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

[Andy Chiu]

Hi Vineet,

On Wed, Jan 4, 2023 at 3:17 AM Vineet Gupta <vineetg at rivosinc.com> wrote:
> The prctl support in there is really rudimentary and incomplete. There's
> more work needed to use the dynamic state of enablement - for say signal
> frame etc.
Yes, I agree that signal and ptrace need special handling if we'd turn
off Vector with prctl. For example, we may not need to save/restore
vector context on context switches and signal handlings. And we may
have to prevent ptrace from setting/getting vector context in such
case. I can implement this into the series if this is what you're
looking for. Or you could share the code somewhere so that I could
merge it.

> The new Kconfig CONFIG_RISCV_VSTATE_INIT_ALL seems like a
> hack bolted on top.
IIUC, most opinions suggested that we should keep the default Vector
state to ON in thread:
https://lore.kernel.org/all/20220921214439.1491510-17-stillson@rivosinc.com/T/#u
So IMHO adding a build option to those who prefer not to
unconditionally enable V should be sufficient.

> I would also suggesting dropping the 2 patches for in-kernel enablement
> for your submission as it might require some more thinking/design and
> builds naturally on top of the baseline patches.
Yes, I agree. Those patches were heavily copied from arm neon, which
will not benefit from hardware feature on riscv-V. I will refine those
patches and submit independently, on top of the baseline patch.

Thanks,
Andy