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

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Date:   Thu,  9 Nov 2023 18:28:57 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Konstantin Khorenko <khorenko@...tuozzo.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 10/10] KVM: x86/pmu: Avoid CPL lookup if PMC enabline for USER
 and KERNEL is the same

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@...gle.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