Re: Migration and CPU type?

[Anthony Liguori <aliguori@us.ibm.com>]

John Byrne wrote:
>
> There are various differences between x86 CPU types that I believe 
> would cause a guest to fail after being migrated. Are there checks in 
> the migration code to prevent this from happening? Does it check for 
> an "incompatible CPU" and fail early, leaving the guest running on the 
> source host? If there is a check, what is its nature? (Exact match of 
> CPU type/rev or something based on CPU-features?)
This, unfortunately, is always going to be a heuristic.  There's no way 
to guarantee CPU compatibility (except, of course, with VT/SVM).  The 
major problem is the CPUID instruction.  If an application obtains CPUID 
info (which I think can even contain a serial number for the processor), 
and relies on this info (even for something simple like CPU frequency), 
it will break when moved to another CPU that doesn't have the same exact 
information reported for CPUID.

This is a bit contrived but important as a base case to assert that 
there's no completely safe heuristic for migration.

There are going to, of course, be common cases of incompatibility mostly 
related to things like the presence of SIMD instructions.  One can argue 
that for a lot server work loads, this isn't going to be an issue.  I 
don't think it's horrible that Xend doesn't do any checking right now 
because of this.  It certainly wouldn't be harmful to do some checking 
and issue a warning to the user before migration.

Personally, I think it's far easier for high level tools to take care of 
this since they will have a better idea of the cluster's hardware layout 
and can make appropriate decisions based on risk compared to availability.

Regards,

Anthony Liguori
> Thanks,
>
> John Byrne
>
>
>
>
>
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