Re: [PATCH v2] target/riscv: Fix LMUL check to use VLEN

[Alistair Francis <alistair23@gmail.com>]

On Tue, Jul 18, 2023 at 11:14 PM Rob Bradford <rbradford@rivosinc.com> wrote:
>
> The previous check was failing with:
>
> VLEN=128 ELEN = 64 SEW = 16 and LMUL = 1/8 which is a
> valid combination.
>
> Fix the check to allow valid combinations when VLEN is a multiple of
> ELEN.
>
> From the specification:
>
> "In general, the requirement is to support LMUL ≥ SEWMIN/ELEN, where
> SEWMIN is the narrowest supported SEW value and ELEN is the widest
> supported SEW value. In the standard extensions, SEWMIN=8. For standard
> vector extensions with ELEN=32, fractional LMULs of 1/2 and 1/4 must be
> supported. For standard vector extensions with ELEN=64, fractional LMULs
> of 1/2, 1/4, and 1/8 must be supported." Elsewhere in the specification
> it makes clear that VLEN>=ELEN.
>
> From inspection this new check allows:
>
> VLEN=ELEN=64 1/2, 1/4, 1/8 for SEW >=8
> VLEN=ELEN=32 1/2, 1/4 for SEW >=8
>
> Fixes: d9b7609a1fb2 ("target/riscv: rvv-1.0: configure instructions")
>
> Signed-off-by: Rob Bradford <rbradford@rivosinc.com>

Thanks!

Applied to riscv-to-apply.next

Alistair

> ---
> V2: Switch check to use VLEN and active SEW vs ELEN and minimum SEW
> ---
>  target/riscv/vector_helper.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/target/riscv/vector_helper.c b/target/riscv/vector_helper.c
> index cfacf2ebba..4d06754826 100644
> --- a/target/riscv/vector_helper.c
> +++ b/target/riscv/vector_helper.c
> @@ -43,9 +43,9 @@ target_ulong HELPER(vsetvl)(CPURISCVState *env, target_ulong s1,
>                                              xlen - 1 - R_VTYPE_RESERVED_SHIFT);
>
>      if (lmul & 4) {
> -        /* Fractional LMUL. */
> +        /* Fractional LMUL - check LMUL * VLEN >= SEW */
>          if (lmul == 4 ||
> -            cpu->cfg.elen >> (8 - lmul) < sew) {
> +            cpu->cfg.vlen >> (8 - lmul) < sew) {
>              vill = true;
>          }
>      }
> --
> 2.41.0
>
>