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

[Markus Armbruster]

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

> On 4/26/24 11:39 AM, 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.
>>>
>>> Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
>>> ---
>>>   hw/rtc/mc146818rtc.c         | 15 +++++++++++++++
>>>   include/hw/rtc/mc146818rtc.h |  1 +
>>>   qapi/misc-target.json        | 16 ++++++++++++++++
>>>   3 files changed, 32 insertions(+)
>>>
>>> diff --git a/hw/rtc/mc146818rtc.c b/hw/rtc/mc146818rtc.c
>>> index f4c1869232..6980a78d5f 100644
>>> --- a/hw/rtc/mc146818rtc.c
>>> +++ b/hw/rtc/mc146818rtc.c
>>> @@ -116,6 +116,21 @@ void qmp_rtc_reset_reinjection(Error **errp)
>>>       }
>>>   }
>>>   
>>> +void qmp_rtc_notify(Error **errp)
>>> +{
>>> +    MC146818RtcState *s;
>>> +
>>> +    /*
>>> +     * See:
>>> +     * https://www.kernel.org/doc/Documentation/virtual/kvm/timekeeping.txt
>>> +     */
>>> +    QLIST_FOREACH(s, &rtc_devices, link) {
>>> +        s->cmos_data[RTC_REG_B] |= REG_B_UIE;
>>> +        s->cmos_data[RTC_REG_C] |= REG_C_IRQF | REG_C_UF;
>>> +        qemu_irq_raise(s->irq);
>>> +    }
>>> +}
>>> +
>>
>> Note for later: qmp_rtc_notify() works on all realized mc146818rtc
>> devices.  Other kinds of RTC devices are silently ignored.  Just like
>> qmp_rtc_reset_reinjection().
>>
>>>   static bool rtc_policy_slew_deliver_irq(MC146818RtcState *s)
>>>   {
>>>       kvm_reset_irq_delivered();
>>> diff --git a/include/hw/rtc/mc146818rtc.h b/include/hw/rtc/mc146818rtc.h
>>> index 97cec0b3e8..5229dffbbd 100644
>>> --- a/include/hw/rtc/mc146818rtc.h
>>> +++ b/include/hw/rtc/mc146818rtc.h
>>> @@ -56,5 +56,6 @@ MC146818RtcState *mc146818_rtc_init(ISABus *bus, int 
>>> base_year,
>>>   void mc146818rtc_set_cmos_data(MC146818RtcState *s, int addr, int val);
>>>   int mc146818rtc_get_cmos_data(MC146818RtcState *s, int addr);
>>>   void qmp_rtc_reset_reinjection(Error **errp);
>>> +void qmp_rtc_notify(Error **errp);
>>>   
>>>   #endif /* HW_RTC_MC146818RTC_H */
>>> diff --git a/qapi/misc-target.json b/qapi/misc-target.json
>>> index 4e0a6492a9..20457b0acc 100644
>>> --- a/qapi/misc-target.json
>>> +++ b/qapi/misc-target.json
>>> @@ -19,6 +19,22 @@
>>     ##
>>     # @rtc-reset-reinjection:
>>     #
>>     # This command will reset the RTC interrupt reinjection backlog.  Can
>>     # be used if another mechanism to synchronize guest time is in effect,
>>     # for example QEMU guest agent's guest-set-time command.
>>     #
>>     # Since: 2.1
>>     #
>>     # Example:
>>     #
>>     #     -> { "execute": "rtc-reset-reinjection" }
>>     #     <- { "return": {} }
>>     ##
>>>   { 'command': 'rtc-reset-reinjection',
>>>     'if': 'TARGET_I386' }
>>>   
>>> +##
>>> +# @rtc-notify:
>>> +#
>>> +# Generate an RTC interrupt.
>>
>> Our QMP command to generate NMIs is called inject-nmi.  Call this one
>> inject-rtc-irq for consistency?  rtc-inject-irq?
>
> This makes sense, I'll rename in the next version. Thanks.
>
>>> +#
>>> +# Since: 9.1
>>> +#
>>> +# Example:
>>> +#
>>> +#     -> { "execute": "rtc-notify" }
>>> +#     <- { "return": {} }
>>> +#
>>> +##
>>> +{ 'command': 'rtc-notify',
>>> +  'if': 'TARGET_I386' }
>>> +
>>
>> As noted above, both commands silently ignore RTCs other than
>> mc146818rtc.
>>
>> They're only available with TARGET_I386.
>>
>> As long as all machines available with TARGET_I386 can only ever contain
>> mc146818rtc RTCs, ignoring other RTCs is a non-problem in practice.
>>
>> Feels a bit fragile to me.  Thoughts?
>
> Feels a bit fragile indeed, but this code has been there since 2.1, and 
> I guess no one really found this to be a problem.

Needs a comment at least.  I'd put it before the two fragile functions
in mc146818rtc.c.

Even better would be to make the build fail if TARGET_I386 picks up
another RTC, but I don't know how.

>>>   ##
>>>   # @SevState:
>>>   #