[PATCH MINI-OS] xenbus: notify the other end when necessary

[Wei Liu <wei.liu2@citrix.com>]

The xenbus thread didn't send notification to other end when it expected
more data or consumed responses, which led to stalling the ring from
time to time.

This is the culprit that guest was less responsive when using stubdom
because the device model was stalled.

Fix this by sending notification to the other end at the right places.

Signed-off-by: Wei Liu <wei.liu2@citrix.com>
---
Cc: Ian Campbell <ian.campbell@citrix.com>
Cc: Ian Jackson <ian.jackson@eu.citrix.com>
Cc: Samuel Thibault <samuel.thibault@ens-lyon.org>

With this path I can migrate a guest with stubdom a few thousand times
without any issue, while before I could easily trigger time out
within a few iterations. This should make OSSTest stubdom test case more
reliable.

Ian J, this is a patch suitable for backporting to 4.6. It's good time
to branch mini-os now.
---
 xenbus/xenbus.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/xenbus/xenbus.c b/xenbus/xenbus.c
index 4613ed6..7451161 100644
--- a/xenbus/xenbus.c
+++ b/xenbus/xenbus.c
@@ -205,8 +205,10 @@ static void xenbus_thread_func(void *ign)
             prod = xenstore_buf->rsp_prod;
             DEBUG("Rsp_cons %d, rsp_prod %d.\n", xenstore_buf->rsp_cons,
                     xenstore_buf->rsp_prod);
-            if (xenstore_buf->rsp_prod - xenstore_buf->rsp_cons < sizeof(msg))
+            if (xenstore_buf->rsp_prod - xenstore_buf->rsp_cons < sizeof(msg)) {
+                notify_remote_via_evtchn(start_info.store_evtchn);
                 break;
+            }
             rmb();
             memcpy_from_ring(xenstore_buf->rsp,
                     &msg,
@@ -217,8 +219,10 @@ static void xenbus_thread_func(void *ign)
                     xenstore_buf->rsp_prod - xenstore_buf->rsp_cons,
                     msg.req_id);
             if (xenstore_buf->rsp_prod - xenstore_buf->rsp_cons <
-                    sizeof(msg) + msg.len)
+                    sizeof(msg) + msg.len) {
+                notify_remote_via_evtchn(start_info.store_evtchn);
                 break;
+            }
 
             DEBUG("Message is good.\n");
 
@@ -265,6 +269,9 @@ static void xenbus_thread_func(void *ign)
                 xenstore_buf->rsp_cons += msg.len + sizeof(msg);
                 wake_up(&req_info[msg.req_id].waitq);
             }
+
+            wmb();
+            notify_remote_via_evtchn(start_info.store_evtchn);
         }
     }
 }
-- 
2.1.4