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

Nicholas Piggin <[email protected]> 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)

> 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 <[email protected]>
> ---
>  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 <[email protected]>