[PATCH v2] qemu: Fix domxml-to-native command failure

[Zhenzhong Duan <zhenzhong.duan@xxxxxxxxx>]

virsh command domxml-to-native failed with below error but start
command succeed for same domain xml.

"internal error: invalid PCI passthrough type 'default'"

If host PCI device's driver attribute isn't defined in domain xml,
qemu driver will choose a proper value based on host environment
before starting qemu process. But this is missed in domxml-to-native
command, add the same logic so domxml-to-native could pass and replace
the relevant code in qemuHostdevPreparePCIDevicesCheckSupport() with
an error report.

Signed-off-by: Zhenzhong Duan <zhenzhong.duan@xxxxxxxxx>
---
v2: replace setting with an error report, suggested by Michal

 src/qemu/qemu_domain.c  | 11 +++++++++++
 src/qemu/qemu_hostdev.c |  8 ++++----
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
index 63b13b6875c9..517c90f93f0a 100644
--- a/src/qemu/qemu_domain.c
+++ b/src/qemu/qemu_domain.c
@@ -11274,6 +11274,17 @@ qemuDomainPrepareHostdev(virDomainHostdevDef *hostdev,
             }
         }
     }
+    if (hostdev->mode == VIR_DOMAIN_HOSTDEV_MODE_SUBSYS &&
+        hostdev->source.subsys.type == VIR_DOMAIN_HOSTDEV_SUBSYS_TYPE_PCI) {
+        bool supportsPassthroughVFIO = qemuHostdevHostSupportsPassthroughVFIO();
+        virDomainHostdevSubsysPCIBackendType *backend = &hostdev->source.subsys.u.pci.backend;
+
+        if (*backend == VIR_DOMAIN_HOSTDEV_PCI_BACKEND_DEFAULT &&
+            supportsPassthroughVFIO &&
+            virQEMUCapsGet(priv->qemuCaps, QEMU_CAPS_DEVICE_VFIO_PCI)) {
+            *backend = VIR_DOMAIN_HOSTDEV_PCI_BACKEND_VFIO;
+        }
+    }
 
     return 0;
 }
diff --git a/src/qemu/qemu_hostdev.c b/src/qemu/qemu_hostdev.c
index 45cd1066f0a5..305a323a0769 100644
--- a/src/qemu/qemu_hostdev.c
+++ b/src/qemu/qemu_hostdev.c
@@ -175,15 +175,15 @@ qemuHostdevPreparePCIDevicesCheckSupport(virDomainHostdevDef **hostdevs,
         case VIR_DOMAIN_HOSTDEV_PCI_BACKEND_DEFAULT:
             if (supportsPassthroughVFIO &&
                 virQEMUCapsGet(qemuCaps, QEMU_CAPS_DEVICE_VFIO_PCI)) {
-                *backend = VIR_DOMAIN_HOSTDEV_PCI_BACKEND_VFIO;
+                virReportError(VIR_ERR_INTERNAL_ERROR, "%s",
+                               _("qemuDomainPrepareHostdev() should have set backend "
+                                 "to VIR_DOMAIN_HOSTDEV_PCI_BACKEND_VFIO"));
             } else {
                 virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                                _("host doesn't support passthrough of "
                                  "host PCI devices"));
-                return false;
             }
-
-            break;
+            return false;
 
         case VIR_DOMAIN_HOSTDEV_PCI_BACKEND_VFIO:
             if (!supportsPassthroughVFIO) {
-- 
2.25.1