Re: [Qemu-devel] allow sysenter on 32bit guests running on vmx host

[Andrea Arcangeli <aarcange@redhat.com>]

Hi Jamie,

thanks for review!

On Wed, Jun 24, 2009 at 06:29:34PM +0100, Jamie Lokier wrote:
> So your patch should make no difference to Linux guests.  Did you
> check Linux behaviour?  Does Windows ignore the vendor id?

Maybe I wasn't clear enough, this patch only makes a difference for
KVM. KVM switches the vendor id to Intel when run on vmx host. It
definitely makes a difference to linux guest, without this patch 32bit
linux run as guest on top of KVM has sep disabled in /proc/cpuinfo and
isn't using sysenter (just like windows, hence the skype crash). I
verified 32bit linux guest has sep enabled with this patch applied
when run on KVM on top of intel.

For AMD the change is a noop because it changes the model from Athlon
to Duron.

> Only on 64-bit host, of course.
> 
> I see the "qemu32" version has model=3, stepping =3 already :-)

Yes, problem only happens on 64bit intel host running a 32bit guest
(windows skype crash or linux slowdown) on KVM.

> It makes sense to change the model number for AMD anyway, as AMD
> Athlons don't have the sep feature, even though this isn't a real CPU id.

No I don't think Duron has sep either... this should be noop for qemu
and a noop for KVM run on AMD.

>     /* Intel sep feature is broken before family/model/stepping 6/3/3,
>        and Windows guests wrongly ignore AMD vendor id when checking. */

I think we should specify this makes the cpu a duron on AMD and a PII
on intel and your comment removed that info but sure you're welcome to
change it ;).

Another approach is to switch model to 3 along with vendor_id in KVM
but because qemu is already using 6/3/3 when emulating a 32bit x86
hardware, I don't think this is simpler and more consistent.