GPU vendors are moving away from using mdev to create virtual GPUs
towards using SRIOV VFs that are vGPUs. In both cases, once created
the vGPUs are assigned to guests via <hostdev> (i.e. VFIO device
assignment), and inside the guest the devices look identical, but mdev
vGPUs are located by QEMU/VFIO using a uuid, while VF vGPUs are
located with a PCI address. So although we generally require the
device on the source host to exactly match the device on the
destination host, in the case of mdev-created vGPU vs. VF vGPU
migration *can* potentially work, except that libvirt has a hard-coded
check that prevents us from even trying.

This patch loosens up that check so that we will allow attempts to
migrate a guest from a source host that has mdev-created vGPUs to a
destination host that has VF vGPUs (and vice versa). The expectation
is that if this doesn't actually work then QEMU will fail and generate
an error that we can report.

Based-on-patch-by: Zhiyi Guo <zhguo@redhat.com>
Signed-off-by: Laine Stump <laine@redhat.com>
---

Zhiyi's original patch removed the check for subsys type completely,
and this worked. My modified patch keeps the check in place, but
allows it to pass if the src type is pci and dst is mdev, or vice
versa.

 src/conf/domain_conf.c | 28 +++++++++++++++++++++-------
 1 file changed, 21 insertions(+), 7 deletions(-)

diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 4ad8289b89..9d5fda0469 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -20647,13 +20647,27 @@ virDomainHostdevDefCheckABIStability(virDomainHostdevDef *src,
         return false;
     }
 
-    if (src->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS &&
-        src->source.subsys.type != dst->source.subsys.type) {
-        virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
-                       _("Target host device subsystem %1$s does not match source %2$s"),
-                       virDomainHostdevSubsysTypeToString(dst->source.subsys.type),
-                       virDomainHostdevSubsysTypeToString(src->source.subsys.type));
-        return false;
+    if (src->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS) {
+        virDomainHostdevSubsysType srcType = src->source.subsys.type;
+        virDomainHostdevSubsysType dstType = dst->source.subsys.type;
+
+        /* If the source and destination subsys types aren't the same,
+         * then migration can't be supported, *except* that it might
+         * be supported to migrate from subsys type 'pci' to 'mdev'
+         * and vice versa. (libvirt can't know for certain whether or
+         * not it will actually work, so we have to just allow it and
+         * count on QEMU to provide us with an error if it fails)
+         */
+
+        if (srcType != dstType
+            && ((srcType != VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_PCI && srcType != VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_MDEV)
+                || (dstType != VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_PCI && dstType != VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_MDEV))) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                           _("Target host device subsystem type %1$s is not compatible with source subsystem type %2$s"),
+                           virDomainHostdevSubsysTypeToString(dstType),
+                           virDomainHostdevSubsysTypeToString(srcType));
+            return false;
+        }
     }
 
     if (!virDomainDeviceInfoCheckABIStability(src->info, dst->info))
-- 
2.47.1