[PATCH] riscv: Add sysctl to control discard of vstate during syscall

[Drew Fustini]

On Mon, Jul 21, 2025 at 01:59:39PM -0700, Drew Fustini wrote:
> On Mon, Jul 21, 2025 at 08:13:48AM -0400, Darius Rad wrote:
> > On Fri, Jul 18, 2025 at 08:39:13PM -0700, Drew Fustini wrote:
> > > From: Drew Fustini <dfustini at tenstorrent.com>
> > > 
> > > Clobbering the vector registers can significantly increase system call
> > > latency for some implementations. To mitigate this performance impact, a
> > > policy mechanism is provided to administrators, distro maintainers, and
> > > developers to control vector state discard in the form of a sysctl knob:
> > > 
> > > /proc/sys/abi/riscv_v_vstate_discard
> > > 
> > > Valid values are:
> > > 
> > > 0: Do not discard vector state during syscall
> > > 1: Discard vector state during syscall
> > > 
> > 
> > Is the intention for this this mean "don't guarantee vector state is
> > clobbered" or "preserve vector state"?  I suspect it is the former, but the
> > wording seems unclear.  Additionally, if that's indeed the case, maybe the
> > documentation should more clearly articulate the tradeoff (performance vs.
> > security/robustness).
> 
> Thanks for your comment. I agree it could be worded better and the trade
> off should be explained. I believe the motivation behind clobbering in
> all syscalls was to make sure that userspace does not rely on the vector
> being preserved through a syscall. The goal of this strictness was to
> catch any programs that were misbehaving.
> 
> Thus, it might be better if I described it as:
> 
> 0: Vector state is not strictly clobbered in all syscalls
> 1: Mandatory clobbering of vector state in all syscalls

Darius - sorry, I seem to have failed to put you in the To: field of my
reply.

Thanks,
Drew