Re: [PATCH] RISC-V: Correct legacy misa register number.

[Palmer Dabbelt <palmer@sifive.com>]

On Tue, 03 Jul 2018 17:13:34 PDT (-0700), Jim Wilson wrote:
> The main purpose of this patch is to fix the legacy misa register number, which
> is missing the +65 added to all of the other CSRs.
>
> This also changes DECLARE_CSR to use RISCV_FIRST_CSR_REGNUM instead of
> RISCV_LAST_FP_REGNUM+1 to be consistent with riscv-tdep.c.
>
> I don't have access to legacy hardware that I can test the misa number change
> with, but it has been tested on a riscv64-linux system using patched gdb and
> patched kernel, since it isn't usable otherwise.

FWIW, there's no legacy RISC-V hardware that can run a native GDB.  Thus the 
only mechanism to access this would be via a JTAG debugger, and there's no ABI 
spec for those.

Is there even a reason to have a legacy MISA CSR exposed to GDB?  I feel like 
we can just handle this in the JTAG debugger and keep this oddity from slipping 
into an ABI.

I'm adding Tim as he might have more context.

> Jim
>
> 	gdb/
> 	* riscv-tdep.h (DECLARE_CSR): Use RISCV_FIRST_CSR_REGNUM instead of
> 	RISCV_LAST_FP_REGNUM + 1.
> 	(RSICV_CSR_LEGACY_MISA_REGNUM): Add RISCV_FIRST_CSR_REGNUM.
> ---
>  gdb/riscv-tdep.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/gdb/riscv-tdep.h b/gdb/riscv-tdep.h
> index ab5e278759..4fc05976ba 100644
> --- a/gdb/riscv-tdep.h
> +++ b/gdb/riscv-tdep.h
> @@ -39,11 +39,11 @@ enum
>    RISCV_LAST_FP_REGNUM = 64,	/* Last Floating Point Register */
>
>    RISCV_FIRST_CSR_REGNUM = 65,  /* First CSR */
> -#define DECLARE_CSR(name, num) RISCV_ ## num ## _REGNUM = RISCV_LAST_FP_REGNUM + 1 + num,
> +#define DECLARE_CSR(name, num) RISCV_ ## num ## _REGNUM = RISCV_FIRST_CSR_REGNUM + num,
>  #include "opcode/riscv-opc.h"
>  #undef DECLARE_CSR
>    RISCV_LAST_CSR_REGNUM = 4160,
> -  RISCV_CSR_LEGACY_MISA_REGNUM = 0xf10,
> +  RISCV_CSR_LEGACY_MISA_REGNUM = 0xf10 + RISCV_FIRST_CSR_REGNUM,
>
>    RISCV_PRIV_REGNUM = 4161,

If we can't nuke RISCV_CSR_LEGACY_MISA_REGNUM them I'm fine with either way.  
Sorry to keep throwing a wrench in the works :)