Re: [PATCH 1/5] target/riscv: Ignore the S and U letters when formatting ISA strings

[Tsukasa OI]

On 2022/08/06 0:54, Conor Dooley wrote:
> From: Palmer Dabbelt <palmer@sifive.com>
> 
> The ISA strings we're providing from QEMU aren't actually legal RISC-V
> ISA strings, as both S and U 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 extensions, so just strip
> out the S and U extensions when formatting ISA strings.
> 
> Signed-off-by: Palmer Dabbelt <palmer@sifive.com>
> [Conor: rebased on 7.1.0-rc1 & slightly tweaked the commit message]
> Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
> ---
>  target/riscv/cpu.c | 18 +++++++++++++++++-
>  1 file changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/target/riscv/cpu.c b/target/riscv/cpu.c
> index ac6f82ebd0..95fdc03b3d 100644
> --- a/target/riscv/cpu.c
> +++ b/target/riscv/cpu.c
> @@ -1122,7 +1122,23 @@ 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_single_letter_exts) - 1; i++) {
>          if (cpu->env.misa_ext & RV(riscv_single_letter_exts[i])) {
> -            *p++ = qemu_tolower(riscv_single_letter_exts[i]);
> +            char lower = qemu_tolower(riscv_single_letter_exts[i]);
> +            switch (lower) {
> +                case 's':
> +                case 'u':
> +                    /*
> +                    * The 's' and 'u' letters shouldn't show up in ISA 
> strings as
> +                    * they're not extensions, but they should show up in 
> MISA.
> +                    * Since we use these letters interally as a pseudo ISA 
> string
> +                    * to set MISA it's easier to just strip them out when
> +                    * formatting the ISA string.
> +                    */
> +                    break;
> +
> +                default:
> +                    *p++ = lower;
> +                    break;
> +            }
>          }
>      }
>      *p = '\0';

I agree with Alistair.  **I** removed 'S' and 'U' from the ISA string
and it should be working in the latest development branch (I tested).

I tested it on master and QEMU 7.1-rc1 (tag: v7.1-rc1).

Example:
/opt/riscv/bin/qemu-system-riscv64
         -machine virt
         -nographic
         -cpu rv64
         -smp 1
         -kernel images/linux.bin
         -initrd images/busybox.cpio.gz
         -append 'console=hvc0 earlycon=sbi'
         -bios images/opensbi-fw_jump.elf
         -gdb tcp::9000

Replacing -machine virt with -machine virt,dumpdtb=sample.dtb dumps the
binary DeviceTree as sample.dtb and generated CPU-related parts like...

                cpu@0 {
                        phandle = <0x01>;
                        device_type = "cpu";
                        reg = <0x00>;
                        status = "okay";
                        compatible = "riscv";
                        riscv,isa =
"rv64imafdch_zicsr_zifencei_zba_zbb_zbc_zbs";
                        mmu-type = "riscv,sv48";

                        interrupt-controller {
                                #interrupt-cells = <0x01>;
                                interrupt-controller;
                                compatible = "riscv,cpu-intc";
                                phandle = <0x02>;
                        };
                };

Besides, this function alone generates the ISA string for DTB and
there's no way such ISA strings with invalid 'S' and 'U' can be
generated.  It's definitely a behavior of QEMU 7.0 or before.

Please. Please make sure that you are testing the right version of QEMU.

Thanks,
Tsukasa