Re: [Qemu-devel] [PATCH v5] MC146818 RTC: coordinate guest clock base to destination host after migration

[Michael S. Tsirkin]

On Mon, Sep 26, 2016 at 08:41:01PM +0800, Junlian Bell wrote:
> qemu tracks guest time based on vector [base_rtc, last_update], in which
> last_update stands for a monotonic tick which is actually uptime of the
> host.
> according to rtc implementation codes of recent releases and upstream,
> after
> migration, the time base vector [base_rtc, last_update] isn't updated to
> coordinate with the destionation host, ie. qemu doesnt update last_update
> to
> uptime of the destination host.
> what problem have we got because of this bug? after migration, guest time
> may
> jump back to several days ago, that will make some critical business
> applications,
> such as lotus notes, malfunction.
> this patch is trying to fix the problem. first, when vmsave in progress,
> we
> rtc_update_time to refresh time stamp in cmos array, then during
> vmrestore,
> we rtc_set_time to update qemu base_rtc and last_update variable according
> to time
> stamp in cmos array.
> 
> Signed-off-by: Junlian Bell <address@hidden>

Seems to make sense superficially but I'm not a good judge
of the timer code.

Paolo, any comment on this?

Acked-by: Michael S. Tsirkin <address@hidden>

> ---
>  hw/timer/mc146818rtc.c | 11 ++++++++++-
>  1 file changed, 10 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
> index ea625f2..4e4af43 100644
> --- a/hw/timer/mc146818rtc.c
> +++ b/hw/timer/mc146818rtc.c
> @@ -717,11 +717,19 @@ static void rtc_set_date_from_host(ISADevice *dev)
>      rtc_set_cmos(s, &tm);
>  }
>  
> +static void rtc_pre_save(void *opaque)
> +{
> +    RTCState *s = opaque;
> +
> +    rtc_update_time(s);
> +}
> +
>  static int rtc_post_load(void *opaque, int version_id)
>  {
>      RTCState *s = opaque;
>  
> -    if (version_id <= 2) {
> +    if (version_id <= 2 ||
> +            rtc_clock == QEMU_CLOCK_REALTIME){
>          rtc_set_time(s);
>          s->offset = 0;
>          check_update_timer(s);
> @@ -764,6 +772,7 @@ static const VMStateDescription vmstate_rtc = {
>      .name = "mc146818rtc",
>      .version_id = 3,
>      .minimum_version_id = 1,
> +    .pre_save = rtc_pre_save,
>      .post_load = rtc_post_load,
>      .fields = (VMStateField[]) {
>          VMSTATE_BUFFER(cmos_data, RTCState),
> -- 
> 2.9.0.windows.1