[Qemu-devel] [PATCH 3/4] libvhost-user: implement VHOST_USER_PROTOCOL_F_MQ

[Stefan Hajnoczi <stefanha@redhat.com>]

Existing vhost-user device backends, including vhost-user-scsi and
vhost-user-blk, support multiqueue but libvhost-user currently does not
advertise this.

VHOST_USER_PROTOCOL_F_MQ enables the VHOST_USER_GET_QUEUE_NUM request
needed for a vhost-user master to query the number of queues.  For
example, QEMU's vhost-user-net master depends on
VHOST_USER_PROTOCOL_F_MQ for multiqueue.

If you're wondering how any device backend with more than one virtqueue
functions today, it's because device types with a fixed number of
virtqueues do not require querying the number of queues.  Therefore the
vhost-user master for vhost-user-input with 2 virtqueues, for example,
doesn't actually depend on VHOST_USER_PROTOCOL_F_MQ.  It just enables
virtqueues 0 and 1 without asking.

Let there be multiqueue!

Suggested-by: Sebastien Boeuf <sebastien.boeuf@intel.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 contrib/libvhost-user/libvhost-user.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/contrib/libvhost-user/libvhost-user.c b/contrib/libvhost-user/libvhost-user.c
index 0c88431e8f..312c54f260 100644
--- a/contrib/libvhost-user/libvhost-user.c
+++ b/contrib/libvhost-user/libvhost-user.c
@@ -1160,7 +1160,8 @@ vu_set_vring_err_exec(VuDev *dev, VhostUserMsg *vmsg)
 static bool
 vu_get_protocol_features_exec(VuDev *dev, VhostUserMsg *vmsg)
 {
-    uint64_t features = 1ULL << VHOST_USER_PROTOCOL_F_LOG_SHMFD |
+    uint64_t features = 1ULL << VHOST_USER_PROTOCOL_F_MQ |
+                        1ULL << VHOST_USER_PROTOCOL_F_LOG_SHMFD |
                         1ULL << VHOST_USER_PROTOCOL_F_SLAVE_REQ |
                         1ULL << VHOST_USER_PROTOCOL_F_HOST_NOTIFIER |
                         1ULL << VHOST_USER_PROTOCOL_F_SLAVE_SEND_FD;
@@ -1200,8 +1201,8 @@ vu_set_protocol_features_exec(VuDev *dev, VhostUserMsg *vmsg)
 static bool
 vu_get_queue_num_exec(VuDev *dev, VhostUserMsg *vmsg)
 {
-    DPRINT("Function %s() not implemented yet.\n", __func__);
-    return false;
+    vmsg_set_reply_u64(vmsg, dev->max_queues);
+    return true;
 }
 
 static bool
-- 
2.21.0