Re: IOMMU faults after S3

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 01/04/2026 9:14 am, Jan Beulich wrote:
> On 27.03.2026 11:19, Marek Marczykowski-Górecki wrote:
>> I noticed that on some systems, there are a lot of IOMMU faults after
>> S3. I can see it also on a laptop with MTL, but it affects also the ADL
>> gitlab runner:
>>
>>     https://gitlab.com/xen-project/hardware/xen/-/jobs/13661033722
>>     (XEN) [   37.201160] [VT-D]DMAR:[DMA Write] Request device 
>> [0000:00:1e.6] fault addr 0
>>     (XEN) [   37.201164] [VT-D]DMAR: reason 02 - Present bit in context 
>> entry is clear
>>     (XEN) [   37.202332] [VT-D]DMAR:[DMA Write] Request device 
>> [0000:00:1e.6] fault addr 0
>>     (XEN) [   37.202339] [VT-D]DMAR: reason 02 - Present bit in context 
>> entry is clear
>>
>> Interestingly, the 0000:00:1e.6 device is not even listed by lspci.
>>
>> The issue is present only on staging, not staging-4.21.
>>
>> Bisect says:
>>
>> 5ec93b2f19ff8873fca65d38c1164b0a56d3898b is the first bad commit
>> commit 5ec93b2f19ff8873fca65d38c1164b0a56d3898b
>> Author: Jan Beulich <jbeulich@xxxxxxxx>
>> Date:   Thu Jan 22 14:13:35 2026 +0100
>>
>>     x86/HPET: drop .set_affinity hook
> Looking into this, I find several things I can't quite understand (yet).
> First there is
>
> (XEN) [000000456c0fe39f] Disabling HPET for being unreliable
>
> which looks to only affect clocksource selection, but not use as
> broadcast source for CPU-idle management. (This may be an independent
> issue.)
>
> Then there is
>
> (XEN) [    2.760248] HPET: 8 timers usable for broadcast (8 total)
>
> which should only occur on ARAT-incapable systems. That should only be
> older hardware.

I'm not sure that's a reasonable assertion to draw.  The number of HPET
channels is down to the HPET alone, not anything to do with the CPU
capabilities.

>  (On my much older Skylake I don't see this line, for
> example.) What does CPUID leaf 6 have on this system? Sadly xen-cpuid
> is purely featureset based, and hence doesn't expose info about that
> leaf.

xen-cpuid -p

That will get you leaf 6, but there's no human-readable decode of it.

~Andrew