[PATCH 10/10] KVM: x86/pmu: Avoid CPL lookup if PMC enabline for USER and KERNEL is the same

Sean Christopherson
2023-11-10T02:29:21Z

Don't bother querying the CPL if a PMC is (not) counting for both USER and
KERNEL, i.e. if the end result is guaranteed to be the same regardless of
the CPL. Querying the CPL on Intel requires a VMREAD, i.e. isn't free,
and a single CMP+Jcc is cheap.

Signed-off-by: Sean Christopherson <seanjc@google.com>
---
arch/x86/kvm/pmu.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
index a5ea729b16f2..231604d6dc86 100644
--- a/arch/x86/kvm/pmu.c
+++ b/arch/x86/kvm/pmu.c
@@ -828,6 +828,13 @@ static inline bool cpl_is_matched(struct kvm_pmc *pmc)
select_user = config & 0x2;
}

+ /*
+ * Skip the CPL lookup, which isn't free on Intel, if the result will
+ * be the same regardless of the CPL.
+ */
+ if (select_os == select_user)
+ return select_os;
+
return (static_call(kvm_x86_get_cpl)(pmc->vcpu) == 0) ? select_os : select_user;
}

--
2.42.0.869.gea05f2083d-goog