Re: [PATCH v3] vfio/pci: migration: Skip config space check for Vendor Specific Information in VSC during restore/load

["Cédric Le Goater" <clg@redhat.com>]

On 3/27/24 21:52, Alex Williamson wrote:
> On Wed, 27 Mar 2024 16:11:37 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
>> On Wed, Mar 27, 2024 at 11:39:15AM -0600, Alex Williamson wrote:
>>> On Fri, 22 Mar 2024 12:12:10 +0530
>>> Vinayak Kale <vkale@nvidia.com> wrote:
>>>    
>>>> In case of migration, during restore operation, qemu checks config space of the
>>>> pci device with the config space in the migration stream captured during save
>>>> operation. In case of config space data mismatch, restore operation is failed.
>>>>
>>>> config space check is done in function get_pci_config_device(). By default VSC
>>>> (vendor-specific-capability) in config space is checked.
>>>>
>>>> Due to qemu's config space check for VSC, live migration is broken across NVIDIA
>>>> vGPU devices in situation where source and destination host driver is different.
>>>> In this situation, Vendor Specific Information in VSC varies on the destination
>>>> to ensure vGPU feature capabilities exposed to the guest driver are compatible
>>>> with destination host.
>>>>
>>>> If a vfio-pci device is migration capable and vfio-pci vendor driver is OK with
>>>> volatile Vendor Specific Info in VSC then qemu should exempt config space check
>>>> for Vendor Specific Info. It is vendor driver's responsibility to ensure that
>>>> VSC is consistent across migration. Here consistency could mean that VSC format
>>>> should be same on source and destination, however actual Vendor Specific Info
>>>> may not be byte-to-byte identical.
>>>>
>>>> This patch skips the check for Vendor Specific Information in VSC for VFIO-PCI
>>>> device by clearing pdev->cmask[] offsets. Config space check is still enforced
>>>> for 3 byte VSC header. If cmask[] is not set for an offset, then qemu skips
>>>> config space check for that offset.
>>>>
>>>> Signed-off-by: Vinayak Kale <vkale@nvidia.com>
>>>> ---
>>>> Version History
>>>> v2->v3:
>>>>      - Config space check skipped only for Vendor Specific Info in VSC, check is
>>>>        still enforced for 3 byte VSC header.
>>>>      - Updated commit description with live migration failure scenario.
>>>> v1->v2:
>>>>      - Limited scope of change to vfio-pci devices instead of all pci devices.
>>>>
>>>>   hw/vfio/pci.c | 24 ++++++++++++++++++++++++
>>>>   1 file changed, 24 insertions(+)
>>>
>>>
>>> Acked-by: Alex Williamson <alex.williamson@redhat.com>
>>
>>
>> A very reasonable way to do it.
>>
>> Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
>>
>> Merge through the VFIO tree I presume?
> 
> Yep, Cédric said he´d grab it for 9.1.  Thanks,


Applied to vfio-next.

Thanks,

C.