Adding V-ext regs to signal context w/o expanding kernel struct sigcontext to avoid glibc ABI break

[Andy Chiu]

On Fri, Dec 23, 2022 at 4:28 AM Vineet Gupta <vineetg at rivosinc.com> wrote:
> This approach looks perfect. Lets productize it to fold this patch into
> the respective patch(es).
> We would then need fixups to not unconditionally enable V on fork/execve
> and hook that up to a prctl.
> Let me work on that and provide something on top of your series.

Hi Vineet, I have included the approach into the Vector series
according to suggestions, which makes it formaler than the PoC one.
Additionally, I picked up your prctl patch and added a kconfig to
compile a kernel that won't unconditionally enable V. Please tell me
if this does not seem right to you. I will submit the series if this
seems well to you and let's discuss some more details further in that
thread. Here is the tree, thanks:

https://github.com/sifive/riscv-linux/tree/dev/andyc/for-next-v13.1-newapi-prctl

-Andy