linux-kernel - [PATCH 3/5] KVM: x86: Apply TSX_CTRL_CPUID_CLEAR if and only if the vCPU has RTM or HLE

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Message-ID: <20241211013302.1347853-4-seanjc@google.com>
Date: Tue, 10 Dec 2024 17:33:00 -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, 
	Jim Mattson <jmattson@...gle.com>
Subject: [PATCH 3/5] KVM: x86: Apply TSX_CTRL_CPUID_CLEAR if and only if the
 vCPU has RTM or HLE

When emulating CPUID, retrieve MSR_IA32_TSX_CTRL.TSX_CTRL_CPUID_CLEAR if
and only if RTM and/or HLE feature bits need to be cleared.  Getting the
MSR value is unnecessary if neither bit is set, and avoiding the lookup
saves ~80 cycles for vCPUs without RTM or HLE.

Cc: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 arch/x86/kvm/cpuid.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index f73af4a98c35..7f5fa6665969 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -1998,7 +1998,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
 		*edx = entry->edx;
 		if (function == 7 && index == 0) {
 			u64 data;
-		        if (!__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
+			if ((*ebx & (feature_bit(RTM) | feature_bit(HLE))) &&
+			    !__kvm_get_msr(vcpu, MSR_IA32_TSX_CTRL, &data, true) &&
 			    (data & TSX_CTRL_CPUID_CLEAR))
 				*ebx &= ~(feature_bit(RTM) | feature_bit(HLE));
 		} else if (function == 0x80000007) {
-- 
2.47.0.338.g60cca15819-goog