vhost-user: Add MTU protocol feature and op

This patch implements VHOST_USER_PROTOCOL_F_NET_MTU
protocol feature and VHOST_USER_NET_SET_MTU request so
that the backend gets notified of the user defined host
MTU.

If backend supports VHOST_USER_PROTOCOL_F_REPLY_ACK,
QEMU assumes MTU is valid if success is returned.

Vhost-net driver sends this request through a new
vhost_net_set_mtu vhost_ops entry.

Cc: Michael S. Tsirkin <mst@redhat.com>
Cc: Aaron Conole <aconole@redhat.com>
Signed-off-by: Maxime Coquelin <maxime.coquelin@redhat.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

docs/specs/vhost-user.txt

@@ -259,6 +259,7 @@ Protocol features
 #define VHOST_USER_PROTOCOL_F_LOG_SHMFD      1
 #define VHOST_USER_PROTOCOL_F_RARP           2
 #define VHOST_USER_PROTOCOL_F_REPLY_ACK      3
+#define VHOST_USER_PROTOCOL_F_MTU            4
 
 Message types
 -------------
@@ -470,6 +471,21 @@ Message types
       The first 6 bytes of the payload contain the mac address of the guest to
       allow the vhost user backend to construct and broadcast the fake RARP.
 
+ * VHOST_USER_NET_SET_MTU
+
+      Id: 20
+      Equivalent ioctl: N/A
+      Master payload: u64
+
+      Set host MTU value exposed to the guest.
+      This request should be sent only when VIRTIO_NET_F_MTU feature has been
+      successfully negotiated, VHOST_USER_F_PROTOCOL_FEATURES is present in
+      VHOST_USER_GET_FEATURES and protocol feature bit
+      VHOST_USER_PROTOCOL_F_NET_MTU is present in
+      VHOST_USER_GET_PROTOCOL_FEATURES.
+      If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, slave must respond
+      with zero in case the specified MTU is valid, or non-zero otherwise.
+
 VHOST_USER_PROTOCOL_F_REPLY_ACK:
 -------------------------------
 The original vhost-user specification only demands replies for certain

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_NET_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_NET_SET_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_NET_SET_MTU:
         return true;
     default:
         return false;
@@ -685,6 +688,36 @@ static bool vhost_user_can_merge(struct vhost_dev *dev,
     return mfd == rfd;
 }
 
+static int vhost_user_net_set_mtu(struct vhost_dev *dev, uint16_t mtu)
+{
+    VhostUserMsg msg;
+    bool reply_supported = virtio_has_feature(dev->protocol_features,
+                                              VHOST_USER_PROTOCOL_F_REPLY_ACK);
+
+    if (!(dev->protocol_features & (1ULL << VHOST_USER_PROTOCOL_F_NET_MTU))) {
+        return 0;
+    }
+
+    msg.request = VHOST_USER_NET_SET_MTU;
+    msg.payload.u64 = mtu;
+    msg.size = sizeof(msg.payload.u64);
+    msg.flags = VHOST_USER_VERSION;
+    if (reply_supported) {
+        msg.flags |= VHOST_USER_NEED_REPLY_MASK;
+    }
+
+    if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
+        return -1;
+    }
+
+    /* If reply_ack supported, slave has to ack specified MTU is valid */
+    if (reply_supported) {
+        return process_message_reply(dev, msg.request);
+    }
+
+    return 0;
+}
+
 const VhostOps user_ops = {
         .backend_type = VHOST_BACKEND_TYPE_USER,
         .vhost_backend_init = vhost_user_init,
@@ -708,4 +741,5 @@ const VhostOps user_ops = {
         .vhost_requires_shm_log = vhost_user_requires_shm_log,
         .vhost_migration_done = vhost_user_migration_done,
         .vhost_backend_can_merge = vhost_user_can_merge,
+        .vhost_net_set_mtu = vhost_user_net_set_mtu,
 };

include/hw/virtio/vhost-backend.h

@@ -32,6 +32,7 @@ typedef int (*vhost_backend_memslots_limit)(struct vhost_dev *dev);
 
 typedef int (*vhost_net_set_backend_op)(struct vhost_dev *dev,
                                 struct vhost_vring_file *file);
+typedef int (*vhost_net_set_mtu_op)(struct vhost_dev *dev, uint16_t mtu);
 typedef int (*vhost_scsi_set_endpoint_op)(struct vhost_dev *dev,
                                   struct vhost_scsi_target *target);
 typedef int (*vhost_scsi_clear_endpoint_op)(struct vhost_dev *dev,
@@ -83,6 +84,7 @@ typedef struct VhostOps {
     vhost_backend_cleanup vhost_backend_cleanup;
     vhost_backend_memslots_limit vhost_backend_memslots_limit;
     vhost_net_set_backend_op vhost_net_set_backend;
+    vhost_net_set_mtu_op vhost_net_set_mtu;
     vhost_scsi_set_endpoint_op vhost_scsi_set_endpoint;
     vhost_scsi_clear_endpoint_op vhost_scsi_clear_endpoint;
     vhost_scsi_get_abi_version_op vhost_scsi_get_abi_version;