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

[Maciej W. Rozycki]

On Thu, 10 Aug 2023, Maciej W. Rozycki wrote:

> > 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.

 NB since this stuff went in with v6.5-rc1 and v6.5 hasn't been released 
you can still back out the problematic change as no one is expected to use 
RC stuff in production.  Alternatively you can redefine NT_RISCV_VECTOR 
for a corrected ABI, but I think it shouldn't be necessary.  You just need 
to act quickly as I guess there may be 1-2 further v6.5 RCs only and you 
have to get with that to Linus right away.  We can have a release or two 
without NT_RISCV_VECTOR support for the otherwise included vector stuff, 
it shouldn't be a big deal.  There just won't be support for the debug 
API.

 CC-ing Linux ptrace/RISC-V maintainers now to bring their attention.

  Maciej