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

From: Drew Fustini <fustini@kernel.org>
To: Darius Rad <darius@bluespec.com>
Cc: "Palmer Dabbelt" <palmer@dabbelt.com>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Drew Fustini" <dfustini@tenstorrent.com>,
	"Andy Chiu" <andybnac@gmail.com>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] riscv: Add sysctl to control discard of vstate during syscall
Date: Mon, 21 Jul 2025 14:28:26 -0700	[thread overview]
Message-ID: <aH6w+in5tCe42Kvc@x1> (raw)
In-Reply-To: <aH6qO3c39gmybwEc@x1>

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@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

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