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

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Date:	Tue, 13 Jan 2015 11:17:23 -0500
From:	Boris Ostrovsky <boris.ostrovsky@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>,
	Imre Palik <imrep.amz@...il.com>,
	xen-devel@...ts.xenproject.org
CC:	"Palik, Imre" <imrep@...zon.de>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
	Anthony Liguori <aliguori@...zon.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: 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 11:07 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@...zon.de>
>>>>
>>>> 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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