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Re: [Xen-users] xen 3.1 live migration


When Linux boots it tests what CPU features are available on this machine and 
may choose to make use of them.  For instance, MMX, 3DNow, etc may be used by 
the kernel to accelerate certain operations if they are available.  Also, 
userspace libraries may go through a similar initialisation process in order 
to choose an optimised implementation of a call.

The problem is they don't expect these features to change.  Suppose you boot a 
domain on a CPU that has 3DNow, and Linux decides to use it.  Then you 
suspend/resume or live migrate to a machine without 3DNow.  Linux won't be 
able to change the instructions it's using, so when it invokes an instruction 
this CPU doesn't support it'll crash with an invalid opcode exception.

If you shut down a machine and start it on different hardware then it'll probe 
the CPU of the new hardware on bootup and start using features that that 
system supports.

Migration between systems ought to be OK as long as Linux (and userspace) do 
not "lose" CPU features that they were using in the process.  So e.g. 
migrating between two CPUs with the same features but different speeds or 
cache sizes should not crash the domain.


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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