linux-kernel - [PATCH v2 2/2] KVM: VMX: mark RDMSR path as unlikely

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Message-Id: <20180222154318.20361-3-pbonzini@redhat.com>
Date:   Thu, 22 Feb 2018 16:43:18 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     x86@...nel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        KarimAllah Ahmed <karahmed@...zon.de>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Jim Mattson <jmattson@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, stable@...r.kernel.org
Subject: [PATCH v2 2/2] KVM: VMX: mark RDMSR path as unlikely

vmx_vcpu_run and svm_vcpu_run are large functions, and this can actually
make a substantial cycle difference by keeping the fast path contiguous
in memory.  Without it, the retpoline guest/retpoline host case is about
50 cycles slower.

Cc: x86@...nel.org
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: KarimAllah Ahmed <karahmed@...zon.de>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Jim Mattson <jmattson@...gle.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Ingo Molnar <mingo@...nel.org>
Cc: stable@...r.kernel.org
Reviewed-by: Jim Mattson <jmattson@...gle.com>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/svm.c | 2 +-
 arch/x86/kvm/vmx.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 1598beeda11c..24c9521ebc24 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -5465,7 +5465,7 @@ static void svm_vcpu_run(struct kvm_vcpu *vcpu)
 	 * If the L02 MSR bitmap does not intercept the MSR, then we need to
 	 * save it.
 	 */
-	if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
+	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
 		svm->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
 
 	if (svm->spec_ctrl)
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index af89d377681d..e13fd2a833c4 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9589,7 +9589,7 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 	 * If the L02 MSR bitmap does not intercept the MSR, then we need to
 	 * save it.
 	 */
-	if (!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL))
+	if (unlikely(!msr_write_intercepted(vcpu, MSR_IA32_SPEC_CTRL)))
 		vmx->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
 
 	if (vmx->spec_ctrl)
-- 
1.8.3.1