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

[David Woodhouse <dwmw2@infradead.org>]

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On Thu, 2024-08-01 at 21:06 +0200, Thomas Gleixner wrote:
> On Thu, Aug 01 2024 at 18:49, David Woodhouse wrote:
> > On Thu, 2024-08-01 at 16:21 +0200, Thomas Gleixner wrote:
> > > The stop sequence is wrong:
> > > 
> > >     When there is a count in progress, writing a new LSB before the
> > >     counter has counted down to 0 and rolled over to FFFFh, WILL stop
> > >     the counter.  However, if the LSB is loaded AFTER the counter has
> > >     rolled over to FFFFh, so that an MSB now exists in the counter, then
> > >     the counter WILL NOT stop.
> > > 
> > > The original i8253 datasheet says:
> > > 
> > >     1) Write 1st byte stops the current counting
> > >     2) Write 2nd byte starts the new count
> > 
> > It says that for mode zero ("Interrupt on Terminal Count"), yes. But in
> > that mode, shouldn't the IRQ only fire *one* more time anyway, rather
> > than repeatedly? That should be OK, shouldn't it?
> > 
> > "When terminal count is reached, the output will go high and remain
> > high until the selected count register is reloaded wityh the mode or a
> > new count is loaded".
> 
> I just confirmed that this is the case on KVM.
> 
> > It's OK for it to keep *counting* as long as it stops firing
> > interrupts, isn't it?
> 
> 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.

> > Either way, this is somewhat orthogonal to the patch I posted in 
> > https://lore.kernel.org/kvm/6cd62b5058e11a6262cb2e798cc85cc5daead3b1.camel@infradead.org/T/#u
> > for the fact that we don't shut down the PIT at *all* if we aren't ever
> > going to use it.
> > 
> > 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.

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