linux-kernel - Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable

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Message-ID: <4F8C4818.8070603@citrix.com>
Date:	Mon, 16 Apr 2012 17:26:00 +0100
From:	David Vrabel <david.vrabel@...rix.com>
To:	Dan Magenheimer <dan.magenheimer@...cle.com>
CC:	Jan Beulich <JBeulich@...e.com>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	"Tim (Xen.org)" <tim@....org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	xen-devel <xen-devel@...ts.xen.org>,
	Sheng Yang <sheng@...ker.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable

On 16/04/12 17:05, Dan Magenheimer wrote:
>> From: David Vrabel [mailto:david.vrabel@...rix.com]
>> Subject: Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable
> 
> Nacked-by: Dan Magenheimer <dan.magenheimer@...cle.com>

Fair enough,

> [A stable clock] should be true for Xen 4.0+ (but not for pre-Xen-4.0).

The original customer problem is on a host with Xen 3.4.  What do you
recommend for Linux guests running such hosts?

> In fact, it might be wise for a Xen-savvy kernel to check to see
> if it is running on Xen-4.0+ and, if so, force clocksource=tsc
> and tsc=reliable.

So, should the xen clocksource do:

if Xen 4.0+
    clock is stable, use rdtsc only.
else
    clock is unstable, use existing pvclock implementation.

David
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