Re: [Xen-users] Migrating domU and virtualization extensions

[Mark Williamson <mark.williamson@xxxxxxxxxxxx>]

> I have a dom0 with the Intel virtualization extensions, and one
> without.  For reference, Linux reports the first (with) as being
> "model 6" in /proc/cpuinfo, and the second (without) as being "model
> 4".  The "flags" the first has which the second doesn't are
>
>     de
>     est
>     lahf_lm
>     pge
>     pse
>     sep
>     vme
>     vmx
>
> Can I migrate a domU from the second dom0 (without the vme/vmx
> extensions) to the first (with)?

Yes, I guess you should be able to - are they both the same manufacturer?  
Both Intel?  What models are they, actually?

If that works, you also may be able to migrate in the other direction...  The 
main problem is that certain system software (e.g. the kernel, some 
libraries) check at startup what extensions are available (e.g. SSE version, 
etc).  They get horribly confused if live migration happens - if they 
suddenly find themselves migrated to a system without the capabilities they 
were expecting, receive invalid operation exceptions, and probably crash.

Because of this, migrating to a machine with "more" features ought to work - 
nothing disappears.  However, if the domain reboots after being migrated to a 
system that has more features (or possibly if a library initialises itself 
after being migrated to that system) it won't necessarily be safe to migrate 
it back!

The VMX extensions don't matter for this, since they won't be in use in a domU 
anyhow...  I'm not sure what all of the other flags mean: if any of them are 
used by applications or the kernel this could be a problem...

There was talk about masking out some CPU feature bits for guests in order to 
avoid this but I'm not entirely sure what the status of this is.  Hope that's 
at least helped a bit.

Cheers,
Mark


-- 
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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