[Xen-changelog] [qemu-xen master] xen-bus: Avoid rewriting identical values to xenstore

[patchbot@xxxxxxx]

commit cef9660618a880ced798375a0fd16a8ad80bd0f0
Author:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
AuthorDate: Fri Aug 23 11:15:34 2019 +0100
Commit:     Anthony PERARD <anthony.perard@xxxxxxxxxx>
CommitDate: Tue Aug 27 17:25:52 2019 +0100

    xen-bus: Avoid rewriting identical values to xenstore
    
    When QEMU receives a xenstore watch event suggesting that the "state"
    of the frontend changed, it records this in its own state but it also
    re-write the value back into xenstore even so there were no change.
    This triggers an unnecessary xenstore watch event which QEMU will
    process again (and maybe the frontend as well). Also QEMU could
    potentially write an already old value.
    
    Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
    Reviewed-by: Paul Durrant <paul.durrant@xxxxxxxxxx>
    Message-Id: <20190823101534.465-3-anthony.perard@xxxxxxxxxx>
    (cherry picked from commit 705be570941b38cd1cbebc68f7f671ce7532ecb0)
---
 hw/xen/xen-bus.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/hw/xen/xen-bus.c b/hw/xen/xen-bus.c
index 5929aa4b2e..973c72ef64 100644
--- a/hw/xen/xen-bus.c
+++ b/hw/xen/xen-bus.c
@@ -698,7 +698,8 @@ int xen_device_frontend_scanf(XenDevice *xendev, const char 
*key,
 }
 
 static void xen_device_frontend_set_state(XenDevice *xendev,
-                                          enum xenbus_state state)
+                                          enum xenbus_state state,
+                                          bool publish)
 {
     const char *type = object_get_typename(OBJECT(xendev));
 
@@ -710,7 +711,9 @@ static void xen_device_frontend_set_state(XenDevice *xendev,
                                     xs_strstate(state));
 
     xendev->frontend_state = state;
-    xen_device_frontend_printf(xendev, "state", "%u", state);
+    if (publish) {
+        xen_device_frontend_printf(xendev, "state", "%u", state);
+    }
 }
 
 static void xen_device_frontend_changed(void *opaque)
@@ -726,7 +729,7 @@ static void xen_device_frontend_changed(void *opaque)
         state = XenbusStateUnknown;
     }
 
-    xen_device_frontend_set_state(xendev, state);
+    xen_device_frontend_set_state(xendev, state, false);
 
     if (state == XenbusStateInitialising &&
         xendev->backend_state == XenbusStateClosed &&
@@ -1169,7 +1172,7 @@ static void xen_device_realize(DeviceState *dev, Error 
**errp)
     xen_device_frontend_printf(xendev, "backend-id", "%u",
                                xenbus->backend_id);
 
-    xen_device_frontend_set_state(xendev, XenbusStateInitialising);
+    xen_device_frontend_set_state(xendev, XenbusStateInitialising, true);
 
     xendev->exit.notify = xen_device_exit;
     qemu_add_exit_notifier(&xendev->exit);
--
generated by git-patchbot for /home/xen/git/qemu-xen.git#master

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