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

From: Drew Fustini <pdp7pdp7@gmail.com>
To: "Radim Krčmář" <rkrcmar@ventanamicro.com>
Cc: "Drew Fustini" <fustini@kernel.org>,
	"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,
	linux-riscv <linux-riscv-bounces@lists.infradead.org>
Subject: Re: [PATCH] riscv: Add sysctl to control discard of vstate during syscall
Date: Mon, 21 Jul 2025 14:20:03 -0700	[thread overview]
Message-ID: <aH6vA+e5v7NMMGnc@x1> (raw)
In-Reply-To: <DBHTIDY0HRM0.2B8L1WG7IBCXM@ventanamicro.com>

On Mon, Jul 21, 2025 at 04:54:25PM +0200, Radim Krčmář wrote:
> 2025-07-21T14:35:38+02:00, Radim Krčmář <rkrcmar@ventanamicro.com>:
> > Shouldn't the RISC-V Linux syscall ABI be defined somewhere?
> 
> To clarify this point.  My issue is with the following part in
> Documentation/arch/riscv/vector.rst:
> 
> >>  As indicated by version 1.0 of the V extension [1], vector registers are
> >>  clobbered by system calls.
> >>  [...]
> >>  1: https://github.com/riscv/riscv-v-spec/blob/master/calling-convention.adoc
> 
> The ISA does not say that vector registers are clobbered by system
> calls.  All the ISA says is:
> 
>   "This Appendix is only a placeholder to help explain the conventions
>    used in the code examples, and is not considered frozen or
>    part of the ratification process.  The official RISC-V psABI document
>    is being expanded to specify the vector calling conventions."
> 
> while the RISC-V psABI says:
> 
>   "The calling convention for system calls does not fall within the
>    scope of this document. Please refer to the documentation of the
>    RISC-V execution environment interface (e.g OS kernel ABI, SBI)."
> 
> We made a circular dependency, misinterpreted the ISA, and probably
> implemented a suboptimal syscall ABI -- preserving vector registers
> seems strictly better.

Thanks for providing these references. It does seem like this is
something that an OS can decide and is not mandated by the ISA or psABI.

> > How come we could have broken it with 9657e9b7d253?
> 
> We changed the ABI once, so maybe we can change it back?

Reverting 9657e9b7d253 would solve the performance issue for some
implementations that I've highlighted in this patch. However, I am
interested to hear from others that feel the current mandatory
clobbering behavior is ideal for testing (and maybe security?).

Thanks,
Drew

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