vhost-user: inject kick after SET_VRING_KICK

The vhost-user specification was updated to say that front-ends should
inject a kick after SET_VRING_KICK in case the back-end implements the
old spec wording which said vrings start when a kick is received. Do
this in QEMU's front-end.

An example scenario where this behavior helps: the back-end fails to
check if the vring has available buffers when SET_VRING_KICK is received
and the front-end stopped and then restarted the vring. In the case the
back-end may not notice the available buffers unless the front-end
injects a kick.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-ID: <20260604201029.250450-4-stefanha@redhat.com>

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index d627351f45d7c76c151e3ef9ab2b62c57d8daca5..517cc4ca7168a798b34c0a8733fdb4bafc8d3bd2 100644 (file)
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -1487,7 +1487,29 @@ static int vhost_set_vring_file(struct vhost_dev *dev,
 static int vhost_user_set_vring_kick(struct vhost_dev *dev,
                                      struct vhost_vring_file *file)
 {
-    return vhost_set_vring_file(dev, VHOST_USER_SET_VRING_KICK, file);
+    int ret = vhost_set_vring_file(dev, VHOST_USER_SET_VRING_KICK, file);
+    if (ret < 0) {
+        return ret;
+    }
+
+    /*
+     * Inject a kick in case the back-end only starts vring processing upon
+     * receiving a kick. The spec suggests this to improve compatibility.
+     */
+    if (file->fd != -1) {
+        uint64_t val = 1;
+        ssize_t nwritten;
+
+        do {
+            nwritten = write(file->fd, &val, sizeof(val));
+        } while (nwritten < 0 && errno == EINTR);
+
+        if (nwritten < 0 && errno != EAGAIN /* back-end can already read */) {
+            return -errno;
+        }
+    }
+
+    return 0;
 }
 
 static int vhost_user_set_vring_call(struct vhost_dev *dev,