From: Dan Magenheimer <dan.magenheimer@oracle.com>
To: Jan Beulich <JBeulich@suse.com>, David Vrabel <david.vrabel@citrix.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
xen-devel <xen-devel@lists.xen.org>,
Konrad Wilk <konrad.wilk@oracle.com>,
linux-kernel@vger.kernel.org, "Tim (Xen.org)" <tim@xen.org>,
Sheng Yang <sheng@yasker.org>
Subject: RE: [Xen-devel] [PATCH] xen: always set the sched clock as unstable
Date: Tue, 17 Apr 2012 08:42:48 -0700 (PDT) [thread overview]
Message-ID: <3ea2625c-bbf0-44ad-9047-ecc3b8bce29f@default> (raw)
In-Reply-To: <4F8D3C4E020000780007E658@nat28.tlf.novell.com>
> From: Jan Beulich [mailto:JBeulich@suse.com]
> Subject: RE: [Xen-devel] [PATCH] xen: always set the sched clock as unstable
>
> >>> On 16.04.12 at 19:30, Dan Magenheimer <dan.magenheimer@oracle.com> wrote:
> >> From: David Vrabel [mailto:david.vrabel@citrix.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@citrix.com]
> >> >> Subject: Re: [Xen-devel] [PATCH] xen: always set the sched clock as unstable
> >> >
> >> > Nacked-by: Dan Magenheimer <dan.magenheimer@oracle.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+
>
> _and_ TSC reads are emulated (which I don't think they are by
> default
They are emulated by default on any machine where Xen has
determined that TSC is untrustworthy AND always after migration.
So by definition (if not always in fact, see previous email),
and ignoring Xen bugs, Xen 4.0+ guarantees to guests that TSC
is a stable clock across all vcpus.
next prev parent reply other threads:[~2012-04-17 15:43 UTC|newest]
Thread overview: 45+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-04-13 18:20 [PATCH] xen: always set the sched clock as unstable David Vrabel
2012-04-13 18:20 ` David Vrabel
2012-04-13 18:31 ` Sheng Yang
2012-04-13 18:39 ` David Vrabel
2012-04-13 18:33 ` Sheng Yang
2012-04-16 11:32 ` Jan Beulich
2012-04-16 14:59 ` David Vrabel
2012-04-16 15:16 ` Tim Deegan
2012-04-16 15:16 ` Tim Deegan
2012-04-16 15:17 ` Konrad Rzeszutek Wilk
2012-04-16 15:17 ` Konrad Rzeszutek Wilk
2012-04-16 16:20 ` Dan Magenheimer
2012-04-16 16:20 ` [Xen-devel] " Dan Magenheimer
2012-04-16 16:05 ` Dan Magenheimer
2012-04-16 16:14 ` Jan Beulich
2012-04-16 16:14 ` [Xen-devel] " Jan Beulich
2012-04-16 17:22 ` Dan Magenheimer
2012-04-16 17:22 ` [Xen-devel] " Dan Magenheimer
2012-04-17 7:27 ` Jan Beulich
2012-04-17 15:36 ` Dan Magenheimer
2012-04-17 15:36 ` [Xen-devel] " Dan Magenheimer
2012-04-17 7:27 ` Jan Beulich
2012-04-16 16:26 ` David Vrabel
2012-04-16 16:26 ` [Xen-devel] " David Vrabel
2012-04-16 17:30 ` Dan Magenheimer
2012-04-16 17:30 ` [Xen-devel] " Dan Magenheimer
2012-04-17 7:47 ` Jan Beulich
2012-04-17 7:47 ` [Xen-devel] " Jan Beulich
2012-04-17 15:42 ` Dan Magenheimer [this message]
2012-04-17 15:42 ` Dan Magenheimer
2012-04-16 17:08 ` [Xen-devel] " Tim Deegan
2012-04-16 17:52 ` Dan Magenheimer
2012-04-16 18:17 ` Tim Deegan
2012-04-16 18:17 ` [Xen-devel] " Tim Deegan
2012-04-16 23:01 ` Sheng Yang
2012-04-16 23:01 ` [Xen-devel] " Sheng Yang
2012-04-17 0:29 ` Dan Magenheimer
2012-04-17 0:29 ` [Xen-devel] " Dan Magenheimer
2012-04-17 8:19 ` Tim Deegan
2012-04-17 8:19 ` [Xen-devel] " Tim Deegan
2012-04-16 17:52 ` Dan Magenheimer
2012-04-16 17:08 ` Tim Deegan
2012-04-16 16:05 ` Dan Magenheimer
2012-04-16 14:59 ` David Vrabel
2012-04-16 11:32 ` Jan Beulich
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