linux-kernel - [PATCH] KVM: x86: avoid unnecessary XSETBV on guest entry

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Date:   Wed, 13 Dec 2017 13:51:52 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     wanpeng.li@...mail.com
Subject: [PATCH] KVM: x86: avoid unnecessary XSETBV on guest entry

xsetbv can be expensive when running on nested virtualization, try to
avoid it.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/kvm/x86.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0f82e2cbf64c..daa1918031df 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -702,7 +702,8 @@ static void kvm_load_guest_xcr0(struct kvm_vcpu *vcpu)
 	if (kvm_read_cr4_bits(vcpu, X86_CR4_OSXSAVE) &&
 			!vcpu->guest_xcr0_loaded) {
 		/* kvm_set_xcr() also depends on this */
-		xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
+		if (vcpu->arch.xcr0 != host_xcr0)
+			xsetbv(XCR_XFEATURE_ENABLED_MASK, vcpu->arch.xcr0);
 		vcpu->guest_xcr0_loaded = 1;
 	}
 }
-- 
1.8.3.1