linux-kernel - Re: [PATCH 1/1] RISC-V: Add parameter to unaligned access speed

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Message-ID: <CALs-HsshaOvxkJ1NhagL0TWqLJAjKAwyoHLEkdq-_n7rpXdF7Q@mail.gmail.com>
Date: Mon, 5 Aug 2024 11:56:04 -0700
From: Evan Green <evan@...osinc.com>
To: Charlie Jenkins <charlie@...osinc.com>
Cc: Jesse Taube <jesse@...osinc.com>, linux-riscv@...ts.infradead.org, 
	Jonathan Corbet <corbet@....net>, Paul Walmsley <paul.walmsley@...ive.com>, 
	Palmer Dabbelt <palmer@...belt.com>, Albert Ou <aou@...s.berkeley.edu>, 
	Conor Dooley <conor@...nel.org>, Rob Herring <robh@...nel.org>, 
	Krzysztof Kozlowski <krzk+dt@...nel.org>, Clément Léger <cleger@...osinc.com>, 
	Andrew Jones <ajones@...tanamicro.com>, Xiao Wang <xiao.w.wang@...el.com>, 
	Andy Chiu <andy.chiu@...ive.com>, Eric Biggers <ebiggers@...gle.com>, 
	Greentime Hu <greentime.hu@...ive.com>, Björn Töpel <bjorn@...osinc.com>, 
	Heiko Stuebner <heiko@...ech.de>, Costa Shulyupin <costa.shul@...hat.com>, 
	Andrew Morton <akpm@...ux-foundation.org>, Baoquan He <bhe@...hat.com>, 
	Anup Patel <apatel@...tanamicro.com>, Zong Li <zong.li@...ive.com>, 
	Sami Tolvanen <samitolvanen@...gle.com>, Ben Dooks <ben.dooks@...ethink.co.uk>, 
	Alexandre Ghiti <alexghiti@...osinc.com>, "Gustavo A. R. Silva" <gustavoars@...nel.org>, 
	Erick Archer <erick.archer@....com>, Joel Granados <j.granados@...sung.com>, linux-doc@...r.kernel.org, 
	linux-kernel@...r.kernel.org, devicetree@...r.kernel.org
Subject: Re: [PATCH 1/1] RISC-V: Add parameter to unaligned access speed

On Mon, Aug 5, 2024 at 11:48 AM Charlie Jenkins <charlie@...osinc.com> wrote:
>
> On Mon, Aug 05, 2024 at 11:38:23AM -0700, Evan Green wrote:
> > On Mon, Aug 5, 2024 at 10:38 AM Jesse Taube <jesse@...osinc.com> wrote:
> > >
> > > Add a kernel parameter to the unaligned access speed. This allows
> > > skiping of the speed tests for unaligned accesses, which often is very
> > > slow.
> > >
> > > Signed-off-by: Jesse Taube <jesse@...osinc.com>
> >
> > How come this is a command line parameter rather than a Kconfig
> > option? I could be wrong, so I'll lay out my rationale and people can
> > pick it apart if I've got a bad assumption.
> >
> > I think of commandline parameters as (mostly) something end users
> > twiddle with, versus kconfig options as something system builders set
> > up. I'd largely expect end users not to notice two ticks at boot time.
> > I'd expect its system builders and fleet managers, who know their
> > hardware and build their kernels optimized for it, are the ones who
> > would want to shave off this time and go straight to the known answer.
> > Anecdotally, at ChromeOS we had a strong preference for Kconfig
> > options, as they were easier to compose and maintain than a loose pile
> > of commandline arguments.
> >
> > The commit text doesn't go into the rationale, intended audience, or
> > expected usage, so maybe my guesses miss the mark on what you're
> > thinking.
> > -Evan
>
> There was a brief discussion about this on Jesse's series about vector
> unaligned support [1]. The original idea was to use Zicclsm to allow
> people to set the unaligned access speed on pre-compiled distro kernels.
> However Zicclsm isn't useful so the alternative route was to use a
> kernel arg. There is already support for a Kconfig, the kernel arg is
> just another option for users.
>
> Link:
> https://lore.kernel.org/lkml/af3152b6-adf7-40fa-b2a1-87e66eec45b0@rivosinc.com/
> [1]

Ah got it, thanks for the explanation Charlie! If there are consumers
for this then the concept seems fine with me.
-Evan