Re: [Qemu-devel] [PATCH for 4.1] RISC-V: Ignore the S and U extensions when formatting ISA strings

[Alistair Francis <alistair23@gmail.com>]

On Wed, Aug 7, 2019 at 8:00 AM Palmer Dabbelt <palmer@sifive.com> wrote:
>
> The ISA strings we're providing from QEMU aren't actually legal RISC-V
> ISA strings, as both the S and U extensions cannot exist as
> single-letter extensions and must instead be multi-letter strings.
> We're still using the ISA strings inside QEMU to track the availiable

s/availiable/available/g

> extensions, so this patch just strips out the S and U extensions when
> formatting ISA strings.

Atish and I were talking about this and we concluded that S and U
aren't extensions, but should be reported in the misa CSR.

>
> This boots Linux on top of 4.1-rc3, which no longer has the U extension
> in /proc/cpuinfo.
>
> Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
> ---
> This is another late one, but I'd like to target it for 4.1 as we're
> providing illegal ISA strings and I don't want to bake that into a bunch
> of other code.
> ---
>  target/riscv/cpu.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
>
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index f8d07bd20ad7..4df14433d789 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -501,7 +501,22 @@ char *riscv_isa_string(RISCVCPU *cpu)
>      char *p = isa_str + snprintf(isa_str, maxlen, "rv%d", TARGET_LONG_BITS);
>      for (i = 0; i < sizeof(riscv_exts); i++) {
>          if (cpu->env.misa & RV(riscv_exts[i])) {
> -            *p++ = qemu_tolower(riscv_exts[i]);
> +            char lower = qemu_tolower(riscv_exts[i]);
> +            switch (lower) {
> +            case 's':
> +            case 'u':
> +                /*
> +                 * The 's' and 'u' extensions shouldn't be passed in the device
> +                 * tree, but we still use them internally to track extension
> +                 * sets.  Here we just explicitly remove them when formatting
> +                 * an ISA string.

This should be updated to note mention 's' and 'u' as extensions, but
clarify that they are correctly include in the misa CSR.

Alistair

> +                 */
> +                break;
> +
> +            default:
> +                *p++ = qemu_tolower(riscv_exts[i]);
> +                break;
> +            }
>          }
>      }
>      *p = '\0';
> --
> 2.21.0
>
>