------- + 2.5.1 Network Device ==================== The virtio network device is a virtual ethernet card, and is the most complex of the devices supported so far by virtio. It has enhanced rapidly and demonstrates clearly how support for new features should be added to an existing device. Empty buffers are placed in one virtqueue for receiving packets, and outgoing packets are enqueued into another for transmission in that order. A third command queue is used to control advanced filtering features. 2.5.1.1 Device ID ----------------- 1 2.5.1.2 Virtqueues ------------------ 0:receiveq. 1:transmitq. 2:controlq Virtqueue 2 only exists if VIRTIO_NET_F_CTRL_VQ set. 2.5.1.3 Feature bits -------------------- VIRTIO_NET_F_CSUM (0) Device handles packets with partial checksum VIRTIO_NET_F_GUEST_CSUM (1) Guest handles packets with partial checksum VIRTIO_NET_F_MAC (5) Device has given MAC address. VIRTIO_NET_F_GSO (6) (Deprecated) device handles packets with any GSO type.[13] VIRTIO_NET_F_GUEST_TSO4 (7) Guest can receive TSOv4. VIRTIO_NET_F_GUEST_TSO6 (8) Guest can receive TSOv6. VIRTIO_NET_F_GUEST_ECN (9) Guest can receive TSO with ECN. VIRTIO_NET_F_GUEST_UFO (10) Guest can receive UFO. VIRTIO_NET_F_HOST_TSO4 (11) Device can receive TSOv4. VIRTIO_NET_F_HOST_TSO6 (12) Device can receive TSOv6. VIRTIO_NET_F_HOST_ECN (13) Device can receive TSO with ECN. VIRTIO_NET_F_HOST_UFO (14) Device can receive UFO. VIRTIO_NET_F_MRG_RXBUF (15) Guest can merge receive buffers. VIRTIO_NET_F_STATUS (16) Configuration status field is available. VIRTIO_NET_F_CTRL_VQ (17) Control channel is available. VIRTIO_NET_F_CTRL_RX (18) Control channel RX mode support. VIRTIO_NET_F_CTRL_VLAN (19) Control channel VLAN filtering. VIRTIO_NET_F_GUEST_ANNOUNCE(21) Guest can send gratuitous packets. Device configuration layout Two configuration fields are currently defined. The mac address field always exists (though is only valid if VIRTIO_NET_F_MAC is set), and the status field only exists if VIRTIO_NET_F_STATUS is set. Two read-only bits are currently defined for the status field: VIRTIO_NET_S_LINK_UP and VIRTIO_NET_S_ANNOUNCE. #define VIRTIO_NET_S_LINK_UP 1 #define VIRTIO_NET_S_ANNOUNCE 2 struct virtio_net_config { u8 mac[6]; u16 status; }; 2.5.1.4 Device Initialization ----------------------------- 1. The initialization routine should identify the receive and transmission virtqueues. 2. If the VIRTIO_NET_F_MAC feature bit is set, the configuration space “mac” entry indicates the “physical” address of the the network card, otherwise a private MAC address should be assigned. All guests are expected to negotiate this feature if it is set. 3. If the VIRTIO_NET_F_CTRL_VQ feature bit is negotiated, identify the control virtqueue. 4. If the VIRTIO_NET_F_STATUS feature bit is negotiated, the link status can be read from the bottom bit of the “status” config field. Otherwise, the link should be assumed active. 5. The receive virtqueue should be filled with receive buffers. This is described in detail below in “Setting Up Receive Buffers”. 6. A driver can indicate that it will generate checksumless packets by negotating the VIRTIO_NET_F_CSUM feature. This “ checksum offload” is a common feature on modern network cards. 7. If that feature is negotiated[14], a driver can use TCP or UDP segmentation offload by negotiating the VIRTIO_NET_F_HOST_TSO4 (IPv4 TCP), VIRTIO_NET_F_HOST_TSO6 (IPv6 TCP) and VIRTIO_NET_F_HOST_UFO (UDP fragmentation) features. It should not send TCP packets requiring segmentation offload which have the Explicit Congestion Notification bit set, unless the VIRTIO_NET_F_HOST_ECN feature is negotiated.[15] 8. The converse features are also available: a driver can save the virtual device some work by negotiating these features.[16] The VIRTIO_NET_F_GUEST_CSUM feature indicates that partially checksummed packets can be received, and if it can do that then the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6, VIRTIO_NET_F_GUEST_UFO and VIRTIO_NET_F_GUEST_ECN are the input equivalents of the features described above. See “Receiving Packets” below. 