Re: [PATCH] KVM: X86: Add mmx movq emulation

[Avi Kivity <avi@redhat.com>]

On 05/07/2012 11:56 AM, Joerg Roedel wrote:
> > Since you've just thought of the issues involved, I'd appreciate a
> > review of the commits above, both wrt correctness and
> > maintainability.
>
> The patches above look correct to me. In fact cbe2c9d30 is more general
> than my implementation because it fetches all possible mmx operands.
>
> My implementation on the other side should be a bit faster because it
> looks for FP exceptions directly when the registers are accessed which
> saves one get_fpu/put_fpu cycle (and an fwait instruction).

The get_fpu/put_fpu are nops (unless we schedule in between), since we
only put_fpu() doesn't really unload the fpu.  You're correct about the
fwait; my motivation was to get the #MF exception early instead of doing
the accesses first.

> > In fact I already see one difference - my patches do reg &= 7, while
> > your patches generate #UD for %mm8-%mm15.
>
> Your version is correct. Documentation says that REX-prefixes are
> ignored where not supported or misplaced. I also tried that directly on
> hardware and it works as documented and implemented in KVM.

Thanks for verifying.

-- 
error compiling committee.c: too many arguments to function