Re: IOMMU faults after S3

[Jan Beulich <jbeulich@xxxxxxxx>]

On 02.04.2026 16:02, Marek Marczykowski-Górecki wrote:
> On Thu, Apr 02, 2026 at 12:23:08PM +0200, Jan Beulich wrote:
>> On 02.04.2026 11:42, Marek Marczykowski-Górecki wrote:
>>> On Thu, Apr 02, 2026 at 10:47:53AM +0200, Jan Beulich wrote:
>>>> On 02.04.2026 10:39, Jan Beulich wrote:
>>>>> On 02.04.2026 10:08, Marek Marczykowski-Górecki wrote:
>>>>>> The xl dmesg output (from MTL this time):
>>>>>>
>>>>>>     (XEN) [  123.477511] Entering ACPI S3 state.
>>>>>>     (XEN) [18446743903.571842] _disable_pit_irq:2649: using_pit: 0, 
>>>>>> cpu_has_apic: 1
>>>>>>     (XEN) [18446743903.571856] _disable_pit_irq:2659: 
>>>>>> cpuidle_using_deep_cstate: 1, boot_cpu_has(X86_FEATURE_XEN_ARAT): 0
>>>>>
>>>>> XEN_ARAT being off is the one odd aspect here. That'll want tracking down
>>>>> separately. As per xen-cpuid output (below) ARAT is available.
>>>>
>>>> For this you may want to also add logging to intel_init_arat(): Since 
>>>> opt_arat
>>>> can be false only due to command line option use, it can only be the 
>>>> function
>>>> not being called (which looks impossible on plain staging code), or 
>>>> cpu_has_arat
>>>> being false despite the xen-cpuid output that you supplied earlier 
>>>> (inexplicable
>>>> as well, at least for now).
>>>
>>> Hm, I got this:
>>>
>>>     (XEN) [   11.403340] intel_init_arat:674: opt_arat: 1, cpu_has_arat: 0
>>>
>>> so, cpu_has_arat=0 ...
>>> next lines are those, to hint when it happened in the boot process:
>>>
>>>     (XEN) [   11.409754] mwait-idle: MWAIT substates: 0x11112020
>>>     (XEN) [   11.416130] mwait-idle: v0.4.1 model 0xaa
>>>     (XEN) [   11.422396] mwait-idle: lapic_timer_reliable_states 0x2
>>>
>>> Looks like calculate_host_policy() runs much later...
>>
>> Hmm, yes, and that's the problem. The reason I don't see this is that a newer
>> version of [1] has this
>>
>> --- a/xen/arch/x86/cpu/common.c
>> +++ b/xen/arch/x86/cpu/common.c
>> @@ -628,6 +628,8 @@ void identify_cpu(struct cpuinfo_x86 *c)
>>      }
>>  
>>      /* Now the feature flags better reflect actual CPU features! */
>> +    if (c == &boot_cpu_data)
>> +            calculate_host_policy();
>>  
>>      xstate_init(c);
>>  
>> --- a/xen/arch/x86/cpu-policy.c
>> +++ b/xen/arch/x86/cpu-policy.c
>> @@ -384,7 +384,7 @@ void calculate_raw_cpu_policy(void)
>>      /* Was already added by probe_cpuid_faulting() */
>>  }
>>  
>> -static void __init calculate_host_policy(void)
>> +void __init calculate_host_policy(void)
>>  {
>>      struct cpu_policy *p = &host_cpu_policy;
>>  
>> @@ -959,6 +959,7 @@ static void __init calculate_hvm_def_pol
>>  
>>  void __init init_guest_cpu_policies(void)
>>  {
>> +    /* Do this a 2nd time to account for setup_{clear,force}_cpu_cap() 
>> uses. */
>>      calculate_host_policy();
>>  
>>      if ( IS_ENABLED(CONFIG_PV) )
>>
>> and of course I'm doing my work (and my analysis) with that in place.
> 
> FWIW, with this patch applied I get:
> (XEN) [18446743899.051851] _disable_pit_irq:2649: using_pit: 0, cpu_has_apic: 
> 1
> (XEN) [18446743899.051865] _disable_pit_irq:2659: cpuidle_using_deep_cstate: 
> 1, boot_cpu_has(X86_FEATURE_XEN_ARAT): 1
> 
> And no IOMMU faults anymore.

I've Cc-ed you on the formal patch submission; please clarify whether I may
translate the above to Tested-by:.

Jan