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

[Markus Armbruster <armbru@redhat.com>]

Daniil Tatianin <d-tatianin@yandex-team.ru> writes:

> On 5/29/24 4:39 PM, Philippe Mathieu-Daudé wrote:
>
>> 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?
>
> Yes, that is correct.
>
>> @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.
>
> Perhaps? I'm not sure how that would be implemented, but let's see what Markus has to say.

You can't listen on an event in QEMU itself.  You can only hook into the
place that generates the event.

The RESUME event is sent from vm_prepare_start() in system/cpus.c.

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