On Fri, Oct 04, 2013 at 01:06:43PM +0930, Rusty Russell wrote:
> "Michael S. Tsirkin" <> writes:
> > On Tue, Sep 24, 2013 at 08:18:20PM +0930, Rusty Russell wrote:
> >> I don't think qemu supports changing a disk underneath a device anyway?
> >
> > QEMU supports resizing I think.
> >
> >> And in practice, I'm not sure it would ever change fast enough to
> >> trigger this race...
> >
> > Well it's easy to make guest very slow :)
> >
> >> So, should we try to fix it?
> >> Rusty.
> >
> > Not really sure but I thought I'd put it on the table.
>
> OK, I got ambitious! How's this look?
>
> Cheers,
> Rusty.
>
> commit 9aebe9c69be1461dd0110dfc61459a4ac7bda6f9
> Author: Rusty Russell <>
> Date: Fri Oct 4 13:01:35 2013 +0930
>
> Configuration (read) atomicity.
>
> Aka issue VIRTIO-35.
>
> This is solved per transport:
> 1) PCI: use the 8 bit reserved field.
> Assume that if you really change that fast, you'll do it lazily on
> config space read.
I guess it's possible ... but isn't it easier to just make it a 32 bit field
than add this lazy update logic to devices?
> 2) MMIO: use offset 0x18.
> Qemu returns 0 when they read here anyway, so it's almost 100%
> backwards compatible (legacy drivers should still multi-read
> for any transport).
> 3) CCW: no transport changes.
> They always read/write the entire thing. This just shows that
> Cornelia is smarter than I am.
>
> Signed-off-by: Rusty Russell <>
>
> diff --git a/virtio-v1.0-wd01-part1-specification.txt b/virtio-v1.0-wd01-part1-specification.txt
> index b5a5835..2945db4 100644
> --- a/virtio-v1.0-wd01-part1-specification.txt
> +++ b/virtio-v1.0-wd01-part1-specification.txt
> @@ -201,11 +201,36 @@ Interface' in the section title.
> -------------------------
>
> Configuration space is generally used for rarely-changing or
> -initialization-time parameters.
> +initialization-time parameters. Drivers must not assume reads from
> +fields greater than 32 bits wide are atomic, nor or reads from
> +multiple fields.
> +
> +Each transport provides a generation count for the configuration
> +space, which must change whenever there is a possibility that two
MUST?
> +accesses to the configuration space can see different versions of that
> +space.
> +
> +Thus drivers should read configuration space fields like so:
SHOULD?
> +
> + u32 before, after;
> + do {
> + before = get_config_generation(device);
> + // read config entry/entries.
> + after = get_config_generation(device);
> + } while (after != before);
BTW this implies reads must never have side
effects (PCI ISR is the only exception, isn't it?).
>
> Note that this space is generally the guest's native endian,
> rather than PCI's little-endian.
>
> +2.1.3.1. Legacy Interface: Configuration Space
> +-------------------------
> +
> +Legacy devices did not have a configuration generation field, thus are
> +susceptible to race conditions if configuration is updated. This
> +effects the block capacity and network mac fields; best practice is to
> +read these fields multiple times until two reads generate a consistent
> +result.
> +
> 2.1.4. Virtqueues
> ----------------
>
> @@ -768,7 +793,7 @@ struct virtio_pci_common_cfg {
> __le16 msix_config; /* read-write */
> __le16 num_queues; /* read-only */
> __u8 device_status; /* read-write */
> - __u8 unused1;
> + __u8 config_generation; /* read-only */
>
> /* About a specific virtqueue. */
> __le16 queue_select; /* read-write */
> @@ -820,6 +845,14 @@ device_status
> Device Status field. Writing 0 into this field resets the
> device.
>
> +config_generation
> +
> + Configuration atomicity value. Changes every time the
> + configuration noticeably changes. This means the device may
> + only change the value after a configuration read operation,
> + but it must change if there is any risk of a device seeing an
> + inconsistent configuration state.
> +
> queue_select
>
> Queue Select. Selects which virtqueue do other fields refer to.
> @@ -941,6 +974,9 @@ Legacy Interface. When used through the Legacy Interface,
> Transitional Devices must assume that Feature Bits 32 to 63
> are not acknowledged by Driver.
>
> +As legacy devices had no configuration generation field, see "2.1.3.1.
> +Legacy Interface: Configuration Space" for workarounds.
> +
> 2.3.1.3. PCI-specific Initialization And Device Operation
> --------------------------------------------------------
>
> @@ -1281,6 +1317,13 @@ configuration space. The following list presents their layout:
> must write a value to the HostFeaturesSel register before
> reading from the HostFeatures register.
>
> +• 0x018 | R | ConfigGeneration
> + Configuration atomicity value.
> + Changes every time the configuration noticeably changes. This means the
> + device may only change the value after a configuration read operation,
> + but it must change if there is any risk of a device seeing an inconsistent
> + configuration state.
> +
> • 0x020 | W | GuestFeatures
> Flags representing device features understood and activated by
> the driver.
> @@ -1396,6 +1439,12 @@ Writing to registers described as “R” and reading from
> registers described as “W” is not permitted and can cause
> undefined behavior.
>
> +2.3.2.2.1. Legacy Interface: MMIO Device Layout
> +--------------------------
> +The ConfigGeneration field does not exist in legacy devices; fortunately
> +it would return 0 if accessed. Nonetheless, the workarounds in
> +"2.1.3.1. Legacy Interface: Configuration Space" should still be used.
> +
> 100.3.2.2.1. Virtqueue Layout
> ------------------------------
>
> @@ -1631,7 +1680,9 @@ For changing configuration information, the guest may use
> CCW_CMD_WRITE_CONF, specifying the guest memory for the host to
> read from.
>
> -In both cases, the complete configuration space is transmitted.
> +In both cases, the complete configuration space is transmitted. This
> +allows the guest to compare the new configuration space with the old
> +version, and keep a generation count internally whenever it changes.
>
> 2.3.3.2.5. Setting Up Indicators
> --------------------------------
> @@ -3415,12 +3466,15 @@ transmit output.
> ---------------------------------------
>
> Configuration space should only be used for initialization-time
> -parameters. It is a limited resource with no synchronization, so for
> -most uses it is better to use a virtqueue to update configuration
> -information (the network device does this for filtering,
> +parameters. It is a limited resource with no synchronization between
> +writable fields, so for most uses it is better to use a virtqueue to update
> +configuration information (the network device does this for filtering,
> otherwise the table in the config space could potentially be very
> large).
>
> +Devices must not assume that configuration fields over 32 bits wide
> +are atomically writable.
> +
> 2.7.3. What Device Number?
> --------------------------
>
