libvhost-user: advertise vring features

libvhost-user implements several vring features without advertising
them. There is no way for the vhost-user master to detect support for
these features.

Things more or less work today because QEMU assumes the vhost-user
backend always implements certain feature bits like
VIRTIO_RING_F_EVENT_IDX. This is not documented anywhere.

This patch explicitly advertises features implemented in libvhost-user
so that the vhost-user master does not need to make undocumented
assumptions.

Feature bits that libvhost-user now advertises can be removed from
vhost-user-blk.c. Devices should not be responsible for advertising
vring feature bits, that is libvhost-user's job.

Cc: Marc-André Lureau <marcandre.lureau@redhat.com>
Cc: Jason Wang <jasowang@redhat.com>
Cc: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20200529161338.456017-1-stefanha@redhat.com>
Reviewed-by: NMichael S. Tsirkin <mst@redhat.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>
Reviewed-by: NMarc-André Lureau <marcandre.lureau@redhat.com>

contrib/libvhost-user/libvhost-user.c

@@ -498,6 +498,16 @@ static bool
 vu_get_features_exec(VuDev *dev, VhostUserMsg *vmsg)
 {
     vmsg->payload.u64 =
+        /*
+         * The following VIRTIO feature bits are supported by our virtqueue
+         * implementation:
+         */
+        1ULL << VIRTIO_F_NOTIFY_ON_EMPTY |
+        1ULL << VIRTIO_RING_F_INDIRECT_DESC |
+        1ULL << VIRTIO_RING_F_EVENT_IDX |
+        1ULL << VIRTIO_F_VERSION_1 |
+
+        /* vhost-user feature bits */
         1ULL << VHOST_F_LOG_ALL |
         1ULL << VHOST_USER_F_PROTOCOL_FEATURES;
 

contrib/vhost-user-blk/vhost-user-blk.c

@@ -382,9 +382,7 @@ vub_get_features(VuDev *dev)
                1ull << VIRTIO_BLK_F_DISCARD |
                1ull << VIRTIO_BLK_F_WRITE_ZEROES |
                #endif
-               1ull << VIRTIO_BLK_F_CONFIG_WCE |
-               1ull << VIRTIO_F_VERSION_1 |
-               1ull << VHOST_USER_F_PROTOCOL_FEATURES;
+               1ull << VIRTIO_BLK_F_CONFIG_WCE;
 
     if (vdev_blk->enable_ro) {
         features |= 1ull << VIRTIO_BLK_F_RO;