Re: [Xen-devel] [PATCH] xen-time: decreasing the rating of the xen clocksource below that of the tsc clocksource for dom0's

[Boris Ostrovsky]

On 01/13/2015 11:07 AM, David Vrabel wrote:
> On 13/01/15 15:42, Boris Ostrovsky wrote:
> > On 01/13/2015 04:52 AM, David Vrabel wrote:
> > > On 13/01/15 08:14, Imre Palik wrote:
> > > > From: "Palik, Imre" <imrep@xxxxxxxxx>
> > > >
> > > > In Dom0's the use of the TSC clocksource (whenever it is stable
> > > > enough to
> > > > be used) instead of the Xen clocksource should not cause any issues, as
> > > > Dom0 VMs never live-migrated.  The TSC clocksource is somewhat more
> > > > efficient than the Xen paravirtualised clocksource, thus it should have
> > > > higher rating.
> > > >
> > > > This patch decreases the rating of the Xen clocksource in Dom0s to 275.
> > > > Which is half-way between the rating of the TSC clocksource (300) and
> > > > the
> > > > hpet clocksource (250).
> > > I'm happy with this but would like to see acks from those who objected
> > > to v1.
> > >
> > > David
> > >
> > > > --- a/arch/x86/xen/time.c
> > > > +++ b/arch/x86/xen/time.c
> > > > @@ -487,6 +487,10 @@ static void __init xen_time_init(void)
> > > >        int cpu = smp_processor_id();
> > > >        struct timespec tp;
> > > >    +    /* As Dom0 is never moved, no penalty on using TSC there */
> > Again, why not any PV guest with TSC_MODE_NEVER_EMULATE?
> Surely if TSC_MODE_NEVER_EMULATE is set then the TSC is /not/ stable
> across a guest save/restore thus the PV clocksource must be used?

TSC is declared stable when !d->disable_migrate && !d->arch.vtsc, with 
vtsc being 0 with TSC_MODE_NEVER_EMULATE (per domain_cpuid()).

And if TSC is not stable as seen by CPUID (which would be the case if 
disable_migrate is not set) then kernel won't use TSC as clocksource 
anyway, regardless of rating value, won't it?

-boris

> I don't think we want to assume that TSC_MODE_NEVER_EMULATE => never
> migrate.
>
> David

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