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

[Gautam Menghani]

On Thu, Nov 07, 2024 at 10:54:29PM +1100, Michael Ellerman wrote:
> Gautam Menghani <gautam at 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.

> 
> > 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.

> 
> > 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?

> 
> Can you identify the commit that broke this and include a Fixes: tag
> please.

Yes will do

Thanks,
Gautam