[PATCH v3 10/52] KVM: PPC: Book3S HV: Don't always save PMU for guest capable of nesting

[Nicholas Piggin]

Excerpts from Fabiano Rosas's message of October 16, 2021 10:38 pm:
> Nicholas Piggin <npiggin at gmail.com> writes:
> 
>> Provide a config option that controls the workaround added by commit
>> 63279eeb7f93 ("KVM: PPC: Book3S HV: Always save guest pmu for guest
>> capable of nesting"). The option defaults to y for now, but is expected
>> to go away within a few releases.
>>
>> 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
> 
> I think the commit reference is now: 178266389794 (KVM: PPC: Book3S HV
> Nested: Reflect guest PMU in-use to L0 when guest SPRs are live)

Ah yes good point, would be good to that in the changelog. I guess we
can add a References: tag as well, just in case anybody mines this 
stuff.

> 
>> in-use status of their guests, which means the parent does not need to
>> unconditionally save the PMU for nested capable guests.
>>
>> After this latest round of performance optimisations, this option costs
>> about 540 cycles or 10% entry/exit performance on a POWER9 nested-capable
>> guest.
>>
>> Signed-off-by: Nicholas Piggin <npiggin at gmail.com>
>> ---
>>  arch/powerpc/kvm/Kconfig     | 15 +++++++++++++++
>>  arch/powerpc/kvm/book3s_hv.c | 10 ++++++++--
>>  2 files changed, 23 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/kvm/Kconfig b/arch/powerpc/kvm/Kconfig
>> index ff581d70f20c..1e7aae522be8 100644
>> --- a/arch/powerpc/kvm/Kconfig
>> +++ b/arch/powerpc/kvm/Kconfig
>> @@ -130,6 +130,21 @@ config KVM_BOOK3S_HV_EXIT_TIMING
>>
>>  	  If unsure, say N.
>>
>> +config KVM_BOOK3S_HV_NESTED_PMU_WORKAROUND
>> +	bool "Nested L0 host workaround for L1 KVM host PMU handling bug" if EXPERT
>> +	depends on KVM_BOOK3S_HV_POSSIBLE
>> +	default !EXPERT
>> +	help
>> +	  Old nested HV capable Linux guests have a bug where the don't
> 
> s/the/they/
> 
> Reviewed-by: Fabiano Rosas <farosas at linux.ibm.com>
> 

Good catch.

Thanks,
Nick