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
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -519,7 +519,7 @@
{
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 @@
* 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);
+ }
}
}