vhost-user: back SET/GET_CONFIG requests with a protocol feature

Without a dedicated protocol feature, QEMU cannot know whether
the backend can handle VHOST_USER_SET_CONFIG and
VHOST_USER_GET_CONFIG messages.

This patch adds a protocol feature that is only advertised by
QEMU if the device implements the config ops. Vhost user init
fails if the device support the feature but the backend doesn't.

The backend should only send VHOST_USER_SLAVE_CONFIG_CHANGE_MSG
requests if the protocol feature has been negotiated.

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>
Acked-by: Changpeng Liu <changpeng.liu@intel.com>

docs/interop/vhost-user.txt

@@ -379,6 +379,7 @@ Protocol features
 #define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
 #define VHOST_USER_PROTOCOL_F_CRYPTO_SESSION 7
 #define VHOST_USER_PROTOCOL_F_PAGEFAULT      8
+#define VHOST_USER_PROTOCOL_F_CONFIG         9
 
 Master message types
 --------------------
@@ -664,7 +665,8 @@ Master message types
       Master payload: virtio device config space
       Slave payload: virtio device config space
 
-      Submitted by the vhost-user master to fetch the contents of the virtio
+      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
+      submitted by the vhost-user master to fetch the contents of the virtio
       device configuration space, vhost-user slave's payload size MUST match
       master's request, vhost-user slave uses zero length of payload to
       indicate an error to vhost-user master. The vhost-user master may
@@ -677,7 +679,8 @@ Master message types
       Master payload: virtio device config space
       Slave payload: N/A
 
-      Submitted by the vhost-user master when the Guest changes the virtio
+      When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, this message is
+      submitted by the vhost-user master when the Guest changes the virtio
       device configuration space and also can be used for live migration
       on the destination host. The vhost-user slave must check the flags
       field, and slaves MUST NOT accept SET_CONFIG for read-only
@@ -766,13 +769,13 @@ Slave message types
      Slave payload: N/A
      Master payload: N/A
 
-     Vhost-user slave sends such messages to notify that the virtio device's
-     configuration space has changed, for those host devices which can support
-     such feature, host driver can send VHOST_USER_GET_CONFIG message to slave
-     to get the latest content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is
-     negotiated, and slave set the VHOST_USER_NEED_REPLY flag, master must
-     respond with zero when operation is successfully completed, or non-zero
-     otherwise.
+     When VHOST_USER_PROTOCOL_F_CONFIG is negotiated, vhost-user slave sends
+     such messages to notify that the virtio device's configuration space has
+     changed, for those host devices which can support such feature, host
+     driver can send VHOST_USER_GET_CONFIG message to slave to get the latest
+     content. If VHOST_USER_PROTOCOL_F_REPLY_ACK is negotiated, and slave set
+     the VHOST_USER_NEED_REPLY flag, master must respond with zero when
+     operation is successfully completed, or non-zero otherwise.
 
 VHOST_USER_PROTOCOL_F_REPLY_ACK:
 -------------------------------

hw/virtio/vhost-user.c

@@ -46,6 +46,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_CROSS_ENDIAN = 6,
     VHOST_USER_PROTOCOL_F_CRYPTO_SESSION = 7,
     VHOST_USER_PROTOCOL_F_PAGEFAULT = 8,
+    VHOST_USER_PROTOCOL_F_CONFIG = 9,
     VHOST_USER_PROTOCOL_F_MAX
 };
 
@@ -1211,6 +1212,17 @@ static int vhost_user_init(struct vhost_dev *dev, void *opaque)
 
         dev->protocol_features =
             protocol_features & VHOST_USER_PROTOCOL_FEATURE_MASK;
+
+        if (!dev->config_ops || !dev->config_ops->vhost_dev_config_notifier) {
+            /* Don't acknowledge CONFIG feature if device doesn't support it */
+            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
+        } else if (!(protocol_features &
+                    (1ULL << VHOST_USER_PROTOCOL_F_CONFIG))) {
+            error_report("Device expects VHOST_USER_PROTOCOL_F_CONFIG "
+                    "but backend does not support it.");
+            return -1;
+        }
+
         err = vhost_user_set_protocol_features(dev, dev->protocol_features);
         if (err < 0) {
             return err;
@@ -1405,6 +1417,11 @@ static int vhost_user_get_config(struct vhost_dev *dev, uint8_t *config,
         .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + config_len,
     };
 
+    if (!virtio_has_feature(dev->protocol_features,
+                VHOST_USER_PROTOCOL_F_CONFIG)) {
+        return -1;
+    }
+
     if (config_len > VHOST_USER_MAX_CONFIG_SIZE) {
         return -1;
     }
@@ -1448,6 +1465,11 @@ static int vhost_user_set_config(struct vhost_dev *dev, const uint8_t *data,
         .hdr.size = VHOST_USER_CONFIG_HDR_SIZE + size,
     };
 
+    if (!virtio_has_feature(dev->protocol_features,
+                VHOST_USER_PROTOCOL_F_CONFIG)) {
+        return -1;
+    }
+
     if (reply_supported) {
         msg.hdr.flags |= VHOST_USER_NEED_REPLY_MASK;
     }