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    SubjectRE: [Xen-devel] [PATCH] xen: always set the sched clock as unstable
    > 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+
    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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