Re: [PATCH v1 14/55] KVM: PPC: Book3S HV: Don't always save PMU for guest capable of nesting

Excerpts from Michael Ellerman's message of August 6, 2021 5:34 pm:
> Nicholas Piggin <[email protected]> writes:
>> Revert the workaround added by commit 63279eeb7f93a ("KVM: PPC: Book3S
>> HV: Always save guest pmu for guest capable of nesting").
>>
>> Nested capable guests running with the earlier commit ("KVM: PPC: Book3S
>> HV Nested: Indicate guest PMU in-use in VPA") will now indicate the PMU
>> in-use status of their guests, which means the parent does not need to
>> unconditionally save the PMU for nested capable guests.
>>
>> This will cause the PMU to break for nested guests when running older
>> nested hypervisor guests under a kernel with this change. It's unclear
>> there's an easy way to avoid that, so this could wait for a release or
>> so for the fix to filter into stable kernels.
> 
> I'm not sure PMU inside nested guests is getting much use, but I don't
> think we can break this quite so casually :)
> 
> Especially as the failure mode will be PMU counts that don't match
> reality, which is hard to diagnose. It took nearly a year for us to find
> the original bug.
> 
> I think we need to hold this back for a while.
> 
> We could put it under a CONFIG option, and then flip that option to off
> at some point in the future.

Okay that might be a good compromise, I'll do that.

Thanks,
Nick