Re: [PATCH v4] mc146818rtc: add a way to generate RTC interrupts via QMP

["Philippe Mathieu-Daudé" <philmd@linaro.org>]

On 29/5/24 14:43, Daniil Tatianin wrote:
> On 5/29/24 3:36 PM, Philippe Mathieu-Daudé wrote:
> 
>> On 29/5/24 14:03, Markus Armbruster wrote:
>>> Daniil Tatianin <d-tatianin@yandex-team.ru> writes:
>>>
>>>> This can be used to force-synchronize the time in guest after a long
>>>> stop-cont pause, which can be useful for serverless-type workload.
>>>>
>>>> Also add a comment to highlight the fact that this (and one other QMP
>>>> command) only works for the MC146818 RTC controller.
>>>>
>>>> Acked-by: Philippe Mathieu-Daudé <philmd@linaro.org>
>>>> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
>>>> ---
>>>>
>>>> Changes since v0:
>>>> - Rename to rtc-inject-irq to match other similar API
>>>> - Add a comment to highlight that this only works for the I386 RTC
>>>>
>>>> Changes since v1:
>>>> - Added a description below the QMP command to explain how it can be
>>>>    used and what it does.
>>>>
>>>> Changes since v2:
>>>> - Add a 'broadcast' suffix.
>>>> - Change the comments to explain the flags we're setting.
>>>> - Change the command description to fix styling & explain that it's 
>>>> a broadcast command.
>>>>
>>>> Changes since v3:
>>>> - Fix checkpatch complaints about usage of C99 comments
>>>>
>>>> ---
>>>>   hw/rtc/mc146818rtc.c         | 20 ++++++++++++++++++++
>>>>   include/hw/rtc/mc146818rtc.h |  1 +
>>>>   qapi/misc-target.json        | 19 +++++++++++++++++++
>>>>   3 files changed, 40 insertions(+)


>>>> diff --git a/qapi/misc-target.json b/qapi/misc-target.json
>>>> index 4e0a6492a9..7d388a3753 100644
>>>> --- a/qapi/misc-target.json
>>>> +++ b/qapi/misc-target.json
>>>> @@ -19,6 +19,25 @@
>>>>   { 'command': 'rtc-reset-reinjection',
>>>>     'if': 'TARGET_I386' }
>>>>   +##
>>>> +# @rtc-inject-irq-broadcast:
>>>> +#
>>>> +# Inject an RTC interrupt for all existing RTCs on the system.
>>>> +# The interrupt forces the guest to synchronize the time with RTC.
>>>> +# This is useful after a long stop-cont pause, which is common for
>>>> +# serverless-type workload.
>>
>> In previous version you said:
>>
>>   > This isn't really related to migration though. Serverless is based
>>   > on constantly stopping and resuming the VM on e.g. every HTTP
>>   > request to an endpoint.
>>
>> Which made some sense. Maybe mention HTTP? And point to that use case
>> (possibly with QMP commands) in the commit description?
> 
> Hmm, maybe it would be helpful for people who don't know what serverless 
> means.
> 
> How about:
>      This is useful after a long stop-const pause, which is common for 
> serverless-type workloads,
>      e.g. stopping/resuming the VM on every HTTP request to an endpoint, 
> which might involve
>      a long pause in between the requests, causing time drift in the guest.

Please help me understand your workflow. Your management layer call
@stop and @cont QMP commands, is that right?

@cont will emit a @RESUME event.

If we could listen to QAPI events from C code, we could have the
mc146818rtc device automatically sync on VM resume, and no need for
this async command.

I'll let our QAPI expert enlighten me on this :)