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

[Anthony Liguori]

Michael S. Tsirkin wrote:
> On Mon, Dec 14, 2009 at 02:54:49PM -0600, Anthony Liguori wrote:
>   
> > Michael S. Tsirkin wrote:
> >     
> > > On Mon, Dec 14, 2009 at 02:18:33PM -0600, Anthony Liguori wrote:
> > >   
> > >       
> > > > Michael S. Tsirkin wrote:
> > > >     
> > > >         
> > > > > This might help 32 bit guests, but not guests with 64 bit
> > > > > kernel and 32 bit userspace (my case) because all 64 bit
> > > > > CPUs advertise syscall bit in cpuid. Thus 64 bit guests
> > > > > do not seem to even bother checking this bit:
> > > > > AMD + 64 bit -> syscall.
> > > > >         
> > > > >           
> > > > Okay, I don't see a great option other than migrating the vendor_id string.
> > > >     
> > > >         
> > > This won't help with kernels <2.6.32 though.  I guess we can switch
> > > default vendor to Intel, this likely has some other side effects.
> > >   
> > >       
> > That's a kernel bug.  If we think it effects a lot of users, we should  
> > introduce a CAP such that we can detect this in userspace and fail  
> > gracefully.
> >     
>
> Not emulating feature host CPU does not have is a kernel bug?
> Okay ...
> Yes, almost no one runs 2.6.32 yet.
>   

The kernel has the ability to filter feature bits from cpuid.  We assume 
it's going to filter out things it doesn't support.

Regards,

Anthony Liguori