Re: [PATCH v3 10/52] KVM: PPC: Book3S HV: Don't always save PMU for guest capable of nesting
From: Fabiano Rosas <hidden>
Date: 2021-10-16 12:39:59
Nicholas Piggin [off-list ref] 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 PMUI think the commit reference is now: 178266389794 (KVM: PPC: Book3S HV Nested: Reflect guest PMU in-use to L0 when guest SPRs are live)
quoted hunk ↗ jump to hunk
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>