2.5.1.5 Device Operation ------------------------ Packets are transmitted by placing them in the transmitq, and buffers for incoming packets are placed in the receiveq. In each case, the packet itself is preceeded by a header: struct virtio_net_hdr { #define VIRTIO_NET_HDR_F_NEEDS_CSUM 1 u8 flags; #define VIRTIO_NET_HDR_GSO_NONE 0 #define VIRTIO_NET_HDR_GSO_TCPV4 1 #define VIRTIO_NET_HDR_GSO_UDP 3 #define VIRTIO_NET_HDR_GSO_TCPV6 4 #define VIRTIO_NET_HDR_GSO_ECN 0x80 u8 gso_type; u16 hdr_len; u16 gso_size; u16 csum_start; u16 csum_offset; /* Only if VIRTIO_NET_F_MRG_RXBUF: */ u16 num_buffers }; The controlq is used to control device features such as filtering. 2.5.1.5.1 Packet Transmission ----------------------------- Transmitting a single packet is simple, but varies depending on the different features the driver negotiated. 1. If the driver negotiated VIRTIO_NET_F_CSUM, and the packet has not been fully checksummed, then the virtio_net_hdr's fields are set as follows. Otherwise, the packet must be fully checksummed, and flags is zero. • flags has the VIRTIO_NET_HDR_F_NEEDS_CSUM set, • csum_start is set to the offset within the packet to begin checksumming, and • csum_offset indicates how many bytes after the csum_start the new (16 bit ones' complement) checksum should be placed.[17] 2. If the driver negotiated VIRTIO_NET_F_HOST_TSO4, TSO6 or UFO, and the packet requires TCP segmentation or UDP fragmentation, then the “gso_type” field is set to VIRTIO_NET_HDR_GSO_TCPV4, TCPV6 or UDP. (Otherwise, it is set to VIRTIO_NET_HDR_GSO_NONE). In this case, packets larger than 1514 bytes can be transmitted: the metadata indicates how to replicate the packet header to cut it into smaller packets. The other gso fields are set: • hdr_len is a hint to the device as to how much of the header needs to be kept to copy into each packet, usually set to the length of the headers, including the transport header.[18] • gso_size is the maximum size of each packet beyond that header (ie. MSS). • If the driver negotiated the VIRTIO_NET_F_HOST_ECN feature, the VIRTIO_NET_HDR_GSO_ECN bit may be set in “gso_type” as well, indicating that the TCP packet has the ECN bit set.[19] 3. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, the num_buffers field is set to zero. 4. The header and packet are added as one output buffer to the transmitq, and the device is notified of the new entry (see 2.4.1.4 Notifying The Device).[20] 2.5.1.5.1.1 Packet Transmission Interrupt ----------------------------------------- Often a driver will suppress transmission interrupts using the VRING_AVAIL_F_NO_INTERRUPT flag (see 2.4.2 Receiving Used Buffers From The Device) and check for used packets in the transmit path of following packets. However, it will still receive interrupts if the VIRTIO_F_NOTIFY_ON_EMPTY feature is negotiated, indicating that the transmission queue is completely emptied. The normal behavior in this interrupt handler is to retrieve and new descriptors from the used ring and free the corresponding headers and packets. 2.5.1.5.2 Setting Up Receive Buffers It is generally a good idea to keep the receive virtqueue as fully populated as possible: if it runs out, network performance will suffer. If the VIRTIO_NET_F_GUEST_TSO4, VIRTIO_NET_F_GUEST_TSO6 or VIRTIO_NET_F_GUEST_UFO features are used, the Guest will need to accept packets of up to 65550 bytes long (the maximum size of a TCP or UDP packet, plus the 14 byte ethernet header), otherwise 1514 bytes. So unless VIRTIO_NET_F_MRG_RXBUF is negotiated, every buffer in the receive queue needs to be at least this length [20a] If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer must be at least the size of the struct virtio_net_hdr. 2.5.1.5.2.1 Packet Receive Interrupt ------------------------------------ When a packet is copied into a buffer in the receiveq, the optimal path is to disable further interrupts for the receiveq (see [sub:Receiving-Used-Buffers]) and process packets until no more are found, then re-enable them. Processing packet involves: 1. If the driver negotiated the VIRTIO_NET_F_MRG_RXBUF feature, then the “num_buffers” field indicates how many descriptors this packet is spread over (including this one). This allows receipt of large packets without having to allocate large buffers. In this case, there will be at least “num_buffers” in the used ring, and they should be chained together to form a single packet. The other buffers will not begin with a struct virtio_net_hdr. 2. If the VIRTIO_NET_F_MRG_RXBUF feature was not negotiated, or the “num_buffers” field is one, then the entire packet will be contained within this buffer, immediately following the struct virtio_net_hdr. 3. If the VIRTIO_NET_F_GUEST_CSUM feature was negotiated, the VIRTIO_NET_HDR_F_NEEDS_CSUM bit in the “flags” field may be set: if so, the checksum on the packet is incomplete and the “ csum_start” and “csum_offset” fields indicate how to calculate it (see Packet Transmission point 1). 