[Qemu-devel] [PATCH 1/1] vhost-user: modify SET_LOG_BASE only if VHOST_USER_PROTOCOL_F_LOG_SHMFD is set

[Thibaut Collet]

Fixes: 2b8819c6eee5 ("vhost-user: modify SET_LOG_BASE to pass mmap size and
offset")

For compatibility with old vhost backend content of the SET_LOG_BASE message
can not be modified.
The SET_LOG_BASE message payload is modified only if the
VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature has been negociated.

The documentation has been updated accordingly with remarks from Marc André
Lureau.

Signed-off-by: Thibaut Collet <address@hidden>
---
 docs/specs/vhost-user.txt | 16 ++++++++++++++--
 hw/virtio/vhost-user.c    | 12 +++++++++---
 2 files changed, 23 insertions(+), 5 deletions(-)

diff --git a/docs/specs/vhost-user.txt b/docs/specs/vhost-user.txt
index 26dde2e..da4bf9c 100644
--- a/docs/specs/vhost-user.txt
+++ b/docs/specs/vhost-user.txt
@@ -87,6 +87,14 @@ Depending on the request type, payload can be:
    User address: a 64-bit user address
    mmap offset: 64-bit offset where region starts in the mapped memory
 
+ * vhost user log description
+   -----------------
+   | size | offset |
+   -----------------
+
+   size: a 64-bit size
+   Offset: a 64-bit offset where log starts in the mapped memory
+
 In QEMU the vhost-user message is implemented with the following struct:
 
 typedef struct VhostUserMsg {
@@ -280,14 +288,18 @@ Message types
 
       Id: 6
       Equivalent ioctl: VHOST_SET_LOG_BASE
-      Master payload: u64
+      Master payload: - u64 if slave has not the 
VHOST_USER_PROTOCOL_F_LOG_SHMFD
+                        protocol feature
+                      - vhost user log if slave has the
+                        VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol feature
       Slave payload: N/A
 
       Sets logging shared memory space.
       When slave has VHOST_USER_PROTOCOL_F_LOG_SHMFD protocol
       feature, the log memory fd is provided in the ancillary data of
       VHOST_USER_SET_LOG_BASE message, the size and offset of shared
-      memory area provided in the message.
+      memory area provided in the message and the message reply is an
+      empty message (size of 0).
 
 
  * VHOST_USER_SET_LOG_FD
diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index c443602..dcdfd40 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -206,11 +206,17 @@ static int vhost_user_set_log_base(struct vhost_dev *dev, 
uint64_t base,
     VhostUserMsg msg = {
         .request = VHOST_USER_SET_LOG_BASE,
         .flags = VHOST_USER_VERSION,
-        .payload.log.mmap_size = log->size,
-        .payload.log.mmap_offset = 0,
-        .size = sizeof(msg.payload.log),
     };
 
+    if (shmfd) {
+        msg.payload.log.mmap_size = log->size;
+        msg.payload.log.mmap_offset = 0;
+        msg.size = sizeof(msg.payload.log);
+    } else {
+        msg.payload.u64 = base;
+        msg.size = sizeof(msg.payload.u64);
+    }
+
     if (shmfd && log->fd != -1) {
         fds[fd_num++] = log->fd;
     }
-- 
2.1.4