Gautam Menghani <gau...@linux.ibm.com> writes: > On Thu, Nov 07, 2024 at 10:54:29PM +1100, Michael Ellerman wrote: >> Gautam Menghani <gau...@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
