Re: [Xen-devel] [PATCH v2 1/3] x86: also allow REP STOS emulation acceleration

[Tim Deegan <tim@xxxxxxx>]

At 10:56 +0000 on 09 Jan (1420797418), Andrew Cooper wrote:
> On 09/01/15 10:27, Jan Beulich wrote:
> > While the REP MOVS acceleration appears to have helped qemu-traditional
> > based guests, qemu-upstream (or really the respective video BIOSes)
> > doesn't appear to benefit from that. Instead the acceleration added
> > here provides a visible performance improvement during very early HVM
> > guest boot.
> >
> > Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> > ---
> > v2: fix asm() constraints in hvmemul_rep_stos(), as pointed out by
> >     Andrew. Add output operand telling the compiler that "buf" is being
> >     written.
> 
> Is writing buf wise? it looks like you will xfree() a wild pointer, and
> appears to interfere the "buf != p_data" logic.

I think the constraints are correct, though the 'memory' clobber makes
the rest of it moot.  But this:

> > +            default:
> > +                xfree(buf);
> > +                ASSERT(!buf);

looks dodgy...

Tim.


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