vhost-user: fix legacy cross-endian configurations

Currently, vhost-user does not implement any means for notifying the
backend about guest endianess. This commit introduces a new message
called VHOST_USER_SET_VRING_ENDIAN which is analogous to the ioctl()
called VHOST_SET_VRING_ENDIAN used for kernel vhost backends. Such
message is necessary for backends supporting legacy (pre-1.0) virtio
devices running in big-endian guests.

Signed-off-by: Felipe Franciosi <felipe@nutanix.com>
Signed-off-by: Mike Cui <cui@nutanix.com>

docs/interop/vhost-user.txt

@@ -326,6 +326,7 @@ Protocol features
 #define VHOST_USER_PROTOCOL_F_REPLY_ACK      3
 #define VHOST_USER_PROTOCOL_F_MTU            4
 #define VHOST_USER_PROTOCOL_F_SLAVE_REQ      5
+#define VHOST_USER_PROTOCOL_F_CROSS_ENDIAN   6
 
 Master message types
 --------------------
@@ -580,6 +581,21 @@ Master message types
       This request should be send only when VIRTIO_F_IOMMU_PLATFORM feature
       has been successfully negotiated.
 
+ * VHOST_USER_SET_VRING_ENDIAN
+
+      Id: 23
+      Equivalent ioctl: VHOST_SET_VRING_ENDIAN
+      Master payload: vring state description
+
+      Set the endianess of a VQ for legacy devices. Little-endian is indicated
+      with state.num set to 0 and big-endian is indicated with state.num set
+      to 1. Other values are invalid.
+      This request should be sent only when VHOST_USER_PROTOCOL_F_CROSS_ENDIAN
+      has been negotiated.
+      Backends that negotiated this feature should handle both endianesses
+      and expect this message once (per VQ) during device configuration
+      (ie. before the master starts the VQ).
+
 Slave message types
 -------------------
 

hw/virtio/vhost-user.c

@@ -33,6 +33,7 @@ enum VhostUserProtocolFeature {
     VHOST_USER_PROTOCOL_F_REPLY_ACK = 3,
     VHOST_USER_PROTOCOL_F_NET_MTU = 4,
     VHOST_USER_PROTOCOL_F_SLAVE_REQ = 5,
+    VHOST_USER_PROTOCOL_F_CROSS_ENDIAN = 6,
 
     VHOST_USER_PROTOCOL_F_MAX
 };
@@ -63,6 +64,7 @@ typedef enum VhostUserRequest {
     VHOST_USER_NET_SET_MTU = 20,
     VHOST_USER_SET_SLAVE_REQ_FD = 21,
     VHOST_USER_IOTLB_MSG = 22,
+    VHOST_USER_SET_VRING_ENDIAN = 23,
     VHOST_USER_MAX
 } VhostUserRequest;
 
@@ -367,8 +369,25 @@ static int vhost_user_set_vring_addr(struct vhost_dev *dev,
 static int vhost_user_set_vring_endian(struct vhost_dev *dev,
                                        struct vhost_vring_state *ring)
 {
-    error_report("vhost-user trying to send unhandled ioctl");
+    bool cross_endian = virtio_has_feature(dev->protocol_features,
-    return -1;
+                                           VHOST_USER_PROTOCOL_F_CROSS_ENDIAN);
+    VhostUserMsg msg = {
+        .request = VHOST_USER_SET_VRING_ENDIAN,
+        .flags = VHOST_USER_VERSION,
+        .payload.state = *ring,
+        .size = sizeof(msg.payload.state),
+    };
+
+    if (!cross_endian) {
+        error_report("vhost-user trying to send unhandled ioctl");
+        return -1;
+    }
+
+    if (vhost_user_write(dev, &msg, NULL, 0) < 0) {
+        return -1;
+    }
+
+    return 0;
 }
 
 static int vhost_set_vring(struct vhost_dev *dev,