linux-kernel - [tip:x86/pti] KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run() by marking the RDMSR path as unlikely()

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Message-ID: <tip-946fbbc13dce68902f64515b610eeb2a6c3d7a64@git.kernel.org>
Date:   Fri, 23 Feb 2018 00:26:59 -0800
From:   tip-bot for Paolo Bonzini <tipbot@...or.com>
To:     linux-tip-commits@...r.kernel.org
Cc:     rkrcmar@...hat.com, peterz@...radead.org, jmattson@...gle.com,
        dwmw@...zon.co.uk, linux-kernel@...r.kernel.org, hpa@...or.com,
        torvalds@...ux-foundation.org, karahmed@...zon.de,
        pbonzini@...hat.com, tglx@...utronix.de, mingo@...nel.org
Subject: [tip:x86/pti] KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run()
 by marking the RDMSR path as unlikely()

Commit-ID:  946fbbc13dce68902f64515b610eeb2a6c3d7a64
Gitweb:     https://git.kernel.org/tip/946fbbc13dce68902f64515b610eeb2a6c3d7a64
Author:     Paolo Bonzini <pbonzini@...hat.com>
AuthorDate: Thu, 22 Feb 2018 16:43:18 +0100
Committer:  Ingo Molnar <mingo@...nel.org>
CommitDate: Fri, 23 Feb 2018 08:24:36 +0100

KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run() by marking the RDMSR path as unlikely()

vmx_vcpu_run() and svm_vcpu_run() are large functions, and giving
branch hints to the compiler can actually make a substantial cycle
difference by keeping the fast path contiguous in memory.

With this optimization, the retpoline-guest/retpoline-host case is
about 50 cycles faster.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: KarimAllah Ahmed <karahmed@...zon.de>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Radim Krčmář <rkrcmar@...hat.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: kvm@...r.kernel.org
Cc: stable@...r.kernel.org
Link: http://lkml.kernel.org/r/20180222154318.20361-3-pbonzini@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
---
 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 0927be315965..7f8401d05939 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -9588,7 +9588,7 @@ 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)