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

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Date:	Wed, 16 Jun 2010 10:22:23 -0300
From:	Glauber Costa <glommer@...hat.com>
To:	Jason Wang <jasowang@...hat.com>
Cc:	Zachary Amsden <zamsden@...hat.com>, avi@...hat.com,
	mtosatti@...hat.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 03/17] Unify vendor TSC logic

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.

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