Re: [PATCH v2] riscv: Add sysctl to control discard of vstate during syscall

From: Florian Weimer <fweimer@redhat.com>
To: Drew Fustini <fustini@kernel.org>
Cc: "Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Samuel Holland" <samuel.holland@sifive.com>,
	"Björn Töpel" <bjorn@rivosinc.com>,
	"Andy Chiu" <andybnac@gmail.com>,
	"Conor Dooley" <conor.dooley@microchip.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	"Drew Fustini" <dfustini@tenstorrent.com>
Subject: Re: [PATCH v2] riscv: Add sysctl to control discard of vstate during syscall
Date: Sat, 09 Aug 2025 10:40:46 +0200	[thread overview]
Message-ID: <lhuo6so7ur5.fsf@oldenburg.str.redhat.com> (raw)
In-Reply-To: <20250806-riscv_v_vstate_discard-v2-1-6bfd61b2c23b@kernel.org> (Drew Fustini's message of "Wed, 06 Aug 2025 07:03:28 -0700")

* Drew Fustini:

> 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
> sysctl knob is provided that controls whether the vector state is
> discarded in the syscall path:
>
> /proc/sys/abi/riscv_v_vstate_discard
>
> Valid values are:
>
> 0: Vector state is not always clobbered in all syscalls
> 1: Mandatory clobbering of vector state in all syscalls
>
> The initial state is controlled by CONFIG_RISCV_ISA_V_VSTATE_DISCARD.

Can this be put into the system call number instead, or make it specific
to some system calls in other ways?

I think C libraries can use this optimization for their system calls
(after adjusting the assembler clobbers) because the vector state is
caller-saved in the standard calling convention.  But there is backwards
compatibility impact for turning this on for the entire process.

Thanks,
Florian

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