Re: [PATCH] arch/powerpc/pseries: Fix KVM guest detection for disabling hardlockup detector

[Michael Ellerman <mpe@ellerman.id.au>]

Gautam Menghani <gautam@linux.ibm.com> writes:
> On Thu, Nov 07, 2024 at 10:54:29PM +1100, Michael Ellerman wrote:
>> Gautam Menghani <gautam@linux.ibm.com> writes:
>> > As per the kernel documentation[1], hardlockup detector should be
>> > disabled in KVM guests as it may give false positives. On PPC, hardlockup
>> > detector is broken inside KVM guests because disable_hardlockup_detector()
>>  
>> Isn't it the opposite? Inside KVM guests, the hardlockup detector should
>> be *disabled*, but it's not it's *enabled*, due to this bug.
>> 
>> ie. it's not broken, it's working, but that's the bug.
>
> Yes right, will change the description in v2.

Thanks.

>> > is marked as early_initcall and it uses is_kvm_guest(), which is
>> > initialized by check_kvm_guest() later during boot as it is a
>> > core_initcall. check_kvm_guest() is also called in pSeries_smp_probe(),
>> > which is called before initcalls, but it is skipped if KVM guest does
>> > not have doorbell support or if the guest is launched with SMT=1.
>> 
>> I'm wondering how no one has noticed. Most KVM guests have SMT=1.
>
> Looking at the commit history, code around hardlockups and
> pSeries_smp_probe() was changed around 2021/2022 timeframe, and I
> believe KVM wasn't being actively tested at the time. 
> Even I noticed this only after coming across the documentation that said
> hardlockups should be disabled. So probably this feature decision isn't
> widely known.

I do test KVM but probably not under enough load to notice something
like that.

>> > Move the check_kvm_guest() call in pSeries_smp_probe() to the initial
>> > part of function before doorbell/SMT checks so that "kvm_guest" static
>> > key is initialized by the time disable_hardlockup_detector() runs.
>> 
>> check_kvm_guest() is safe to be called multiple times so
>> disable_hardlockup_detector() should just call it before it calls
>> is_kvm_guest(). That should avoid future breakage when the order of
>> calls changes, or someone refactors pSeries_smp_probe().
>
> Yeah I did that initially but in the worst case, that results in 3 calls
> to check_kvm_guest() - the core_initcall, pseries_smp_probe() call and 
> then disable_hardlockup_detector(). Will that be fine?

Yeah it's fine. It's not pretty, maybe we can come up with something
cleaner in future, but it's fine for a bug fix.

cheers