Re: [PATCH] KVM: lapic: Reuse auto-adjusted timer advance of first stable vCPU

[Sean Christopherson <sean.j.christopherson@intel.com>]

On Mon, May 20, 2019 at 03:03:10PM +0200, Paolo Bonzini wrote:
> On 08/05/19 23:47, Sean Christopherson wrote:
> > +
> > +		/*
> > +		 * The first vCPU to get a stable advancement time "wins" and
> > +		 * sets the advancement time that is used for *new* vCPUS that
> > +		 * are created with auto-adjusting enabled.
> > +		 */
> > +		if (apic->lapic_timer.timer_advance_adjust_done)
> > +			(void)cmpxchg(&adjusted_timer_advance_ns, -1,
> > +				      timer_advance_ns);
> 
> This is relatively expensive, so it should only be done after setting
> timer_advance_adjust_done to true.

That's already the case, or am I missing something?

This code is inside "if (!apic->lapic_timer.timer_advance_adjust_done)",
and apic->lapic_timer.timer_advance_adjust_done is set to true at creation
time if "adjusted_timer_advance_ns != -1", i.e. the cmpxchg() will only
be reached on vCPUs that are created before adjusted_timer_advance_ns is
set and will be reached at most once per vCPU.