Patch "KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run() by marking the RDMSR path as unlikely()" has been added to the 4.15-stable tree

This is a note to let you know that I've just added the patch titled

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

to the 4.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
kvm-vmx-optimize-vmx_vcpu_run-and-svm_vcpu_run-by-marking-the-rdmsr-path-as-unlikely.patch
and it can be found in the queue-4.15 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Paolo Bonzini <[email protected]>
Date: Thu, 22 Feb 2018 16:43:18 +0100
Subject: KVM/VMX: Optimize vmx_vcpu_run() and svm_vcpu_run() by marking the RDMSR path as unlikely()
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From: Paolo Bonzini <[email protected]>

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.

---
 arch/x86/kvm/svm.c |    2 +-
 arch/x86/kvm/vmx.c |    2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

@@ -5139,7 +5139,7 @@ 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)

@@ -9579,7 +9579,7 @@ static void __noclone vmx_vcpu_run(struc
 	 * 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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.15/kvm-x86-fix-smram-accessing-even-if-vm-is-shutdown.patch queue-4.15/kvm-x86-remove-indirect-msr-op-calls-from-spec_ctrl.patch queue-4.15/kvm-x86-move-lapic-initialization-after-vmcs-creation.patch queue-4.15/kvm-mmu-fix-overlap-between-public-and-private-memslots.patch queue-4.15/kvm-vmx-optimize-vmx_vcpu_run-and-svm_vcpu_run-by-marking-the-rdmsr-path-as-unlikely.patch