4. If the VIRTIO_NET_F_GUEST_TSO4, TSO6 or UFO options were negotiated, then the “gso_type” may be something other than VIRTIO_NET_HDR_GSO_NONE, and the “gso_size” field indicates the desired MSS (see Packet Transmission point 2). 2.5.1.5.3 Control Virtqueue --------------------------- The driver uses the control virtqueue (if VIRTIO_NET_F_VTRL_VQ is negotiated) to send commands to manipulate various features of the device which would not easily map into the configuration space. All commands are of the following form: struct virtio_net_ctrl { u8 class; u8 command; u8 command-specific-data[]; u8 ack; }; /* ack values */ #define VIRTIO_NET_OK 0 #define VIRTIO_NET_ERR 1 The class, command and command-specific-data are set by the driver, and the device sets the ack byte. There is little it can do except issue a diagnostic if the ack byte is not VIRTIO_NET_OK. 2.5.1.5.3.1 Packet Receive Filtering ------------------------------------ If the VIRTIO_NET_F_CTRL_RX feature is negotiated, the driver can send control commands for promiscuous mode, multicast receiving, and filtering of MAC addresses. Note that in general, these commands are best-effort: unwanted packets may still arrive. Setting Promiscuous Mode #define VIRTIO_NET_CTRL_RX 0 #define VIRTIO_NET_CTRL_RX_PROMISC 0 #define VIRTIO_NET_CTRL_RX_ALLMULTI 1 The class VIRTIO_NET_CTRL_RX has two commands: VIRTIO_NET_CTRL_RX_PROMISC turns promiscuous mode on and off, and VIRTIO_NET_CTRL_RX_ALLMULTI turns all-multicast receive on and off. The command-specific-data is one byte containing 0 (off) or 1 (on). 2.5.1.5.3.2 Setting MAC Address Filtering ----------------------------------------- struct virtio_net_ctrl_mac { u32 entries; u8 macs[entries][ETH_ALEN]; }; #define VIRTIO_NET_CTRL_MAC 1 #define VIRTIO_NET_CTRL_MAC_TABLE_SET 0 The device can filter incoming packets by any number of destination MAC addresses.[21] This table is set using the class VIRTIO_NET_CTRL_MAC and the command VIRTIO_NET_CTRL_MAC_TABLE_SET. The command-specific-data is two variable length tables of 6-byte MAC addresses. The first table contains unicast addresses, and the second contains multicast addresses. 2.5.1.5.3.3 VLAN Filtering -------------------------- If the driver negotiates the VIRTION_NET_F_CTRL_VLAN feature, it can control a VLAN filter table in the device. #define VIRTIO_NET_CTRL_VLAN 2 #define VIRTIO_NET_CTRL_VLAN_ADD 0 #define VIRTIO_NET_CTRL_VLAN_DEL 1 Both the VIRTIO_NET_CTRL_VLAN_ADD and VIRTIO_NET_CTRL_VLAN_DEL command take a 16-bit VLAN id as the command-specific-data. 2.5.1.5.3.4 Gratuitous Packet Sending ------------------------------------- If the driver negotiates the VIRTIO_NET_F_GUEST_ANNOUNCE (depends on VIRTIO_NET_F_CTRL_VQ), it can ask the guest to send gratuitous packets; this is usually done after the guest has been physically migrated, and needs to announce its presence on the new network links. (As hypervisor does not have the knowledge of guest network configuration (eg. tagged vlan) it is simplest to prod the guest in this way). #define VIRTIO_NET_CTRL_ANNOUNCE 3 #define VIRTIO_NET_CTRL_ANNOUNCE_ACK 0 The Guest needs to check VIRTIO_NET_S_ANNOUNCE bit in status field when it notices the changes of device configuration. The command VIRTIO_NET_CTRL_ANNOUNCE_ACK is used to indicate that driver has recevied the notification and device would clear the VIRTIO_NET_S_ANNOUNCE bit in the status filed after it received this command. Processing this notification involves: 1. Sending the gratuitous packets or marking there are pending gratuitous packets to be sent and letting deferred routine to send them. 2. Sending VIRTIO_NET_CTRL_ANNOUNCE_ACK command through control vq. 2.5.2 Block Device ================== The virtio block device is a simple virtual block device (ie. disk). Read and write requests (and other exotic requests) are placed in the queue, and serviced (probably out of order) by the device except where noted. 