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Re: [PATCH 1/2] x86/time: use relative counts in calibration loops
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Thu, 13 Jan 2022 11:44:08 +0100
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
- Delivery-date: Thu, 13 Jan 2022 10:44:21 +0000
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On Thu, Jan 13, 2022 at 10:37:53AM +0100, Jan Beulich wrote:
> On 12.01.2022 09:55, Jan Beulich wrote:
> > @@ -504,11 +501,8 @@ static s64 __init init_pmtimer(struct pl
> >
> > count = inl(pmtmr_ioport) & mask;
> > start = rdtsc_ordered();
> > - target = count + CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
> > - if ( target < count )
> > - while ( (inl(pmtmr_ioport) & mask) >= count )
> > - continue;
> > - while ( (inl(pmtmr_ioport) & mask) < target )
> > + target = CALIBRATE_VALUE(ACPI_PM_FREQUENCY);
> > + while ( (elapsed = (inl(pmtmr_ioport) & mask) - count) < target )
>
> I think this is wrong, and instead needs to be
>
> while ( (elapsed = (inl(pmtmr_ioport) - count) & mask) < target )
>
> There no similar issue with HPET as there we always have full 32 bits
> available.
>
> Roger - you gave your R-b. If you agree, I'd like to retain that with
> the fix in place. But I'm not going to commit either variant ahead of
> hearing back from you.
Indeed, or else overflows past the mask boundary could make the loop
exit early.
Please keep the R-b.
Roger.
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