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 [off-list ref] writes:

quoted

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