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

[Paolo Bonzini]

On 07/10/2016 06:24, Michael S. Tsirkin wrote:
> 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>

Yes, I have it queued already.

Paolo

> 
>> ---
>>  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
> 
>