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

[Gleb Natapov]

On Sun, Dec 20, 2009 at 06:29:11PM +0100, Alexander Graf wrote:
> 
> On 20.12.2009, at 18:23, Gleb Natapov wrote:
> 
> > On Sun, Dec 20, 2009 at 07:18:22PM +0200, Michael S. Tsirkin wrote:
> >> On Sun, Dec 20, 2009 at 06:17:02PM +0100, Alexander Graf wrote:
> >>> 
> >>> On 20.12.2009, at 17:56, Michael S. Tsirkin wrote:
> >>> 
> >>>> On Sun, Dec 20, 2009 at 05:59:33PM +0200, Avi Kivity wrote:
> >>>>> On 12/20/2009 05:51 PM, Michael S. Tsirkin wrote:
> >>>>>> 
> >>>>>>> Maybe we should make -cpu host the default.  That will give the best
> >>>>>>> performance for casual users, more testing for newer features, and 
> >>>>>>> will
> >>>>>>> force management apps to treat migration much more seriously.  The
> >>>>>>> downside is that casual users upgrading their machines might 
> >>>>>>> experience
> >>>>>>> issues with Windows.  Feature compatibility is not just about 
> >>>>>>> migration.
> >>>>>>> 
> >>>>>> This seems very aggressive.  Can't we whitelist features that we know
> >>>>>> about?  Further, doesn't KVM already do this?
> >>>>>> 
> >>>>>> 
> >>>>> 
> >>>>> It does, but without -cpuid host you're stuck with qemu64 (kvm.ko  
> >>>>> doesn't add features userspace didn't request).
> >>>> 
> >>>> Hmm, then, shouldn't either kvm or qemu mask features that we do not
> >>>> emulate well enough to make windows not crash?
> >>> 
> >>> -cpu host does that already, no?
> >>> 
> >>> Alex
> >> 
> >> I expected so, but Avi here seems to say windows will crash if you
> >> use a new CPU with it ...
> >> 
> > Windows _may_ crash if you'll _upgrade_ your _host_ CPU.
> 
> Uh. It may lose activation I guess. But apart from that I can't see how it'd 
> break as long as you don't expect loadvm to work.
> 
> Anybody mind to go into a bit more detail here? :-)
> 
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.

--
                        Gleb.