KVM: VMX: Don't adjust tsc offset forward

Most Intel hosts have a stable tsc, and playing with the offset only
reduces accuracy.  By limiting tsc offset adjustment only to forward updates,
we effectively disable tsc offset adjustment on these hosts.

Signed-off-by: Avi Kivity <avi@qumranet.com>

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 7ef710a..fb0389d 100644 (file)
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -519,7 +519,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 {
        struct vcpu_vmx *vmx = to_vmx(vcpu);
        u64 phys_addr = __pa(vmx->vmcs);
-       u64 tsc_this, delta;
+       u64 tsc_this, delta, new_offset;
 
        if (vcpu->cpu != cpu) {
                vcpu_clear(vmx);
@@ -559,8 +559,11 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
                 * Make sure the time stamp counter is monotonous.
                 */
                rdtscll(tsc_this);
-               delta = vcpu->arch.host_tsc - tsc_this;
-               vmcs_write64(TSC_OFFSET, vmcs_read64(TSC_OFFSET) + delta);
+               if (tsc_this < vcpu->arch.host_tsc) {
+                       delta = vcpu->arch.host_tsc - tsc_this;
+                       new_offset = vmcs_read64(TSC_OFFSET) + delta;
+                       vmcs_write64(TSC_OFFSET, new_offset);
+               }
        }
 }