Excerpts from Fabiano Rosas's message of October 16, 2021 10:38 pm:

Nicholas Piggin [off-list ref] writes:

quoted

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.

quoted

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@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 <redacted>

Good catch.

Thanks,
Nick