The virtio-gpu 'blob' support was insufficiently validated. Qemu
requires a memfd memory backing in order to use udmabuf and enable blob
support. Example error:

    $ virsh start rhel9
    error: Failed to start domain 'rhel9'
    error: internal error: qemu unexpectedly closed the monitor: 2023-07-18T02:33:57.083178Z qemu-kvm: -device {"driver":"virtio-vga","id":"video0","max_outputs":1,"blob":true,"bus":"pcie.0","addr":"0x1"}: cannot enable blob resources without udmabuf

Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
---
 src/qemu/qemu_validate.c | 20 ++++++++++++++------
 1 file changed, 14 insertions(+), 6 deletions(-)

diff --git a/src/qemu/qemu_validate.c b/src/qemu/qemu_validate.c
index 7e09e2c52f..139e6f3d62 100644
--- a/src/qemu/qemu_validate.c
+++ b/src/qemu/qemu_validate.c
@@ -2598,6 +2598,7 @@ qemuValidateDomainDeviceDefHostdev(const virDomainHostdevDef *hostdev,
 
 static int
 qemuValidateDomainDeviceDefVideo(const virDomainVideoDef *video,
+                                 const virDomainDef *def,
                                  virQEMUCaps *qemuCaps)
 {
     virDomainCapsDeviceVideo videoCaps = { 0 };
@@ -2714,11 +2715,18 @@ qemuValidateDomainDeviceDefVideo(const virDomainVideoDef *video,
     }
 
     if (video->type == VIR_DOMAIN_VIDEO_TYPE_VIRTIO) {
-        if (video->blob != VIR_TRISTATE_SWITCH_ABSENT &&
-            !virQEMUCapsGet(qemuCaps, QEMU_CAPS_VIRTIO_GPU_BLOB)) {
-            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
-                           _("this QEMU does not support 'blob' for virtio-gpu devices"));
-            return -1;
+        if (video->blob != VIR_TRISTATE_SWITCH_ABSENT) {
+            if (!virQEMUCapsGet(qemuCaps, QEMU_CAPS_VIRTIO_GPU_BLOB)) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                               _("this QEMU does not support 'blob' for virtio-gpu devices"));
+                return -1;
+            }
+            if (video->blob == VIR_TRISTATE_SWITCH_ON
+                && def->mem.source != VIR_DOMAIN_MEMORY_SOURCE_MEMFD) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                               _("'blob' support for virtio-gpu devices requires a memfd memory backend"));
+                return -1;
+            }
         }
     }
 
@@ -5217,7 +5225,7 @@ qemuValidateDomainDeviceDef(const virDomainDeviceDef *dev,
                                                  qemuCaps);
 
     case VIR_DOMAIN_DEVICE_VIDEO:
-        return qemuValidateDomainDeviceDefVideo(dev->data.video, qemuCaps);
+        return qemuValidateDomainDeviceDefVideo(dev->data.video, def, qemuCaps);
 
     case VIR_DOMAIN_DEVICE_DISK:
         return qemuValidateDomainDeviceDefDisk(dev->data.disk, def, qemuCaps);
-- 
2.41.0

On 7/18/23 16:48, Jonathon Jongsma wrote:
> The virtio-gpu 'blob' support was insufficiently validated. Qemu
> requires a memfd memory backing in order to use udmabuf and enable blob
> support. Example error:
> 
>     $ virsh start rhel9
>     error: Failed to start domain 'rhel9'
>     error: internal error: qemu unexpectedly closed the monitor: 2023-07-18T02:33:57.083178Z qemu-kvm: -device {"driver":"virtio-vga","id":"video0","max_outputs":1,"blob":true,"bus":"pcie.0","addr":"0x1"}: cannot enable blob resources without udmabuf
> 
> Signed-off-by: Jonathon Jongsma <jjongsma@redhat.com>
> ---
>  src/qemu/qemu_validate.c | 20 ++++++++++++++------
>  1 file changed, 14 insertions(+), 6 deletions(-)
> 

You forgot to update tests. Squash this in:


diff --git i/tests/qemuxml2argvdata/video-virtio-blob-on.x86_64-latest.args w/tests/qemuxml2argvdata/video-virtio-blob-on.x86_64-latest.args
index ef37e32e5e..577422426b 100644
--- i/tests/qemuxml2argvdata/video-virtio-blob-on.x86_64-latest.args
+++ w/tests/qemuxml2argvdata/video-virtio-blob-on.x86_64-latest.args
@@ -14,7 +14,7 @@ XDG_CONFIG_HOME=/var/lib/libvirt/qemu/domain--1-QEMUGuest1/.config \
 -accel tcg \
 -cpu qemu64 \
 -m size=1048576k \
--object '{"qom-type":"memory-backend-ram","id":"pc.ram","size":1073741824}' \
+-object '{"qom-type":"memory-backend-memfd","id":"pc.ram","x-use-canonical-path-for-ramblock-id":false,"size":1073741824}' \
 -overcommit mem-lock=off \
 -smp 1,sockets=1,cores=1,threads=1 \
 -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \
diff --git i/tests/qemuxml2argvdata/video-virtio-blob-on.xml w/tests/qemuxml2argvdata/video-virtio-blob-on.xml
index 2b8a913f49..96ccf13079 100644
--- i/tests/qemuxml2argvdata/video-virtio-blob-on.xml
+++ w/tests/qemuxml2argvdata/video-virtio-blob-on.xml
@@ -3,6 +3,9 @@
   <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
   <memory unit='KiB'>1048576</memory>
   <currentMemory unit='KiB'>1048576</currentMemory>
+  <memoryBacking>
+    <source type='memfd'/>
+  </memoryBacking>
   <vcpu placement='static'>1</vcpu>
   <os>
     <type arch='x86_64' machine='pc'>hvm</type>
diff --git i/tests/qemuxml2xmloutdata/video-virtio-blob-on.x86_64-latest.xml w/tests/qemuxml2xmloutdata/video-virtio-blob-on.x86_64-latest.xml
index 410db67592..40f40b4132 100644
--- i/tests/qemuxml2xmloutdata/video-virtio-blob-on.x86_64-latest.xml
+++ w/tests/qemuxml2xmloutdata/video-virtio-blob-on.x86_64-latest.xml
@@ -3,6 +3,9 @@
   <uuid>c7a5fdbd-edaf-9455-926a-d65c16db1809</uuid>
   <memory unit='KiB'>1048576</memory>
   <currentMemory unit='KiB'>1048576</currentMemory>
+  <memoryBacking>
+    <source type='memfd'/>
+  </memoryBacking>
   <vcpu placement='static'>1</vcpu>
   <os>
     <type arch='x86_64' machine='pc'>hvm</type>


Reviewed-by: Michal Privoznik <mprivozn@redhat.com>

Michal