[PATCH] riscv: add NT_RISCV_CSR constant

[PATCH] riscv: add NT_RISCV_CSR constant

[Andrew Burgess]

I am currently working on adding support to GDB for dumping bare metal
core files for RISC-V.  As part of this work I would like to include
support for dumping RISC-V CSRs.

The format of these bare metal core files is planned to be ELF + NOTES
very similar to the core file format use by Linux.

As such I need to pick a number to use for the CSR note.  Currently
Linux for RISC-V doesn't support writing out CSRs, and so no number
has been assigned for a CSR note.

In this patch I propose reserving a note number for RISC-V CSRs.  This
note isn't currently used within Linux, but might be one day.  Given
how the per-architecture notes are allocated this patch not only
reserves a note number for the CSRs, but as the first RISC-V specific
note number, it reserves the whole 0x9?? number range for RISC-V
specific notes.

Reserving this constant now will allow for coordination between Linux
and user-space tools that work with core files (like GDB) without
risking possible number conflicts in the future.

I don't know if it's acceptable to have reserved, but otherwise
unused, constants like this in the kernel or not.
---
 include/uapi/linux/elf.h | 1 +
 1 file changed, 1 insertion(+)

diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
index 34c02e4290fe..7db3752108c8 100644
--- a/include/uapi/linux/elf.h
+++ b/include/uapi/linux/elf.h
@@ -428,6 +428,7 @@ typedef struct elf64_shdr {
 #define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers */
 #define NT_MIPS_FP_MODE	0x801		/* MIPS floating-point mode */
 #define NT_MIPS_MSA	0x802		/* MIPS SIMD registers */
+#define NT_RISCV_CSR	0x900		/* RISC-V Control Status Registers */
 
 /* Note header in a PT_NOTE section */
 typedef struct elf32_note {
-- 
2.25.4


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[PING] Re: [PATCH] riscv: add NT_RISCV_CSR constant

[Andrew Burgess]

Ping!

I'm not sure if this is the correct mailing list for this patch, or if
this should be posted to a more common list for consideration?

Does anyone have any thoughts on this?  As RISC-V gains more
extensions (like vectors) it seems inevitable that at some point we
would need to claim a RISC-V specific number range for notes.  It
would be super useful if we could coordinate the note numbers used by
Linux with those used in other tools to avoid conflicts.

Thanks,
Andrew


> I am currently working on adding support to GDB for dumping bare metal
> core files for RISC-V.  As part of this work I would like to include
> support for dumping RISC-V CSRs.
>
> The format of these bare metal core files is planned to be ELF + NOTES
> very similar to the core file format use by Linux.
>
> As such I need to pick a number to use for the CSR note.  Currently
> Linux for RISC-V doesn't support writing out CSRs, and so no number
> has been assigned for a CSR note.
>
> In this patch I propose reserving a note number for RISC-V CSRs.  This
> note isn't currently used within Linux, but might be one day.  Given
> how the per-architecture notes are allocated this patch not only
> reserves a note number for the CSRs, but as the first RISC-V specific
> note number, it reserves the whole 0x9?? number range for RISC-V
> specific notes.
>
> Reserving this constant now will allow for coordination between Linux
> and user-space tools that work with core files (like GDB) without
> risking possible number conflicts in the future.
>
> I don't know if it's acceptable to have reserved, but otherwise
> unused, constants like this in the kernel or not.
> ---
>  include/uapi/linux/elf.h | 1 +
>  1 file changed, 1 insertion(+)
>
> diff --git a/include/uapi/linux/elf.h b/include/uapi/linux/elf.h
> index 34c02e4290fe..7db3752108c8 100644
> --- a/include/uapi/linux/elf.h
> +++ b/include/uapi/linux/elf.h
> @@ -428,6 +428,7 @@ typedef struct elf64_shdr {
>  #define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers */
>  #define NT_MIPS_FP_MODE	0x801		/* MIPS floating-point mode */
>  #define NT_MIPS_MSA	0x802		/* MIPS SIMD registers */
> +#define NT_RISCV_CSR	0x900		/* RISC-V Control Status Registers */
>
>  /* Note header in a PT_NOTE section */
>  typedef struct elf32_note {
> --
> 2.25.4

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Re: [PING] Re: [PATCH] riscv: add NT_RISCV_CSR constant

[Maciej W. Rozycki]

On Wed, 20 Jan 2021, Andrew Burgess wrote:

> I'm not sure if this is the correct mailing list for this patch, or if
> this should be posted to a more common list for consideration?

 You might consider cc-ing <linux-arch@vger.kernel.org> and obviously LKML 
for a wider audience.  There might be too little expertise present here.

> Does anyone have any thoughts on this?  As RISC-V gains more
> extensions (like vectors) it seems inevitable that at some point we
> would need to claim a RISC-V specific number range for notes.  It
> would be super useful if we could coordinate the note numbers used by
> Linux with those used in other tools to avoid conflicts.

 FWIW, these numbers are solely for Linux use (note name "LINUX"), so I'm 
not sure how including privileged registers would be relevant here.  These 
notes reflect OS registers, which may not necessarily bijectively map to 
hardware registers.  See the NT_MIPS_FP_MODE note for a good example.

 As I say I suggest reposting your change with the lists named above cc-ed 
and making the inclusion of LKML with all submissions routine.  You might 
be able to gather more feedback there.

  Maciej

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