Re: [RESEND] target/riscv: fix RV128 lq encoding

[Frédéric Pétrot]

Le 18/01/2022 à 17:32, Christoph Muellner a écrit :
> If LQ has func3==010 and is located in the MISC-MEM opcodes,
> then it conflicts with the CBO opcode space.
> However, since LQ is specified as: "LQ is added to the MISC-MEM major
> opcode", we have an implementation bug, because 'major opcode'
> refers to func3, which must be 111.
>
> This results in the following instruction encodings:
>
> lq        ........ ........ .111.... .0001111
> cbo_clean 00000000 0001.... .0100000 00001111
> cbo_flush 00000000 0010.... .0100000 00001111
> cbo_inval 00000000 0000.... .0100000 00001111
> cbo_zero  00000000 0100.... .0100000 00001111
>                               ^^^-func3
>                                        ^^^^^^^-opcode

  Hello Christoph,
  I see page table 26.1 of the last riscv-isa-manual.pdf what is called major
  opcodes in my understanding, and MISC-MEM is one of them with value 00_111_11.
  The value for func3 that I chose comes from
  https://github.com/michaeljclark/riscv-meta/blob/master/opcodes
  which admittedly is out-dated, but I don't see any particular value for
  LQ/SQ in the new spec either (I mean, riscv-isa-manual.pdf, any pointer we
  could refer to ?).
  I have nothing against changing the opcode, but then we need to change
  disas/riscv.c which also uses the previous opcode to dump instructions when
  running with -d in_asm.

  Frédéric
> Signed-off-by: Christoph Muellner <cmuellner@linux.com>
> ---
>   target/riscv/insn32.decode | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/target/riscv/insn32.decode b/target/riscv/insn32.decode
> index 5bbedc254c..d3f798ca10 100644
> --- a/target/riscv/insn32.decode
> +++ b/target/riscv/insn32.decode
> @@ -168,7 +168,7 @@ sraw     0100000 .....  ..... 101 ..... 0111011 @r
>   
>   # *** RV128I Base Instruction Set (in addition to RV64I) ***
>   ldu      ............   ..... 111 ..... 0000011 @i
> -lq       ............   ..... 010 ..... 0001111 @i
> +lq       ............   ..... 111 ..... 0001111 @i
>   sq       ............   ..... 100 ..... 0100011 @s
>   addid    ............  .....  000 ..... 1011011 @i
>   sllid    000000 ......  ..... 001 ..... 1011011 @sh6

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| Frédéric Pétrot, Pr. Grenoble INP-Ensimag/TIMA,   Ensimag deputy director |
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