Re: [Qemu-devel] Intel AVX instructions

[Richard Henderson <rth@twiddle.net>]

On 01/22/2014 11:35 AM, Xin Tong wrote:
> It seems that BOCHS have AVX instructions support in interpreter. I am
> thinking an easy/reliable way to do this would be generate helper
> calls to emulate every AVX instruction and follow how BOCHS emulates
> them. Then depending on the expected frequency and difficulties, one
> can decide whether to move some of the instructions into TCG JITted
> code ?
> 
> Also, it would be desirable to use the MMX/SSE structures and
> functions that already exist in QEMU target-i386/translate.c

The way you should start is by re-using the existing SSE helpers.

There are several things one must consider with AVX:

  (1) Old SSE insns.  These do not modify bits 128 and higher.
  (2) SSE insns encoded with VEX.  These zero bits 128 and higher.
  (3) AVX insns.  These (generally) modify all 256 bits.

Case 1 can be handled by doing nothing with the existing helpers.

Case 2 can be handled by using the existing helper, followed by a couple of
stores to zero the high part.

Case 3 can, with only a few exceptions, be handled by using the existing helper
twice on the two halves.  Thankfully the existing helpers work with host
pointers rather than register numbers.

That will cover at least 90% of the AVX2 instruction set.


r~