Re: [Xen-devel] [BUG] panic: "IO-APIC + timer doesn't work" - several people have reproduced

[Jan Beulich <jbeulich@xxxxxxxx>]

On 17.03.2020 16:23, Jason Andryuk wrote:
> Below is the diff.  It was messier and I tidied it up some.

I've looked into this some more. I can see how what we currently
do is not in line with firmware handing off with LegacyReplacement
mode enabled. However, this case doesn't look to apply here:

> It's mainly the change to hpet_resume() to mimic Linux's legacy HPET
> setup on T0.  It turns on HPET_CFG_LEGACY to ensure the timer interrupt
> is running.  And it also includes the printing of the initial HPET
> config:
> HPET_CFG 00000001

While HPET_CFG_ENABLE is set, HPET_CFG_LEGACY is clear.

> HPET_T0_CFG 00008030
> HPET_T0_ROUTE 0000016c

And while firmware must have setup FSB routing for T0, its enable
bits (both HPET_TN_ENABLE and HPET_TN_FSB) are also clear.
Therefore we have, afaics, no indication whatsoever that we ought
to enable LegacyReplacement mode. Of course the spec also says
"Assuming platform does not have 8254/RTC hardware or does not
want to support this legacy timer hardware, for this case, System
BIOS should set the LegacyReplacement Route bit and report IRQ0 &
IRQ8 as being consumed by the HPET block in system name space:"
(followed by a table). What is referred to as "system name space"
is, I assume, ACPI DSDT/SSDTs, which we have no access to this
early (and Linux doesn't either, aiui), so also can't be used as
indicator.

Otoh I also don't think it is correct to blindly enable
LegacyReplacement mode, like - afaics - Linux does, the more with
our split brain model as far as affected devices go (Xen "owns"
the PIT [and of course also the HPET], while Linux "owns" the
RTC). This is because of the effect of this setting on what
actually drives IRQ8. In theory we might be able do so when
ACPI_FADT_NO_CMOS_RTC is set, but Linux may use the CMOS RTC
even when that flag is set.

So right now the only possible approach I see to address your
problem is to add yet another fallback mode to check_timer(),
forcing LegacyReplacement mode to be enabled. But between /
after which step(s) to put this there isn't at all obvious to me.

Jan