Re: [PATCH] RISC-V: support for vector register accesses via ptrace() in RISC-V Linux native

["Maciej W. Rozycki" <macro@orcam.me.uk>]

Hi Andy,

> > > Does it make sense to you if we encapsulate this with a hwprobe syscall?
> > > e.g provide a hwprobe entry to get system's VLENB. We will have to
> > > increase and rearrange the buffer for NT_RISCV_VECTOR if we want to use
> > > ptrace as the entry point for this purpose. I am not very sure if it'd be
> > > too late to do though.
> > 
> >  No, how do you expect it to work with a core dump (that can be examined 
> > on a different system, or with a cross-debugger)?  You need to change the 
> > API I'm afraid; it's unusable anyway.  It's a pity the toolchain community 
> > wasn't consulted if you weren't sure how to design the interface.  Better 
> > yet it would have been to implement the GDB side before the kernel part 
> > has been committed.
> 
> Conor just reminded me that we may still have a chance to get it right
> since 6.5 has not been released yet. I will send a fix patch to address
> this issue once the discussion settle down. After looking into some
> code, I think it is possbile to steal the unused space in datap and
> change the uapi with something like this:
> 
> diff --git a/arch/riscv/include/uapi/asm/ptrace.h b/arch/riscv/include/uapi/asm/ptrace.h
> index e17c550986a6..ba6ddf4f9dc9 100644
> --- a/arch/riscv/include/uapi/asm/ptrace.h
> +++ b/arch/riscv/include/uapi/asm/ptrace.h
> @@ -97,14 +97,17 @@ struct __riscv_v_ext_state {
>  	unsigned long vl;
>  	unsigned long vtype;
>  	unsigned long vcsr;
> -	void *datap;
> +	union {
> +		void *datap;
> +		unsigned long vlenb;
> +	};
>  	/*
>  	 * In signal handler, datap will be set a correct user stack offset
>  	 * and vector registers will be copied to the address of datap
>  	 * pointer.
>  	 *
> -	 * In ptrace syscall, datap will be set to zero and the vector
> -	 * registers will be copied to the address right after this
> +	 * In ptrace syscall, the space for datap will be set to vlenb and the
> +	 * vector registers will be copied to the address right after this
>  	 * structure.
>  	 */
>  };
> 
> Now ptrace will have the knowlege of vlen to parse V rsgisters. And this
> will not cause any size change to the original data structure that is
> shared by both signal and ptrace because vlenb is XLEN, which has the
> same size as a pointer in both ilp32/lp64.

 Barring details such as field naming (perhaps `vregp' rather than opaque 
`datap'?), or whether we want to have a union embedded such as above or 
distinct UAPI data types for the two use cases I think your proposal for 
the updated contents makes sense to me, thanks.

  Maciej