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

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 01/13/2015 11:33 AM, Boris Ostrovsky wrote:
> On 01/13/2015 11:17 AM, Boris Ostrovsky wrote:
> > 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()).
> I think I inverted domain_cpuid's logic here but my point was that we 
> use'd use combination of TSC_MODE_NEVER_EMULATE and CPUID flag 
> indicating TSC stability to pick the clocksource.
>
> -boris
>
> > 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?

So this is not going to work because early_init_intel() may set 
X86_FEATURE_CONSTANT_TSC unconditionally, without consulting CPUID.
Imre, you can ignore my question about non-dom0 guests then.

-boris



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