linux-kernel - [PATCH] KVM: SVM: Adjust tsc_offset only if tsc

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Date:	Mon, 14 Dec 2009 12:22:20 +0100
From:	Joerg Roedel <joerg.roedel@....com>
To:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Joerg Roedel <joerg.roedel@....com>
Subject: [PATCH] KVM: SVM: Adjust tsc_offset only if tsc_unstable

The tsc_offset adjustment in svm_vcpu_load is executed
unconditionally even if Linux considers the host tsc as
stable. This causes a Linux guest detecting an unstable tsc
in any case.
This patch removes the tsc_offset adjustment if the host tsc
is stable. The guest will now get the benefit of a stable
tsc too.

Signed-off-by: Joerg Roedel <joerg.roedel@....com>
---
 arch/x86/kvm/svm.c |   18 ++++++++++--------
 1 files changed, 10 insertions(+), 8 deletions(-)

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 3de0b37..cb04378 100644
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -767,14 +767,16 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 	if (unlikely(cpu != vcpu->cpu)) {
 		u64 delta;
 
-		/*
-		 * Make sure that the guest sees a monotonically
-		 * increasing TSC.
-		 */
-		delta = vcpu->arch.host_tsc - native_read_tsc();
-		svm->vmcb->control.tsc_offset += delta;
-		if (is_nested(svm))
-			svm->nested.hsave->control.tsc_offset += delta;
+		if (check_tsc_unstable()) {
+			/*
+			 * Make sure that the guest sees a monotonically
+			 * increasing TSC.
+			 */
+			delta = vcpu->arch.host_tsc - native_read_tsc();
+			svm->vmcb->control.tsc_offset += delta;
+			if (is_nested(svm))
+				svm->nested.hsave->control.tsc_offset += delta;
+		}
 		vcpu->cpu = cpu;
 		kvm_migrate_timers(vcpu);
 		svm->asid_generation = 0;
-- 
1.6.5.4


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