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

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
hpet clocksource (250).
I'm happy with this but would like to see acks from those who objected
to v1.


--- 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.


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.


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