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Re: [PATCH] x86/time: minor adjustments to init_pit()
- To: Andrew Cooper <amc96@xxxxxxxx>
- From: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- Date: Tue, 25 Jan 2022 13:18:04 +0100
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- Cc: Jan Beulich <jbeulich@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>
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On Thu, Jan 20, 2022 at 04:17:37PM +0000, Andrew Cooper wrote:
> On 17/01/2022 10:36, Jan Beulich wrote:
> > For one, "using_pit" shouldn't be set ahead of the function's last
> > (for now: only) error path. Otherwise "clocksource=pit" on the command
> > line can lead to misbehavior when actually taking that error path.
> >
> > And then make an implicit assumption explicit: CALIBRATE_FRAC cannot,
> > for example, simply be changed to 10. The way init_pit() works, the
> > upper bound on the calibration period is about 54ms.
> >
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>
> > ---
> > Really I've noticed this while considering what would happen if someone
> > specified "clocksource=pit" on the shim's command line. Unlike "hpet"
> > and "acpi", "pit" presently wouldn't be (explicitly) ignored. While,
> > aiui, right now the only error path would be taken (due to port 0x61
> > reads being supposed to get back 0xff), I don't think we can build on
> > that longer term: Seeing what we use port 0x61 for in traps.c, I think
> > sooner or later we will need to have some form of emulation for it. Such
> > emulation is then not unlikely to continuously report 0 in the bit in
> > question. That would leed to an infinite loop here.
>
> If we're not already doing it, pv shim really ought to set the FADT
> hardware reduced bits. There should be no need to depend on heuristics
> around ~0.
We have talked about setting the hardware reduced flag for PVH,
it's however tricky. For once hardware-reduced ACPI mandates the usage
of UEFI firmware, which we don't yet fully support for PVH.
And then we cannot set that flag for a PVH dom0, because we use a mix
of crafted and native ACPI tables, so we risk that setting the bit
creates incompatibilities with the native tables we expose.
Roger.
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