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[PATCH RFC] virtio-pci: new configuration layout
"Michael S. Tsirkin" <> writes:
> This is the new configuration layout. >
> Notes: > - Everything is LE > - There's a feature bit that means spec 1.0 compliant. > - Both devices and drivers can either require the 1.0 interface
>
or try to include compatibility support. The spec isn't forcing
>
this decision. Hmm, this kind includes other changes already proposed, like the LE change and the framing change. I think this conceptually splits nicely: 1) Feature bit 32 proposal. 2) Endian change. 3) Framing change. 4) PCI layout change. > - I kept documentation of the legacy interface around, and added notes
>
on transition inline. They are in separate sections each clearly marked >
"Legacy Interface" so we'll be able to separate them out
>
from the final document as necessary - for now I think it's easier
>
to keep it all together. Good thinking: most of us know the current spec so it's definitely clearer. And makes sure we're thinking about the transition.
> Only virtio PCI has been converted.
> Let's discuss this on the meeting tonight, once we figure out PCI
> we can do something similar for MMIO and CCW. > @@ -137,6 +139,11 @@ Feature bits are allocated as follows:
>
24 to 31: Feature bits reserved for extensions to the queue and
>
feature negotiation mechanisms >
> + 32: Feature bit must be set for any device compliant with this > + revision of the specification, and acknowledged by all device drivers. > + > + 33 to 63: Feature bits reserved for future extensions > +
> For example, feature bit 0 for a network device (i.e. Subsystem
> Device ID 1) indicates that the device supports checksumming of
> packets. Why stop at 63? If we go to a more decentralized feature-assignment model, we'll run through those very fast. > @@ -145,13 +152,63 @@ 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. >
> +2.1.2.1 Legacy Interface: A Note on transitions from earlier drafts > +
--------------------------------------
> + > +Earlier drafts of this specification (up to 0.9.X) defined a similar, but > +different interface between the hypervisor and the guest. > +Since these are widely deployed in the field, this specification > +accomodates optional features to simplify transition > +from these earlier draft interfaces. Specifically: > + > +Legacy Interface > + is an interface specified by an earlier draft of this specification > +
(up to 0.9.X) > +Legacy Device > + is a device implemented before this specification was released, > +
and implementing a legacy interface on the host side > +Legacy Driver > + is a driver implemented before this specification was released, > +
and implementing a legacy interface on the guest side > + > +to simplify transition from these earlier draft interfaces, > +it is possible to implement > + > +Transitional Device > + a device supporting both drivers conforming to this > +
specification, and legacy drivers > + > +Transitional Driver > + a driver supporting both devices conforming to this > + specification, and legacy devices > + > +Device and driver that require support for revision 1.0 or newer of > +the specification to function, are called non-transitional device and driver, > +respectively. > + > +Transitional Drivers can detect Legacy Devices by detecting that > +Feature bit 32 is not offered. > +Transitional devices can detect Legacy drivers by detecting that > +Feature bit 32 has not been acknowledged by driver. > + > +To make them easier to locate, specification sections documenting these > +transitional features all explicitly marked with > +'Legacy Interface' in the section title. > + > +
> 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, > +Note that configuration space generally uses the little-endian format > +for multi-byte fields. > + > +2.1.4.1 Legacy Interface: A Note on Configuration Space endian-ness > +
--------------------------------------
> + > +Note that for legacy interfaces, configuration space is generally the guest's native endian,
> rather than PCI's little-endian. >
> 2.1.4 Virtqueues > @@ -164,6 +221,45 @@ 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 consists of three parts: > + > + Descriptor Table > + Available Ring > + Used Ring > + > +where each part is physically-contiguous in guest memory, > +and has different alignment requirements. > + > +The Queue Size field controls the total number of bytes > +required for each part of the virtqueue. > + > +The memory aligment and size requirements, in bytes, of each part of the > +virtqueue are summarized in the following table (qsz is the Queue Size field): > + > ++
------------
+
---------------------------------
+ > + Virtqueue Part
Alignment
Size
> ++
------------
+
---------------------------------
+ > ++
------------
+
---------------------------------
+ > + Descriptor Table
16
16 * qsz
> ++
------------
+
---------------------------------
+ > + Available Ring
2
6 + 2 * qsz
> ++
------------
+
---------------------------------
+ > + Used Ring
4
6 + 4 * qsz
> ++
------------
+
---------------------------------
+ > + > +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 Legacy Interfaces: A Note on Virtqueue Layout > +
