From 705be570941b38cd1cbebc68f7f671ce7532ecb0 Mon Sep 17 00:00:00 2001 From: Anthony PERARD Date: Fri, 23 Aug 2019 11:15:34 +0100 Subject: [PATCH] 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 Reviewed-by: Paul Durrant Message-Id: <20190823101534.465-3-anthony.perard@citrix.com> --- 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 62c127b926..a04478ad4f 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);