Re: [Qemu-devel] cpuid problem in upstream qemu with kvm

[Anthony Liguori]

Gleb Natapov wrote:
> Windows is a mystery box, so we can speculate as much as we want about it.
> If you don't like something just say "it may break Windows" :) Losing
> activation does sound like an issue too.
>   

Downgrading seems more likely to cause problems.  Considering running 
mplayer in a guest.  If it detects SSE3 in one host and migrate to 
another host that doesn't have SSE3, you'll be running an instruction 
stream that uses instructions the processor doesn't support resulting in 
the application faulting due to an illegal operation.

KVM's cpuid filter doesn't help here because it won't attempt to mask 
things like sse3.  It would be insane to emulate sse3 too.

This is a classic problem with live migration.  You can detect this 
scenario and potentially block the migration, but honestly, how likely 
is it that any given guest is using an obscure feature?  This is a 
classic management tool problem and the solution usually is a set of 
heuristics depending on how conservative the target audience is.

Regards,

Anthony Liguori