ACK: [SRU][I][H][F][PATCH 1/1] virtio: write back F_VERSION_1 before validate

[Krzysztof Kozlowski]

On 09/11/2021 13:47, frank.heimes at canonical.com wrote:
> From: Halil Pasic <pasic at linux.ibm.com>
> 
> BugLink: https://bugs.launchpad.net/bugs/1950144
> 
> The virtio specification virtio-v1.1-cs01 states: "Transitional devices
> MUST detect Legacy drivers by detecting that VIRTIO_F_VERSION_1 has not
> been acknowledged by the driver."  This is exactly what QEMU as of 6.1
> has done relying solely on VIRTIO_F_VERSION_1 for detecting that.
> 
> However, the specification also says: "... the driver MAY read (but MUST
> NOT write) the device-specific configuration fields to check that it can
> support the device ..." before setting FEATURES_OK.
> 
> In that case, any transitional device relying solely on
> VIRTIO_F_VERSION_1 for detecting legacy drivers will return data in
> legacy format.  In particular, this implies that it is in big endian
> format for big endian guests. This naturally confuses the driver which
> expects little endian in the modern mode.
> 
> It is probably a good idea to amend the spec to clarify that
> VIRTIO_F_VERSION_1 can only be relied on after the feature negotiation
> is complete. Before validate callback existed, config space was only
> read after FEATURES_OK. However, we already have two regressions, so
> let's address this here as well.
> 
> The regressions affect the VIRTIO_NET_F_MTU feature of virtio-net and
> the VIRTIO_BLK_F_BLK_SIZE feature of virtio-blk for BE guests when
> virtio 1.0 is used on both sides. The latter renders virtio-blk unusable
> with DASD backing, because things simply don't work with the default.
> See Fixes tags for relevant commits.
> 
> For QEMU, we can work around the issue by writing out the feature bits
> with VIRTIO_F_VERSION_1 bit set.  We (ab)use the finalize_features
> config op for this. This isn't enough to address all vhost devices since
> these do not get the features until FEATURES_OK, however it looks like
> the affected devices actually never handled the endianness for legacy
> mode correctly, so at least that's not a regression.
> 
> No devices except virtio net and virtio blk seem to be affected.
> 
> Long term the right thing to do is to fix the hypervisors.
> 
> Cc: <stable at vger.kernel.org> #v4.11
> Signed-off-by: Halil Pasic <pasic at linux.ibm.com>
> Fixes: 82e89ea077b9 ("virtio-blk: Add validation for block size in config space")
> Fixes: fe36cbe0671e ("virtio_net: clear MTU when out of range")
> Reported-by: markver at us.ibm.com
> Reviewed-by: Cornelia Huck <cohuck at redhat.com>
> Link: https://lore.kernel.org/r/20211011053921.1198936-1-pasic@linux.ibm.com
> Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
> (cherry picked from commit 2f9a174f918e29608564c7a4e8329893ab604fb4)
> Signed-off-by: Frank Heimes <frank.heimes at canonical.com>
> ---
>  drivers/virtio/virtio.c | 11 +++++++++++
>  1 file changed, 11 insertions(+)
> 


Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski at canonical.com>


Best regards,
Krzysztof