--------------------------------------
> + > +For Legacy Interfaces, several additional > +restrictions are placed on the virtqueue layout: > +
> Each virtqueue occupies two or more physically-contiguous pages > (usually defined as 4096 bytes, but depending on the transport)
> and consists of three parts: > @@ -182,9 +278,8 @@ required for the virtqueue according to the following formula: >
+ ALIGN(sizeof(u16)*3 + sizeof(struct vring_used_elem)*qsz); > } >
> -This currently wastes some space with padding, but also allows future > -extensions such as the VIRTIO_RING_F_EVENT_IDX extension. The > -virtqueue layout structure looks like this: > +This wastes some space with padding. > +The legacy virtqueue layout structure therefore looks like this: >
> struct vring { > // The actual descriptors (16 bytes each) > @@ -200,25 +295,17 @@ virtqueue layout structure looks like this:
> 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 > +2.1.4.1 Legacy Interfaces: 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. > +endian of the guest, not little-endian as PCI normally is. > +It is assumed that the host is already aware of the guest endian. >
> 2.1.4.2 Message Framing >
-----------------------
> -The original intent of the specification was that message framing (the > -particular layout of descriptors) be independent of the contents of > +Generally, the intent of the specification is for message framing (the > +particular layout of descriptors) to be independent of the contents of
> the buffers. For example, a network transmit buffer consists of a 12
> byte header followed by the network packet. This could be most simply
> placed in the descriptor table as a 12 byte output descriptor followed > @@ -227,16 +314,21 @@ single 1526 byte output descriptor in the case where the header and
> packet are adjacent, or even three or more descriptors (possibly with
> loss of efficiency in that case). >
> -Regrettably, initial driver implementations used simple layouts, and > -devices came to rely on it, despite this specification wording[10]. It > -is thus recommended that drivers be conservative in their assumptions, > -unless the VIRTIO_F_ANY_LAYOUT feature is accepted. In addition, some > +In addition, some
> implementations may have large-but-reasonable restrictions on total
> descriptor size (such as based on IOV_MAX in the host OS). This has
> not been a problem in practice: little sympathy will be given to
> drivers which create unreasonably-sized descriptors such as by
> dividing a network packet into 1500 single-byte descriptors! >
> +2.1.4.2.1 Legacy Interfaces: A Note on Message Framing > +
-----------------------
> +Regrettably, initial driver implementations used simple layouts, and > +devices came to rely on it, despite this specification wording[10]. It > +is thus recommended that when using legacy interfaces, > +drivers should be conservative in their assumptions, > +unless the VIRTIO_F_ANY_LAYOUT feature is accepted. > +
> 2.1.4.3 The Virtqueue Descriptor Table >
--------------------------------------
>
> @@ -386,23 +478,27 @@ how to communicate with the specific device.
> 2.2.1 Device Initialization >
---------------------------
>
> -1. Reset the device. This is not required on initial start up. > +1. Device discovery. This is only required for some transports. > + > +2. Reset the device. This is not required on initial start up. >
> -2. The ACKNOWLEDGE status bit is set: we have noticed the device. > +3. Device layout detection. This is only required for some transports. >
> -3. The DRIVER status bit is set: we know how to drive the device. > +4. The ACKNOWLEDGE status bit is set: we have noticed the device. >
> -4. Device-specific setup, including reading the device feature > +5. The DRIVER status bit is set: we know how to drive the device. > + > +6. 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 > +7. The subset of device feature bits understood by the driver is
>
written to the device. >
> -6. The DRIVER_OK status bit is set. > +8. The DRIVER_OK status bit is set. >
> -7. The device can now be used (ie. buffers added to the > +9. The device can now be used (ie. buffers added to the
>
virtqueues)[4] >
> If any of these steps go irrecoverably wrong, the guest should > @@ -622,35 +718,183 @@ Virtio devices are commonly implemented as PCI devices. >
> 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. > +Revision ID of 0 or Revision ID of 1 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). >
> +Drivers must not match devices where Revision ID does not match 0 or 1. > + > +2.4.1.1.1 Legacy Interface: A Note on PCI Device Discovery > +
----------------------------
> +Transitional devices must have a Revision ID of 0. > + > +Non-transitional devices must have a Revision ID of 1. > + > +Transitional drivers must match a Revision ID of 0 or 1. > + > +Non-transitional drivers must only match a Revision ID of 1. > + I think we should stop abusing Revision IDs, and start using them to reflect device version changes as intended. We could reserve revision id 0 for legacy devices, however, which should work nicely.
