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

[Gleb Natapov <gleb@redhat.com>]

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.
> 
I am confused. We explicitly not talking about migration, why bring it
here? We are talking about casual user who can't care less about
migration, but he upgrades his home computer and Windows VM stops
working for him.

--
			Gleb.