[Qemu-devel] [PATCH 2/2] vhost-user: back SET/GET_CONFIG requests with a protocol feature

[Maxime Coquelin]

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

Signed-off-by: Maxime Coquelin <address@hidden>
---
 docs/interop/vhost-user.txt | 21 ++++++++++++---------
 hw/virtio/vhost-user.c      | 17 +++++++++++++++++
 2 files changed, 29 insertions(+), 9 deletions(-)

diff --git a/docs/interop/vhost-user.txt b/docs/interop/vhost-user.txt
index c058c407df..534caab18a 100644
--- a/docs/interop/vhost-user.txt
+++ b/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:
 -------------------------------
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index 44aea5c0a8..a045203b26 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 +1212,12 @@ 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) {
+            /* Dont acknowledge CONFIG feature if device doesn't support it */
+            dev->protocol_features &= ~(1ULL << VHOST_USER_PROTOCOL_F_CONFIG);
+        }
+
         err = vhost_user_set_protocol_features(dev, dev->protocol_features);
         if (err < 0) {
             return err;
@@ -1405,6 +1412,11 @@ 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 +1460,11 @@ 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;
     }
-- 
2.14.3