linux-kernel - [PATCH 3.16 061/410] KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run() by marking the RDMSR path as unlikely()

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Message-ID: <lsq.1528380321.701822002@decadent.org.uk>
Date:   Thu, 07 Jun 2018 15:05:21 +0100
From:   Ben Hutchings <ben@...adent.org.uk>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
CC:     akpm@...ux-foundation.org, "Paolo Bonzini" <pbonzini@...hat.com>,
        "Ingo Molnar" <mingo@...nel.org>,
        "David Woodhouse" <dwmw@...zon.co.uk>,
        "Peter Zijlstra" <peterz@...radead.org>,
        "Jim Mattson" <jmattson@...gle.com>,
        "Radim Krčmář" <rkrcmar@...hat.com>,
        "KarimAllah Ahmed" <karahmed@...zon.de>, kvm@...r.kernel.org,
        "Thomas Gleixner" <tglx@...utronix.de>,
        "Linus Torvalds" <torvalds@...ux-foundation.org>
Subject: [PATCH 3.16 061/410] KVM/VMX: Optimize vmx_vcpu_run() and
 svm_vcpu_run() by marking the  RDMSR path as unlikely()

3.16.57-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Paolo Bonzini <pbonzini@...hat.com>

commit 946fbbc13dce68902f64515b610eeb2a6c3d7a64 upstream.

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
Link: http://lkml.kernel.org/r/20180222154318.20361-3-pbonzini@redhat.com
Signed-off-by: Ingo Molnar <mingo@...nel.org>
[bwh: Backported to 3.16: adjust context]
Signed-off-by: Ben Hutchings <ben@...adent.org.uk>
---
 arch/x86/kvm/svm.c | 2 +-
 arch/x86/kvm/vmx.c | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -4077,7 +4077,7 @@ static void svm_vcpu_run(struct kvm_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)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -7673,7 +7673,7 @@ static void __noclone vmx_vcpu_run(struc
 	 * If the L01 MSR bitmap does not intercept the MSR, then we need to
 	 * save it.
 	 */
-	if (!msr_write_intercepted_l01(vcpu, MSR_IA32_SPEC_CTRL))
+	if (unlikely(!msr_write_intercepted_l01(vcpu, MSR_IA32_SPEC_CTRL)))
 		vmx->spec_ctrl = native_read_msr(MSR_IA32_SPEC_CTRL);
 
 	if (vmx->spec_ctrl)