On Thu, Aug 01 2024 at 20:21, David Woodhouse wrote:

On Thu, 2024-08-01 at 21:06 +0200, Thomas Gleixner wrote:

quoted

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.

quoted

quoted

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