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

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Message-ID: <8a78da52-26b2-42a2-80c7-c1ab8aee86eb@default>
Date:	Mon, 16 Apr 2012 10:30:35 -0700 (PDT)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Jan Beulich <JBeulich@...e.com>,
	Konrad Wilk <konrad.wilk@...cle.com>,
	"Tim (Xen.org)" <tim@....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

> From: David Vrabel [mailto:david.vrabel@...rix.com]
> 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?

For pre-Xen-4.0 and an unchanged PV guest, I don't know.  If you can
back-patch the guest kernel with a workaround such as your patch, great!
I'm only arguing against the patch getting perpetuated upstream.
 
> > 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.

Yes, that's what I propose.  To clarify:

if the guest can and does determine it is running on Xen 4.0+
	TSC is guaranteed by Xen to be stable, use clocksource=tsc tsc=reliable
else
	Xen only guarantees that pvclock is stable, use pvclock
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