Re: [Qemu-devel] [PATCH v2]MC146818 RTC: coordinate guest clock base to destination host after migration

[Paolo Bonzini <pbonzini@redhat.com>]

On 26/09/2016 05:37, zhongjun@sangfor.com.cn wrote:
> Hi, Paolo
> This is a simplified patch according to your advice. Would you please reiview it again.
> 
> 
> ------------------------------------separation line--------------------------------------------
> 
> 
> MC146818 RTC: coordinate guest clock base to destination host after migration
> 
> 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 <zhongjun@sangfor.com.cn>
> ---
>  hw/timer/mc146818rtc.c | 15 ++++++++++-----
>  1 file changed, 10 insertions(+), 5 deletions(-)
> 
> diff --git a/hw/timer/mc146818rtc.c b/hw/timer/mc146818rtc.c
> index ea625f2..1df17af 100644
> --- a/hw/timer/mc146818rtc.c
> +++ b/hw/timer/mc146818rtc.c
> @@ -717,15 +717,20 @@ 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) {
> -        rtc_set_time(s);
> -        s->offset = 0;
> -        check_update_timer(s);
> -    }
> +    rtc_set_time(s);
> +    s->offset = 0;
> +    check_update_timer(s);

This is wrong if rtc_clock is not QEMU_CLOCK_REALTIME.

Paolo