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

[Michael S. Tsirkin]

On Sun, Dec 20, 2009 at 11:59:43AM -0600, Anthony Liguori wrote:
> 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.

Yes, but I suspect it is we that need to implement these heuristics,
so that management tools only have a small set of knobs to turn.
Duplicating knowledge of CPU features in management tools
does not make much sense to me.
Does not have to be linked into qemu though, we could have
a library or a set of utilities for management to use.


> Regards,
>
> Anthony Liguori