2.5.2.1 Device ID ----------------- 2 2.5.2.2 Virtqueues ------------------ 0:requestq 2.5.2.3 Feature bits -------------------- VIRTIO_BLK_F_BARRIER (0) Host supports request barriers. VIRTIO_BLK_F_SIZE_MAX (1) Maximum size of any single segment is in “size_max”. VIRTIO_BLK_F_SEG_MAX (2) Maximum number of segments in a request is in “seg_max”. VIRTIO_BLK_F_GEOMETRY (4) Disk-style geometry specified in “ geometry”. VIRTIO_BLK_F_RO (5) Device is read-only. VIRTIO_BLK_F_BLK_SIZE (6) Block size of disk is in “blk_size”. VIRTIO_BLK_F_SCSI (7) Device supports scsi packet commands. VIRTIO_BLK_F_FLUSH (9) Cache flush command support. Device configuration layout The capacity of the device (expressed in 512-byte sectors) is always present. The availability of the others all depend on various feature bits as indicated above. struct virtio_blk_config { u64 capacity; u32 size_max; u32 seg_max; struct virtio_blk_geometry { u16 cylinders; u8 heads; u8 sectors; } geometry; u32 blk_size; }; 2.5.2.4 Device Initialization ----------------------------- 1. The device size should be read from the “capacity” configuration field. No requests should be submitted which goes beyond this limit. 2. If the VIRTIO_BLK_F_BLK_SIZE feature is negotiated, the blk_size field can be read to determine the optimal sector size for the driver to use. This does not effect the units used in the protocol (always 512 bytes), but awareness of the correct value can effect performance. 3. If the VIRTIO_BLK_F_RO feature is set by the device, any write requests will fail. 2.5.2.5 Device Operation ------------------------ The driver queues requests to the virtqueue, and they are used by the device (not necessarily in order). Each request is of form: struct virtio_blk_req { u32 type; u32 ioprio; u64 sector; char data[][512]; u8 status; }; If the device has VIRTIO_BLK_F_SCSI feature, it can also support scsi packet command requests, each of these requests is of form: struct virtio_scsi_pc_req { u32 type; u32 ioprio; u64 sector; char cmd[]; char data[][512]; #define SCSI_SENSE_BUFFERSIZE 96 u8 sense[SCSI_SENSE_BUFFERSIZE]; u32 errors; u32 data_len; u32 sense_len; u32 residual; u8 status; }; The type of the request is either a read (VIRTIO_BLK_T_IN), a write (VIRTIO_BLK_T_OUT), a scsi packet command (VIRTIO_BLK_T_SCSI_CMD or VIRTIO_BLK_T_SCSI_CMD_OUT[22]) or a flush (VIRTIO_BLK_T_FLUSH or VIRTIO_BLK_T_FLUSH_OUT[23]). If the device has VIRTIO_BLK_F_BARRIER feature the high bit (VIRTIO_BLK_T_BARRIER) indicates that this request acts as a barrier and that all preceeding requests must be complete before this one, and all following requests must not be started until this is complete. Note that a barrier does not flush caches in the underlying backend device in host, and thus does not serve as data consistency guarantee. Driver must use FLUSH request to flush the host cache. #define VIRTIO_BLK_T_IN 0 #define VIRTIO_BLK_T_OUT 1 #define VIRTIO_BLK_T_SCSI_CMD 2 #define VIRTIO_BLK_T_SCSI_CMD_OUT 3 #define VIRTIO_BLK_T_FLUSH 4 #define VIRTIO_BLK_T_FLUSH_OUT 5 #define VIRTIO_BLK_T_BARRIER 0x80000000 The ioprio field is a hint about the relative priorities of requests to the device: higher numbers indicate more important requests. The sector number indicates the offset (multiplied by 512) where the read or write is to occur. This field is unused and set to 0 for scsi packet commands and for flush commands. The cmd field is only present for scsi packet command requests, and indicates the command to perform. This field must reside in a single, separate read-only buffer; command length can be derived from the length of this buffer. Note that these first three (four for scsi packet commands) fields are always read-only: the data field is either read-only or write-only, depending on the request. The size of the read or write can be derived from the total size of the request buffers. The sense field is only present for scsi packet command requests, and indicates the buffer for scsi sense data. The data_len field is only present for scsi packet command requests, this field is deprecated, and should be ignored by the driver. Historically, devices copied data length there. The sense_len field is only present for scsi packet command requests and indicates the number of bytes actually written to the sense buffer. The residual field is only present for scsi packet command requests and indicates the residual size, calculated as data length - number of bytes actually transferred. The final status byte is written by the device: either VIRTIO_BLK_S_OK for success, VIRTIO_BLK_S_IOERR for host or guest error or VIRTIO_BLK_S_UNSUPP for a request unsupported by host: #define VIRTIO_BLK_S_OK 0 #define VIRTIO_BLK_S_IOERR 1 #define VIRTIO_BLK_S_UNSUPP 2 Historically, devices assumed that the fields type, ioprio and sector reside in a single, separate read-only buffer; the fields errors, data_len, sense_len and residual reside in a single, separate write-only buffer; the sense field in a separate write-only buffer of size 96 bytes, by itself; the fields errors, data_len, sense_len and residual in a single write-only buffer; and the status field is a separate read-only buffer of size 1 byte, by itself. 