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

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Message-Id: <54AD7387020000780005271C@mail.emea.novell.com>
Date:	Wed, 07 Jan 2015 16:57:27 +0000
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Ian Campbell" <Ian.Campbell@...rix.com>,
	"Imre Palik" <imrep.amz@...il.com>
Cc:	"Anthony Liguori" <aliguori@...zon.com>,
	"Imre Palik" <imrep@...zon.de>,
	"David Vrabel" <david.vrabel@...rix.com>, <x86@...nel.org>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	<xen-devel@...ts.xenproject.org>,
	"Boris Ostrovsky" <boris.ostrovsky@...cle.com>,
	"Ingo Molnar" <mingo@...hat.com>, <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [Xen-devel] [PATCH RFC] xen-time: decreasing the rating of
 the xen clocksource below that of the tsc clocksource for dom0's

>>> On 07.01.15 at 17:30, <Ian.Campbell@...rix.com> wrote:
> On Wed, 2015-01-07 at 17:16 +0100, 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.
> 
> Is this still true given that dom0's vcpus are migrated amongst pcpus on
> the host? The tsc are not synchronised on some generations of hardware
> so the result there would be the TSC appearing to do very odd things
> under dom0's feet. Does Linux cope with that or does it not matter for
> some other reason?

Indeed. The textual qualification above ("whenever it is stable enough")
isn't being expressed in the code change at all.

Jan

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