x86: HPET as CPU-local timer + interrupt remapping = lockup

x86: HPET as CPU-local timer + interrupt remapping = lockup

[Jan Kiszka]

Hi all,

just noticed an apparent regression of x86-64 on CPUs without ARAT but a
sufficient number if HPET timers: If you have interrupt remapping
enabled, the kernel will lock up during boot, apparently waiting for
some never-arriving interrupts.

I'm currently adding VT-d interrupt emulation to QEMU and stumbled over
this behavior. It didn't turn out to be an emulation issue, I just
reproduced on real hardware (ARAT patched out) and found some old 3.0
kernel booting fine inside my QEMU version.

FWIW, some further details I found out under QEMU: the HPET timers do
not seem to be switched to MSI mode yet when the lockup happens.

I suppose the issue is uncritical as the combination of hardware feature
(or their absence) is probably untypical, correct? I can't invest much
into bisecting or debugging right now unfortunately. But maybe someone
has an idea what could case this.

Jan

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