[PATCH] virtio-net: clear guest_announce feature if no cvq backend

["Eugenio Pérez" <eperezma@redhat.com>]

Since GUEST_ANNOUNCE is emulated the feature bit could be set without
backend support.  This happens in the vDPA case.

However, backend vDPA parent may not have CVQ support.  This causes an
incoherent feature set, and the driver may refuse to start.  This
happens in virtio-net Linux driver.

This may be solved differently in the future.  Qemu is able to emulate a
CVQ just for guest_announce purposes, helping guest to notify the new
location with vDPA devices that does not support it.  However, this is
left as a TODO as it is way more complex to backport.

Tested with vdpa_net_sim, toggling manually VIRTIO_NET_F_CTRL_VQ in the
driver and migrating it with x-svq=on.

Fixes: 980003debddd ("vdpa: do not handle VIRTIO_NET_F_GUEST_ANNOUNCE in vhost-vdpa")
Reported-by: Dawar, Gautam <gautam.dawar@amd.com>
Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
---
 hw/net/virtio-net.c | 15 +++++++++++++++
 1 file changed, 15 insertions(+)

diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
index 3ae909041a..09d5c7a664 100644
--- a/hw/net/virtio-net.c
+++ b/hw/net/virtio-net.c
@@ -820,6 +820,21 @@ static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
         features |= (1ULL << VIRTIO_NET_F_MTU);
     }
 
+    /*
+     * Since GUEST_ANNOUNCE is emulated the feature bit could be set without
+     * enabled. This happens in the vDPA case.
+     *
+     * Make sure the feature set is not incoherent, as the driver could refuse
+     * to start.
+     *
+     * TODO: QEMU is able to emulate a CVQ just for guest_announce purposes,
+     * helping guest to notify the new location with vDPA devices that does not
+     * support it.
+     */
+    if (!virtio_has_feature(vdev->backend_features, VIRTIO_NET_F_CTRL_VQ)) {
+        virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_ANNOUNCE);
+    }
+
     return features;
 }
 
-- 
2.31.1