KVM: SVM: Adjust tsc_offset only if tsc_unstable

commit 953899b659adce62cbe83d6a7527550ab8797c48 upstream.

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@amd.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c
index 52ffdc68a60a452151239dc794bd97c68bb47ddf..925c326cab5b4f3ab4f5e9b617657856b804584e 100644 (file)
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -795,17 +795,18 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
        int i;
 
        if (unlikely(cpu != vcpu->cpu)) {
-               u64 tsc_this, delta;
-
-               /*
-                * Make sure that the guest sees a monotonically
-                * increasing TSC.
-                */
-               rdtscll(tsc_this);
-               delta = vcpu->arch.host_tsc - tsc_this;
-               svm->vmcb->control.tsc_offset += delta;
-               if (is_nested(svm))
-                       svm->nested.hsave->control.tsc_offset += delta;
+               u64 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;