Re: [RFC PATCH] Emulate MOVBE

[Andre Przywara <andre@andrep.de>]

On Wed, 10 Apr 2013 13:08:46 +0300
Gleb Natapov <gleb@redhat.com> wrote:

> On Wed, Apr 10, 2013 at 11:29:42AM +0200, Andre Przywara wrote:
> > In a real world VendorSpecific should be replaced with something
> > more meaningful. Depends on KVMs intention to emulate instructions,
> > actually out of scope for a pure virtualizer.
> > 
> Something like EmulateOnUD.

Right.

> > What is the opinion from the KVM folks on this? Shall we start to
> > emulate instructions the host does not provide? In this particular
> > case a relatively simple patch fixes a problem (starting Atom
> > optimized kernels on non-Atom machines).
> We can add the emulation, but we should not start announcing the
> instruction availability to a guest if host cpu does not have it by
> default. This may trick a guest into thinking that movbe is the
> fastest way to do something when it is not.

Good point. I'd also like to have a switch which enables this kind of
"non-standard" behavior. Actually this should be requested by QEMU,
right? So that a single guest can override the CPUID masking done by
the kernel if it really really wants to.
 
> > 
> > (And if one can believe the AMD Fam16h SWOG [1], PS4^Wfuture AMD
> > processors have MOVBE, so it's not even actually one CPU anymore).
> If a host CPU has the instruction emulation is not needed unless the
> instruction is used for MMIO access.

I meant to "emulate" such a CPU. -cpu ps4 ;-)

Regards,
Andre.