Re: [PATCH] clockevents/drivers/i8253: Do not zero timer counter in shutdown

[Thomas Gleixner <tglx@linutronix.de>]

On Thu, Aug 01 2024 at 20:21, David Woodhouse wrote:
> On Thu, 2024-08-01 at 21:06 +0200, Thomas Gleixner wrote:
>> Yes. So the sequence should stop KVM from trying to inject
>> interrupts. Maybe someone fixes it to actually stop fiddling with the
>> counter too :)
>
> I don't think we care about the counter value, as that's *calculated*
> on demand when the guest tries to read from it. Or, more to the point,
> *if* the guest tries to read from it.
>
> As opposed to the interrupt, which is a timer in the VMM which takes a
> CPU out of guest mode and incurs steal time, just to waggle a pin on
> the emulated PICs for no good reason.

Well, if the implementation still arms the timer in the background, then
it matters because that has to be processed too. I haven't looked at
that code at all, so what do I know.

>> > I'm glad I decided to export a function from the clocksource driver and
>> > just *call* it from pit_timer_init() though. Means we can bikeshed the
>> > shutdown sequence in *one* place and it isn't duplicated.
>> 
>> Right. Though we don't have to make this conditional on hypervisor I
>> think.
>
> Right, we don't *have* to. I vacillated about that and almost ripped it
> out before sending the patch, but came down on the side of "hardware is
> a steaming pile of crap and if I don't *have* to change its behaviour,
> let's not touch it".
>
> I justify my cowardice on the basis that it doesn't *matter* if a
> hardware implementation is still toggling the IRQ pin; in that case
> it's only a few irrelevant transistors which are busy, and it doesn't
> translate to steal time.

On real hardware it translates to power...

Thanks,

        tglx