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

[Tim Deegan <tim@xxxxxxx>]

At 16:29 +0000 on 08 Jan (1420730974), Jan Beulich wrote:
> >>> On 08.01.15 at 17:16, <tim@xxxxxxx> wrote:
> > At 15:50 +0000 on 08 Jan (1420728649), 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>
> > 
> > If I read this right, it's allocating and memset()ing a buffer and
> > then copying that buffer to the guest?
> 
> In the non-MMIO case yes.
> 
> > Would it be better to map the guest frame and write directly?  Edge
> > cases where the STOS crosses a frame boundary could happen the old way.
> 
> This matches the REP MOVS handling, which also allocates a
> temporary buffer. I.e. if you wanted this for REP STOS, we should
> first make it so for REP MOVS.

Well, REP MOVS is trickier as it would have to handle page crossings
in both source and destination.

Tim.

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