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 10:37 AM, Jan Beulich wrote:
> > > > On 16.04.14 at 16:28, <boris.ostrovsky@xxxxxxxxxx> wrote:
> > On 04/16/2014 07:38 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) )
> > > Doesn't this cpu_has_tsc_ratio check also need to be qualified with
> > > is_pv_domain()? And is the change from && in the old condition to ||
> > > actually valid for PV guests?
> > Hmm, I haven't thought about PV here.
> >
> > So then the condition should be
> >
> > if ( host_tsc_is_safe() )
> > {
> >       if ( (is_hvm_domain() && (arch.tsc_khz == cpu_khz || cpu_has_tsc_ratio)) 
> > ||
> >             (incarnation == 0 && d->arch.tsc_khz == cpu_khz) )
> >                 d->arch.vtsc = 0;
> > }
> Almost - to include PVH you need to either use !is_pv_domain() or
> has_hvm_container_domain().
PVH never makes here, it is forced to use TSC_MODE_NEVER_EMULATE above 
(see pvhfixme above).
PVH need to be looked at anyway. For example, there is a is_hvm_domain() 
check at the bottom which I suspect needs to be PVH-safe. I think it 
should be addressed separately though.
-boris


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