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>

1 files changed, 22 insertions, 0 deletions

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 44aea5c0a8..38da8692bb 100644
--- a/hw/virtio/vhost-user.c
+++ b/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;
     }