2.5.3 Console Device ==================== The virtio console device is a simple device for data input and output. A device may have one or more ports. Each port has a pair of input and output virtqueues. Moreover, a device has a pair of control IO virtqueues. The control virtqueues are used to communicate information between the device and the driver about ports being opened and closed on either side of the connection, indication from the host about whether a particular port is a console port, adding new ports, port hot-plug/unplug, etc., and indication from the guest about whether a port or a device was successfully added, port open/close, etc.. For data IO, one or more empty buffers are placed in the receive queue for incoming data and outgoing characters are placed in the transmit queue. 2.5.3.1 Device ID ----------------- 3 2.5.3.2 Virtqueues ------------------ 0:receiveq(port0). 1:transmitq(port0), 2:control receiveq, 3:control transmitq, 4:receiveq(port1), 5:transmitq(port1), ... Ports 2 onwards only exist if VIRTIO_CONSOLE_F_MULTIPORT is set. 2.5.3.3 Feature bits -------------------- VIRTIO_CONSOLE_F_SIZE (0) Configuration cols and rows fields are valid. VIRTIO_CONSOLE_F_MULTIPORT(1) Device has support for multiple ports; configuration fields nr_ports and max_nr_ports are valid and control virtqueues will be used. 2.5.3.4 Device configuration layout ----------------------------------- The size of the console is supplied in the configuration space if the VIRTIO_CONSOLE_F_SIZE feature is set. Furthermore, if the VIRTIO_CONSOLE_F_MULTIPORT feature is set, the maximum number of ports supported by the device can be fetched. struct virtio_console_config { u16 cols; u16 rows; u32 max_nr_ports; }; 2.5.3.5 Device Initialization ----------------------------- 1. If the VIRTIO_CONSOLE_F_SIZE feature is negotiated, the driver can read the console dimensions from the configuration fields. 2. If the VIRTIO_CONSOLE_F_MULTIPORT feature is negotiated, the driver can spawn multiple ports, not all of which may be attached to a console. Some could be generic ports. In this case, the control virtqueues are enabled and according to the max_nr_ports configuration-space value, the appropriate number of virtqueues are created. A control message indicating the driver is ready is sent to the host. The host can then send control messages for adding new ports to the device. After creating and initializing each port, a VIRTIO_CONSOLE_PORT_READY control message is sent to the host for that port so the host can let us know of any additional configuration options set for that port. 3. The receiveq for each port is populated with one or more receive buffers. 2.5.3.6 Device Operation ------------------------ 1. For output, a buffer containing the characters is placed in the port's transmitq.[25] 2. When a buffer is used in the receiveq (signalled by an interrupt), the contents is the input to the port associated with the virtqueue for which the notification was received. 3. If the driver negotiated the VIRTIO_CONSOLE_F_SIZE feature, a configuration change interrupt may occur. The updated size can be read from the configuration fields. 4. If the driver negotiated the VIRTIO_CONSOLE_F_MULTIPORT feature, active ports are announced by the host using the VIRTIO_CONSOLE_PORT_ADD control message. The same message is used for port hot-plug as well. 5. If the host specified a port `name', a sysfs attribute is created with the name filled in, so that udev rules can be written that can create a symlink from the port's name to the char device for port discovery by applications in the guest. 6. Changes to ports' state are effected by control messages. Appropriate action is taken on the port indicated in the control mess