> 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. > +To configure the device, > +use I/O and/or memory regions and/or PCI configuration space of the PCI device. > +These contain the virtio header registers, the notification register, the > +ISR status register and device specific registers, as specified by Virtio > ++ Structure PCI Capabilities > + > +There may be different widths of accesses to the I/O region; the > +“natural” access method for each field must be > +used (i.e. 32-bit accesses for 32-bit fields, etc). > + > +PCI Device Configuration Layout includes the common configuration, > +ISR, notification and device specific configuration > +structures. > + > +Unless explicitly specified otherwise, all multi-byte fields are little-endian. > + > + > +2.4.1.2.1 Common configuration structure layout > +
-------------------------
> +Common configuration structure layout is documented below: > + > +struct virtio_pci_common_cfg { > + /* About the whole device. */ > + __le32 device_feature_select; /* read-write */ > + __le32 device_feature; /* read-only */ > + __le32 guest_feature_select; /* read-write */ > + __le32 guest_feature; /* read-write */ > + __le16 msix_config; /* read-write */ > + __le16 num_queues; /* read-only */ > + __u8 device_status; /* read-write */ > + __u8 unused1; > + > + /* About a specific virtqueue. */ > + __le16 queue_select; /* read-write */ > + __le16 queue_size; /* read-write, power of 2, or 0. */ > + __le16 queue_msix_vector; /* read-write */ > + __le16 queue_enable; /* read-write */ > + __le16 queue_notify_off; /* read-only */ > + __le64 queue_desc; /* read-write */ > + __le64 queue_avail; /* read-write */ > + __le64 queue_used; /* read-write */ > +}; > + > +device_feature_select > + > + Selects which Feature Bits does device_feature field refer to. > + Value 0x0 selects Feature Bits 0 to 31 > + Value 0x1 selects Feature Bits 32 to 63 > + All other values cause reads from device_feature to return 0. > + > +device_feature > + > + Used by Device to report Feature Bits to Driver. > + Device Feature Bits selected by device_feature_select. > + > +guest_feature_select > + > + Selects which Feature Bits does guest_feature field refer to. > + Value 0x0 selects Feature Bits 0 to 31 > + Value 0x1 selects Feature Bits 32 to 63 > + All other values cause writes to guest_feature to be ignored, > + and reads to return 0. > + > +guest_feature > + > + Used by Driver to acknowledge Feature Bits to Device. > + Guest Feature Bits selected by guest_feature_select. > + > +msix_config > + > + Configuration Vector for MSI-X. > + > +num_queues > + > + Specifies the maximum number of virtqueues supported by device. > + > +device_status > + > + Device Status field. > + > +queue_select > + > + Queue Select. Selects which virtqueue do other fields refer to. > + > +queue_size > + > + Queue Size. On reset, specifies the maximum queue size supported by > + the hypervisor. This can be modified by driver to reduce memory requirements. > + Set to 0 if this virtqueue is unused. > + > +queue_msix_vector > + > + Queue Vector for MSI-X. > + > +queue_enable > + > + Used to selectively prevent host from executing requests from this virtqueue. > + 1 - enabled; 0 - disabled > + > +queue_notify_off > + > + Used to calculate the offset from start of Notification structure at > + which this virtqueue is located. > + Note: this is *not* an offset in bytes. See notify_off_multiplier below. > + > +queue_desc > + > + Physical address of Descriptor Table. > + > +queue_avail > + > + Physical address of Available Ring. > + > +queue_used > + > + Physical address of Used Ring. > + > + > +2.4.1.2.2 ISR status structure layout > +
-------------------------
> +ISR status structure includes a single 8-bite ISR status field 8-bit > + > +2.4.1.2.3 Notification structure layout > +
-------------------------
> +Notification structure is always a multiple of 2 bytes in size. > +It includes 2-byte Queue Notify fields for each virtqueue of > +the device. Note that multiple virtqueues can use the same > +Queue Notify field, if necessary. Hmm, maybe move this down, so you can have a section which starts with "If cfg_type is VIRTIO_PCI_CAP_NOTIFY_CFG" below? That would put it all together. > + > +2.4.1.2.4 Device specific structure > +
-------------------------
> + > +Device specific structure is optional. > + > +2.4.1.2.5 Legacy Interfaces: A Note on PCI Device Layout > +
-------------------------
