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Re: [Qemu-devel] [PATCH v2 2/2] xen-bus: Avoid rewriting identical value
From: |
Paul Durrant |
Subject: |
Re: [Qemu-devel] [PATCH v2 2/2] xen-bus: Avoid rewriting identical values to xenstore |
Date: |
Tue, 27 Aug 2019 09:46:15 +0000 |
> -----Original Message-----
> From: Anthony PERARD <address@hidden>
> Sent: 23 August 2019 11:16
> To: address@hidden
> Cc: Anthony Perard <address@hidden>; Stefano Stabellini <address@hidden>; Paul
> Durrant <address@hidden>; address@hidden
> Subject: [PATCH v2 2/2] 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 <address@hidden>
Reviewed-by: Paul Durrant <address@hidden>
> ---
>
> Notes:
> v2:
> - fix coding style
> - only change frontend_set_state() and use 'publish' instead of 'export'.
>
> 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);
> --
> Anthony PERARD