Re: [Qemu-devel] [PATCH] target-i386: n270 can MOVBE

[Eduardo Habkost]

On Fri, Feb 08, 2013 at 06:35:56PM +0100, Borislav Petkov wrote:
> On Fri, Feb 08, 2013 at 01:58:58PM -0200, Eduardo Habkost wrote:
> 
> [ … ]
> 
> > As we don't have a decent method to do that today, we are using static
> > variables and compatibility-setup functions called from the machine init
> > function. See disable_kvm_pv_eoi() for example.
> > 
> > One day we will be able to do that using properties on the machine-type
> > compat_props tables, but we can't do that yet.
> 
> Ok, understood.
> 
> > People can easily work around the lack of the feature today, using
> > "-cpu n270,+movbe",
> 
> Are you sure?
> 
> $ qemu-system-i386 -snapshot ... -cpu n270,+movbe ...

Using TCG, right?

> 
> from latest qemu master doesn't seem to work here. We still don't see
> bit 22 in ECX of CPUID.EAX(1) advertized to the guest.

Replace "the lack of the feature" with "the lack of this specific patch"
on my message above. For either your patch or the "+movbe" workaround to
work, you first need the feature to be working.


> 
> But that's not the big problem - we still need the actual implementation
> of MOVBE in qemu otherwise the guest kernel #GPs when trying to execute
> that instruction.

If you don't have the implementation of MOVBE working (and included on
TCG_EXT_FEATURES), neither your patch or "+movbe" can help you.

-- 
Eduardo