> + > +Transitional devices should present part of configuration > +registers in a legacy configuration structure in BAR0 in the first I/O > +region of the PCI device, as documented below. >
> 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 > +used (i.e. 32-bit accesses for 32-bit fields, etc), but > +When accessed through the legacy interface 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 > +(i.e. little) endian, when using the legacy interface 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: > +When used through the legacy interface, the virtio header looks as follows: >
> +
------------
++
---------------------
+
---------------------
+
----------
+
--------
+
---------
+
---------
+
---------
+
--------
+
>
Bits
32
32
32
16
16
16
8
8
> @@ -661,7 +905,6 @@ The virtio header looks as follows:
>
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] >
> @@ -689,25 +932,154 @@ device-specific headers: >
> +
------------
++
--------------------
+ >
> +Note that only Feature Bits 0 to 31 are accessible through the > +Legacy Interface. When used through the Legacy Interface, > +Transitional Devices must assume that Feature Bits 32 to 63 > +are not acknowledged by Driver. > + > +
> 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 > +This documents PCI-specific steps executed during Device Initialization. > +As the first step, driver must detect device configuration layout > +to locate configuration fields in memory,I/O or configuration space of the > +device. > + > +2.4.1.3.1.1 Virtio Device Configuration Layout Detection > +
-------------------------------
> + > +As a prerequisite to device initialization, driver executes a > +PCI capability list scan, detecting virtio configuration layout using Virtio > +Structure PCI capabilities. > + > +Virtio Device Configuration Layout includes virtio configuration header, Notification > +and ISR Status and device configuration structures. > +Each structure can be mapped by a Base Address register (BAR) belonging to > +the function, located beginning at 10h in Configuration Space, > +or accessed though PCI configuration space. > + > +Actual location of each structure is specified using vendor-specific PCI capability located > +on capability list in PCI configuration space of the device. > +This virtio structure capability uses little-endian format; all bits are > +read-only: > + > +struct virtio_pci_cap { > + __u8 cap_vndr; /* Generic PCI field: PCI_CAP_ID_VNDR */ > + __u8 cap_next; /* Generic PCI field: next ptr. */ > + __u8 cap_len; /* Generic PCI field: capability length */ > + __u8 cfg_type; /* Identifies the structure. */ > + __u8 bar; /* Where to find it. */ > + __u8 padding[3];/* Pad to full dword. */ > + __le32 offset; /* Offset within bar. */ > + __le32 length; /* Length of the structure, in bytes. */ > +}; > + > +This structure can optionally followed by extra data, depending on > +other fields, as documented below. > + > +The fields are interpreted as follows: > + > +cap_vndr > + 0x09; Identifies a vendor-specific capability. > + > +cap_next > + Link to next capability in the capability list in the configuration space. > + > +cap_len > + Length of the capability structure, including the whole of > + struct virtio_pci_cap, and extra data if any. > + This length might include padding, or fields unused by the driver. > + > +cfg_type > + identifies the structure, according to the following table. > + > + /* Common configuration */ > + #define VIRTIO_PCI_CAP_COMMON_CFG 1 > + /* Notifications */ > + #define VIRTIO_PCI_CAP_NOTIFY_CFG 2 > + /* ISR Status */ > + #define VIRTIO_PCI_CAP_ISR_CFG 3 > + /* Device specific configuration */ > + #define VIRTIO_PCI_CAP_DEVICE_CFG 4 > + > + More than one capability can identify the same structure - this makes it > + possible for the device to expose multiple interfaces to drivers. The order of > + the capabilities in the capability list specifies the order of preference > + suggested by the device; drivers should use the first interface that they can > + support. For example, on some hypervisors, notifications using IO accesses are > + faster than memory accesses. In this case, hypervisor can expose two > + capabilities with cfg_type set to VIRTIO_PCI_CAP_NOTIFY_CFG: > + the first one addressing an I/O BAR, the second one addressing a memory BAR. > + Driver will use the I/O BAR if I/O resources are available, and fall back on > + memory BAR when I/O resources are unavailable. > + > +bar > + > + values 0x0 to 0x5 specify a Base Address register (BAR) belonging to > + the function located beginning at 10h in Configuration Space > + and used