linux-kernel - Re: [PATCH v2 2/8] riscv: Add PLATFORM_MAY_SUPPORT_RISCV_ISA

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Date: Fri, 10 May 2024 14:43:10 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Conor Dooley <conor@...nel.org>
Cc: Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Conor Dooley <conor.dooley@...rochip.com>,
	Song Liu <song@...nel.org>, Xi Wang <xi.wang@...il.com>,
	Björn Töpel <bjorn@...osinc.com>,
	Clément Léger <cleger@...osinc.com>,
	Jessica Clarke <jrtc27@...c27.com>,
	Andy Chiu <andy.chiu@...ive.com>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 2/8] riscv: Add PLATFORM_MAY_SUPPORT_RISCV_ISA_V
 Kconfig option

On Fri, May 10, 2024 at 09:43:33PM +0100, Conor Dooley wrote:
> Hey Charlie,
> 
> On Tue, May 07, 2024 at 06:36:28PM -0700, Charlie Jenkins wrote:
> > Current versions of the kernel add "v" to the march and then immeidately
> > filter it out such that "v" is not passed to CFLAGS.  Instead of doing
> > this filtering, code blocks in the kernel that want to use vector
> > assembly have been changed to locally enable vector (using ".option
> > arch, +v").
> 
> Other content in the series aside, since this is a change that could be
> made independently of the main series objectives, I figured it was worth
> pointing out that this is not a change without downsides: I think that
> it would drop support for vector with most versions of LLVM as
> .option arch support there is much more recent thing than it is for gcc.
> Off the top of my head I don't know exactly the versions involved, but
> it is something like LLVM-14 supports vector but only LLVM-17 and later
> supports .option arch.

Toolchain incompatibilities are always fun. It does look like .option
arch was introduced in LLVM-17. That would be a regression. We do use
option arch for every other extension, but vector was treated special
when it was introduced unfortunately so maybe we have to live with the
weird march parsing hack.

- Charlie

> 
> Thanks,
> Conor.