Re: [PATCH 03/17] Unify vendor TSC logic

Glauber Costa
2010-06-16T13:22:29Z

On Wed, Jun 16, 2010 at 04:10:10PM +0800, Jason Wang wrote:
> Zachary Amsden wrote:
> >
> > void kvm_arch_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
> > {
> > + kvm_x86_ops->vcpu_load(vcpu, cpu);
> > if (unlikely(vcpu->cpu != cpu)) {
> > + /* Make sure TSC doesn't go backwards */
> > + s64 tsc_delta = !vcpu->arch.last_host_tsc ? 0 :
> > + native_read_tsc() - vcpu->arch.last_host_tsc;
> > + if (tsc_delta < 0 || check_tsc_unstable())
> >
> It's better to do the adjustment also when tsc_delta > 0
And why do you think so? Doing it on tsc_delta > 0 would force us to adjust
at every entry but the first. And I guess we want to adjust as few times as
we can.

For example, we would adjust on every cpu bounce even for machines that has
a perfectly sync tsc. This could introduce an error not present before.