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

[Alexander Graf]

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? :-)

Alex