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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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