Re: [Xen-devel] [PATCH v4 1/3] x86: Use native RDTSC(P) execution when guest and host frequencies are the same

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 04/16/2014 05:54 AM, Jan Beulich wrote:
> > > > On 16.04.14 at 03:27, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > @@ -1889,10 +1890,14 @@ void tsc_set_info(struct domain *d,
> >           d->arch.vtsc_offset = get_s_time() - elapsed_nsec;
> >           d->arch.tsc_khz = gtsc_khz ? gtsc_khz : cpu_khz;
> >           set_time_scale(&d->arch.vtsc_to_ns, d->arch.tsc_khz * 1000 );
> > -        /* use native TSC if initial host has safe TSC, has not migrated
> > -         * yet and tsc_khz == cpu_khz */
> > -        if ( host_tsc_is_safe() && incarnation == 0 &&
> > -                d->arch.tsc_khz == cpu_khz )
> > +        /*
> > +         * Use native TSC if initial host has safe TSC and either has not
> > +         * migrated yet or tsc_khz == cpu_khz (either "naturally" or via
> > +         * TSC scaling)
> > +         */
> > +        if ( host_tsc_is_safe() &&
> > +             (incarnation == 0 || d->arch.tsc_khz == cpu_khz ||
> > +              cpu_has_tsc_ratio) )
> Can't you drop checking incarnation to be zero then? In that case,
> afaict d->arch.tsc_khz == cpu_khz due to gtsc_khz being passed
> in as zero from arch_domain_create().
Yes, it's pointless. I was thinking that maybe toolstack may pass 
gtsc_khz != cpu_khz during initial boot (it doesn't do it now, but 
theoretically it could) but then we shouldn't be using vtsc=0 in that 
case anyway.
-boris

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