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[Xen-devel] Re: Live migration fails due to c/s 20627

To: Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>
Subject: [Xen-devel] Re: Live migration fails due to c/s 20627
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Mon, 14 Dec 2009 23:19:45 +0000
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "kurt.hackel@xxxxxxxxxx" <kurt.hackel@xxxxxxxxxx>, "Dugger, Donald D" <donald.d.dugger@xxxxxxxxx>, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
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On 14/12/2009 18:02, "Dan Magenheimer" <dan.magenheimer@xxxxxxxxxx> wrote:

> This may be true in concept, but existing tools (including
> the default xm tools) do NOT check for this... I just
> tested a live migration between a Nehalem (which supports
> rdtscp) and a Conroe (which does not).  The live migration
> works fine and the app using rdtscp runs fine on the
> Nehalem and then crashes when the live migration completes
> on the Conroe.  I *know* of existing code in Oracle
> that will be broken by this!

This is a general problem for migration between dissimilar processors. The
solution is to 'level' the feature sets, by masking CPUID flags from the
more-featured processor. In this case you would mask out RDTSCP (and perhaps
others too). This does need the RDTSCP flag setting/clearing to be moved to
xc_cpuid_x86.c, as currently the user cannot override the policy wedged into
the hypervisor itself. That's an easy thing to fix.

 -- Keir

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