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
> 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@xxxxxxxxxx>
> diff --git a/hw/vfio/pci.c b/hw/vfio/pci.c
> index d7fe06715c..1026cdba18 100644
> --- a/hw/vfio/pci.c
> +++ b/hw/vfio/pci.c
> @@ -2132,6 +2132,27 @@ static void vfio_check_af_flr(VFIOPCIDevice *vdev, uint8_t pos)
> }
> }
>
> +static int vfio_add_vendor_specific_cap(VFIOPCIDevice *vdev, int pos,
> + uint8_t size, Error **errp)
> +{
> + PCIDevice *pdev = &vdev->pdev;
> +
> + pos = pci_add_capability(pdev, PCI_CAP_ID_VNDR, pos, size, errp);
> + if (pos < 0) {
> + return pos;
> + }
> +
> + /*
> + * Exempt config space check for Vendor Specific Information during restore/load.
> + * Config space check is still enforced for 3 byte VSC header.
> + */
> + if (size > 3) {
> + memset(pdev->cmask + pos + 3, 0, size - 3);
> + }
> +
> + return pos;
> +}
> +
> static int vfio_add_std_cap(VFIOPCIDevice *vdev, uint8_t pos, Error **errp)
> {
> PCIDevice *pdev = &vdev->pdev;
> @@ -2199,6 +2220,9 @@ static int vfio_add_std_cap(VFIOPCIDevice *vdev, uint8_t pos, Error **errp)
> vfio_check_af_flr(vdev, pos);
> ret = pci_add_capability(pdev, cap_id, pos, size, errp);
> break;
> + case PCI_CAP_ID_VNDR:
> + ret = vfio_add_vendor_specific_cap(vdev, pos, size, errp);
> + break;
> default:
> ret = pci_add_capability(pdev, cap_id, pos, size, errp);
> break;
