Re: [Qemu-devel] [RISU RFC PATCH v1 4/7] risugen_x86: add module

[Richard Henderson]

On 6/28/19 7:06 PM, Jan Bobek wrote:
> That's true. (Although not in all cases; see Table 2-5 in the Intel Manual,
> Volume 2, Chapter 2, Section 2.2.1 "REX Prefixes" for some cases when REX.B
> is not decoded.) This is a compromise that I've accepted, at least for v1
> of the patch series. Note that this problem is also present in config entries
> such as
> 
> PMOVMSKB        SSE     00001111 11010111 !emit { modrm(mod => MOD_DIRECT, 
> reg => ~REG_ESP); }
> 
> Here, we force MODRM.REG != 4, but this avoids not only ESP/RSP, but
> also R12.
> 
> Hmmm... I suppose I have some ideas on how to do it better. I'll try
> to fix this, though I suspect getting it 100 % right might be
> difficult and time-consuming.

I wonder if it might be better to do the randomization at a higher level:

* Pick full registers, either 3 bits for 32-bit or 4 bits for 64-bit,
  and eventually 5 bits for avx512 z-regs for evex encoding.  Let
  risugen_x86_asm encode those depending on the chosen encoding.

* Pick only register vs memory for MODRM.MOD.  If memory, randomize
  base + index + shift + disp.  Let risugen_x86_asm encode those
  values into the modrm+sib+offset.


r~