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

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Hello Vinayak,

On 3/28/24 10:30, Cédric Le Goater wrote:
On 3/27/24 21:52, Alex Williamson wrote:
On Wed, 27 Mar 2024 16:11:37 -0400
"Michael S. Tsirkin" <mst@xxxxxxxxxx> 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@xxxxxxxxxx> 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@xxxxxxxxxx>
Version History
     - 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.
     - 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@xxxxxxxxxx>

A very reasonable way to do it.

Reviewed-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

Merge through the VFIO tree I presume?

Yep, Cédric said he´d grab it for 9.1.  Thanks,

Could you please resend an update of this change adding a machine
compatibility property for migration ?



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