Commit 2943b53f6 (' virtio: force VIRTIO_F_IOMMU_PLATFORM') made sure
that vhost can't just reject VIRTIO_F_IOMMU_PLATFORM when it was
requested. However, just adding it back to the negotiated flags isn't
right either because it promises support to the guest that the device
actually doesn't support. One example of a vhost-user device that
doesn't have support for the flag is the vhost-user-blk export of QEMU.

Instead of successfully creating a device that doesn't work, just fail
to plug the device when it doesn't support the feature, but it was
requested. This results in much clearer error messages.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1935019
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20210429171316.162022-6-kwolf@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 hw/virtio/virtio-bus.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/hw/virtio/virtio-bus.c b/hw/virtio/virtio-bus.c
index d6332d45c3..859978d248 100644
--- a/hw/virtio/virtio-bus.c
+++ b/hw/virtio/virtio-bus.c
@@ -69,6 +69,11 @@ void virtio_bus_device_plugged(VirtIODevice *vdev, Error **errp)
         return;
     }
 
+    if (has_iommu && !virtio_host_has_feature(vdev, VIRTIO_F_IOMMU_PLATFORM)) {
+        error_setg(errp, "iommu_platform=true is not supported by the device");
+        return;
+    }
+
     if (klass->device_plugged != NULL) {
         klass->device_plugged(qbus->parent, &local_err);
     }
-- 
2.30.2