Re: [Xen-devel] [PATCH 2/2] x86/time: update vtsc_last with cmpxchg and drop vtsc_lock

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Dec 16, 2019 at 12:53:40PM +0000, Igor Druzhinin wrote:
> On 16/12/2019 10:00, Roger Pau Monné wrote:
> > On Fri, Dec 13, 2019 at 10:48:02PM +0000, Igor Druzhinin wrote:
> > I'm not sure if the following would be slightly better performance
> > wise:
> > 
> > do {
> >     old = d->arch.vtsc_last;
> >     if ( d->arch.vtsc_last >= now )
> >     {
> >         new = atomic_inc_return(&d->arch.vtsc_last);
> >         break;
> >     }
> >     else
> >         new = now;
> > } while ( cmpxchg(&d->arch.vtsc_last, old, new) != old );
> > 
> > In any case I'm fine with your version using cmpxchg exclusively.
> 
> That could be marginally better (knowing that atomic increment usually 
> performs
> better than cmpxchg) but it took me some time to work out there is no hidden
> race here. I'd request a third opinion on the matter if it's worth changing.

Anyway, your proposed approach using cmpxchg is fine IMO, we can leave
the atomic increment for a further improvement if there's a need for
it.

Thanks, Roger.

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