[PATCH] vhost-user: fix the issue of vhost deadlock in nested virtualization

[Hao Chen <chenh@yusur.tech>]

I run "dpdk-vdpa" and "qemur-L2" in "qemu-L1".

In a nested virtualization environment, "qemu-L2" vhost-user socket sends
a "VHOST_USER_IOTLB_MSG" message to "dpdk-vdpa" and blocks waiting for
"dpdk-vdpa" to process the message.
If "dpdk-vdpa" doesn't complete the processing of the "VHOST_USER_IOTLB_MSG"
message and sends a message that needs to be replied in another thread,
such as "VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG", "dpdk-vdpa" will also
block and wait for "qemu-L2" to process this message. However, "qemu-L2"
vhost-user's socket is blocking while waiting for a reply from "dpdk-vdpa"
after processing the message "VHOSTr_USER_IOTLB_MSG", and
"VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG" will not be processed.
In this case, both "dpdk-vdpa" and "qemu-L2" are blocked on the
vhost read, resulting in a deadlock.

You can modify "VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG" or
"VHOST_USER_IOTLB_MSG" to "no need reply" to fix this issue.
There are too many messages in dpdk that are similar to
"VHOST_USER_SLAVE_VRING_HOST_NOTIFIER_MSG", and I would prefer the latter.

Fixes: 24e34754eb78 ("vhost-user: factor out msg head and payload")

Signed-off-by: Hao Chen <chenh@yusur.tech>
---
 hw/virtio/vhost-user.c | 10 ++--------
 1 file changed, 2 insertions(+), 8 deletions(-)

diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
index f214df804b..02caa94b6c 100644
--- a/hw/virtio/vhost-user.c
+++ b/hw/virtio/vhost-user.c
@@ -2371,20 +2371,14 @@ static int vhost_user_net_set_mtu(struct vhost_dev *dev, uint16_t mtu)
 static int vhost_user_send_device_iotlb_msg(struct vhost_dev *dev,
                                             struct vhost_iotlb_msg *imsg)
 {
-    int ret;
     VhostUserMsg msg = {
         .hdr.request = VHOST_USER_IOTLB_MSG,
         .hdr.size = sizeof(msg.payload.iotlb),
-        .hdr.flags = VHOST_USER_VERSION | VHOST_USER_NEED_REPLY_MASK,
+        .hdr.flags = VHOST_USER_VERSION,
         .payload.iotlb = *imsg,
     };
 
-    ret = vhost_user_write(dev, &msg, NULL, 0);
-    if (ret < 0) {
-        return ret;
-    }
-
-    return process_message_reply(dev, &msg);
+    return vhost_user_write(dev, &msg, NULL, 0);
 }
 
 
-- 
2.27.0