to map the structure into Memory or I/O Space. > + The BAR is permitted to be either 32-bit or 64-bit, it can map Memory Space > + or I/O Space. > + > + The value 0xF specifies that the structure is in PCI configuration space > + inline with this capability structure, following (not necessarily immediately) > + the length field. Why not immediately? Or how would the driver know where it is? > + > +offset > + indicates where the structure begins relative to the base address associated > + with the BAR. If bar specifies configuration space, offset is relative > + to start of virtio_pci_cap structure. > + > +length > + indicates the length of the structure. > + This size might include padding, or fields unused by the driver. > + Drivers are also recommended to only map part of configuration structure > + large enough for device operation. > + For example, a future device might present a large structure size of several > + MBytes. > + As current devices never utilize structures larger than 4KBytes in size, > + driver can limit the mapped structure size to e.g. > + 4KBytes to allow forward compatibility with such devices without loss of > + functionality and without wasting resources. > + > + > +If cfg_type is VIRTIO_PCI_CAP_NOTIFY_CFG this structure is immediately followed > +by additional fields: > + > +struct virtio_pci_notify_cap { > + struct virtio_pci_cap cap; > + __le32 notify_off_multiplier; /* Multiplier for queue_notify_off. */ > +}; > + > +notify_off_multiplier > + > + Virtqueue offset multiplier, in bytes. Must be even and either a power of two, or 0. > + Value 0x1 is reserved. > + For a given virtqueue, the address to use for notifications is calculated as follows: > + > + queue_notify_off * notify_off_multiplier + offset > + > + If notify_off_multiplier is 0, all virtqueues use the same address in > + the Notifications structure! > + > + > +2.4.1.3.1.1 Legacy Interface: A Note on Device Layout Detection > +
-------------------------------
> + > +Legacy drivers skipped Device Layout Detection step, assuming legacy > +configuration space in BAR0 in I/O space unconditionally. > + > +2.4.1.3.1.3 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. > +(through standard PCI configuration space)
Configuration/Queue > +MSI-X Vector registers are used to map configuration change and queue > +interrupts to MSI-X vectors. In this case, the ISR Status is unused. >
> Writing a valid MSI-X Table entry number, 0 to 0x7FF, to one of
> Configuration/Queue Vector registers, maps interrupts triggered > @@ -732,7 +1104,7 @@ 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 > +2.4.1.3.1.4 Virtqueue Configuration >
-----------------------------------
>
> As a device can have zero or more virtqueues for bulk data > @@ -749,9 +1121,11 @@ This is done as follows, for each virtqueue a device has:
>
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] > +3. Optionally, select a smaller virtqueue size and write it in the Queue Size > +
field. > + > +3. Allocate and zero Descriptor Table, Available and Used rings for the > +
virtqueue in contiguous physical memory. >
> 4. Optionally, if MSI-X capability is present and enabled on the
>
device, select a vector to use to request interrupts triggered > @@ -760,14 +1134,21 @@ This is done as follows, for each virtqueue a device has:
>
Queue Vector field: on success, previously written value is
>
returned; on failure, NO_VECTOR value is returned. >
> + > +2.4.1.3.1.4.1 Legacy Interface: A Note on Virtqueue Configuration > +
-----------------------------------
> +When using the legacy interface, the page size for a virtqueue on a PCI virtio > +device is defined as 4096 bytes. Driver writes the physical address, divided > +by 4096 to the Queue Address field [6]. > +
> 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. > +of this virtqueue to the Queue Notify field. >
> 2.4.1.3.3 Receiving Used Buffers From The Device > +
------------------------------
>
> If an interrupt is necessary: >
> @@ -2798,7 +3179,10 @@ the non-PCI implementations (currently lguest and S/390).
> This is only allowed if the driver does not use any features
> which would alter this early use of the device. >
> -[5] ie. once you enable MSI-X on the device, the other fields move. > +[5] When MSI-X capability is enabled, 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. ie. once you enable MSI-X on the device, the other fields move.
> If you turn it off again, they move back! Thanks, Rusty.
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