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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@citrix.com>
Reviewed-by: Paul Durrant <paul.durrant@citrix.com>
Message-Id: <20190823101534.465-3-anthony.perard@citrix.com>
This commit is contained in:
Anthony PERARD 2019-08-23 11:15:34 +01:00
parent cb32314607
commit 705be57094
1 changed files with 7 additions and 4 deletions
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11
hw/xen/xen-bus.c
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@ -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);