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

[Michael S. Tsirkin]

On Mon, Dec 14, 2009 at 10:08:38PM +0200, Gleb Natapov wrote:
> On Mon, Dec 14, 2009 at 02:02:03PM -0600, Anthony Liguori wrote:
> > Gleb Natapov wrote:
> > >>I thought KVM emulates the syscall instruction?  I swear I've seen
> > >>patches for that.
> > >>
> > >It is. Starting from 2.6.32.
> > 
> > Okay, so this is a performance vs. migration compatibility issue then?
> > 
> > BTW, couldn't we just not advertise syscall in cpuid?  That should
> > fix it too without sacrificing migration compatibility.  We get a
> > slight slowdown on AMD hosts but that's probably minor compared to
> > the cost of using emulated syscall on Intel hosts.
> > 
> As far as I understand not advertising syscall for 64bit CPU is not a
> valid option.

Yes. 64 bit kernel 32 bit userpace crashes with -cpu qemu64,-syscall.

> --
>                       Gleb.