[Qemu-devel] [RFC 2/4] vhost-user: Add new protocol feature MTU

[Maxime Coquelin <maxime.coquelin@redhat.com>]

This patch adds VHOST_USER_PROTOCOL_F_MTU protocol feature.

If supported, QEMU sends VHOST_USER_GET_MTU request to the client,
and expects a u64 reply containing the MTU advised for the guest.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Aaron Conole <aconole@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
---
 hw/virtio/vhost-user.c    | 11 +++++++++++
 include/hw/virtio/vhost.h |  1 +
 2 files changed, 12 insertions(+)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index b57454a..d72dd37 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -32,6 +32,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_LOG_SHMFD = 1,
     VHOST_USER_PROTOCOL_F_RARP = 2,
     VHOST_USER_PROTOCOL_F_REPLY_ACK = 3,
+    VHOST_USER_PROTOCOL_F_MTU = 4,
 
     VHOST_USER_PROTOCOL_F_MAX
 };
@@ -59,6 +60,7 @@ typedef enum VhostUserRequest {
     VHOST_USER_GET_QUEUE_NUM = 17,
     VHOST_USER_SET_VRING_ENABLE = 18,
     VHOST_USER_SEND_RARP = 19,
+    VHOST_USER_GET_MTU = 20,
     VHOST_USER_MAX
 } VhostUserRequest;
 
@@ -186,6 +188,7 @@ static bool vhost_user_one_time_request(VhostUserRequest request)
     case VHOST_USER_RESET_OWNER:
     case VHOST_USER_SET_MEM_TABLE:
     case VHOST_USER_GET_QUEUE_NUM:
+    case VHOST_USER_GET_MTU:
         return true;
     default:
         return false;
@@ -602,6 +605,14 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
                 return err;
             }
         }
+
+        /* query the MTU we support if backend supports MTU feature */
+        if (dev->protocol_features & (1ULL << VHOST_USER_PROTOCOL_F_MTU)) {
+            err = vhost_user_get_u64(dev, VHOST_USER_GET_MTU, &dev->mtu);
+            if (err < 0) {
+                return err;
+            }
+        }
     }
 
     if (dev->migration_blocker == NULL &&
diff --git a/include/hw/virtio/vhost.h b/include/hw/virtio/vhost.h
index e433089..99c8d9b 100644
--- a/include/hw/virtio/vhost.h
+++ b/include/hw/virtio/vhost.h
@@ -50,6 +50,7 @@ struct vhost_dev {
     uint64_t backend_features;
     uint64_t protocol_features;
     uint64_t max_queues;
+    uint64_t mtu;
     bool started;
     bool log_enabled;
     uint64_t log_size;
-- 
2.7.4