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

[Dan Magenheimer <dan.magenheimer@xxxxxxxxxx>]

Hi Dongxiao --

Why would you disable live migraton between two
very widely used Intel processors for ALL HVM domains
just because some domains use the rdtscp instruction?

Why not just add the code to do rdtscp emulation,
which would NOT break live migration?

There are many cases where rdtsc/rdtscp instructions
are emulated and so most of the code is already there.
You only need to intercept illegal instruction traps,
so there is not a significant performance issue.
And the code to do the emulation is necessary
to implement the pvrdtscp algorithm on hvm anyway
(which I think was the reason this whole discussion
started).

Dan

> -----Original Message-----
> From: Xu, Dongxiao [mailto:dongxiao.xu@xxxxxxxxx]
> Sent: Monday, December 14, 2009 9:40 PM
> To: Keir Fraser; Dan Magenheimer
> Cc: Jeremy Fitzhardinge; xen-devel@xxxxxxxxxxxxxxxxxxx; Kurt Hackel;
> Dugger, Donald D; Nakajima, Jun; Zhang, Xiantao
> Subject: RE: Live migration fails due to c/s 20627
> 
> 
> Keir Fraser wrote:
> > 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. 
> 
> I will write a patch for this. Thanks!
> 
> -- Dongxiao
> 
> > 
> >  -- Keir
> > 
> > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxxxxxxxx
> > http://lists.xensource.com/xen-devel

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