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:	Wed, 14 Jan 2015 09:21:18 -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 11:33 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@...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()).
>
> 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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