vhost: enable vrings in vhost_dev_start() for vhost-user devices

Commit 02b61f38d3 ("hw/virtio: incorporate backend features in features")
properly negotiates VHOST_USER_F_PROTOCOL_FEATURES with the vhost-user
backend, but we forgot to enable vrings as specified in
docs/interop/vhost-user.rst:

    If ``VHOST_USER_F_PROTOCOL_FEATURES`` has not been negotiated, the
    ring starts directly in the enabled state.

    If ``VHOST_USER_F_PROTOCOL_FEATURES`` has been negotiated, the ring is
    initialized in a disabled state and is enabled by
    ``VHOST_USER_SET_VRING_ENABLE`` with parameter 1.

Some vhost-user front-ends already did this by calling
vhost_ops.vhost_set_vring_enable() directly:
- backends/cryptodev-vhost.c
- hw/net/virtio-net.c
- hw/virtio/vhost-user-gpio.c

But most didn't do that, so we would leave the vrings disabled and some
backends would not work. We observed this issue with the rust version of
virtiofsd [1], which uses the event loop [2] provided by the
vhost-user-backend crate where requests are not processed if vring is
not enabled.

Let's fix this issue by enabling the vrings in vhost_dev_start() for
vhost-user front-ends that don't already do this directly. Same thing
also in vhost_dev_stop() where we disable vrings.

[1] https://gitlab.com/virtio-fs/virtiofsd
[2] https://github.com/rust-vmm/vhost/blob/240fc2966/crates/vhost-user-backend/src/event_loop.rs#L217

Fixes: 02b61f38d3 ("hw/virtio: incorporate backend features in features")
Reported-by: German Maglione <gmaglione@redhat.com>
Tested-by: German Maglione <gmaglione@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
Acked-by: Raphael Norwitz <raphael.norwitz@nutanix.com>
Message-Id: <20221123131630.52020-1-sgarzare@redhat.com>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <20221130112439.2527228-3-alex.bennee@linaro.org>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

1 files changed, 2 insertions, 2 deletions

diff --git a/backends/cryptodev-vhost.c b/backends/cryptodev-vhost.c
index bc13e466b4..572f87b3be 100644
--- a/backends/cryptodev-vhost.c
+++ b/backends/cryptodev-vhost.c
@@ -94,7 +94,7 @@ cryptodev_vhost_start_one(CryptoDevBackendVhost *crypto,
         goto fail_notifiers;
     }
 
-    r = vhost_dev_start(&crypto->dev, dev);
+    r = vhost_dev_start(&crypto->dev, dev, false);
     if (r < 0) {
         goto fail_start;
     }
@@ -111,7 +111,7 @@ static void
 cryptodev_vhost_stop_one(CryptoDevBackendVhost *crypto,
                                  VirtIODevice *dev)
 {
-    vhost_dev_stop(&crypto->dev, dev);
+    vhost_dev_stop(&crypto->dev, dev, false);
     vhost_dev_disable_notifiers(&crypto->dev, dev);
 }