linux-kernel - Re: [PATCH 2/3] RISC-V: Detect unaligned vector accesses supported.

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Date: Fri, 7 Jun 2024 14:32:48 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Conor Dooley <conor@...nel.org>
Cc: Jesse Taube <jesse@...osinc.com>, linux-riscv@...ts.infradead.org,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Conor Dooley <conor.dooley@...rochip.com>,
	Evan Green <evan@...osinc.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	Xiao Wang <xiao.w.wang@...el.com>,
	Clément Léger <cleger@...osinc.com>,
	Andy Chiu <andy.chiu@...ive.com>,
	Greentime Hu <greentime.hu@...ive.com>,
	Heiko Stuebner <heiko@...ech.de>, Guo Ren <guoren@...nel.org>,
	Björn Töpel <bjorn@...osinc.com>,
	Costa Shulyupin <costa.shul@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Baoquan He <bhe@...hat.com>,
	Sami Tolvanen <samitolvanen@...gle.com>,
	Zong Li <zong.li@...ive.com>, Ben Dooks <ben.dooks@...ethink.co.uk>,
	Erick Archer <erick.archer@....com>,
	Vincent Chen <vincent.chen@...ive.com>,
	Joel Granados <j.granados@...sung.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] RISC-V: Detect unaligned vector accesses supported.

On Fri, Jun 07, 2024 at 10:21:19PM +0100, Conor Dooley wrote:
> On Fri, Jun 07, 2024 at 02:06:27PM -0700, Charlie Jenkins wrote:
> > On Fri, Jun 07, 2024 at 03:53:23PM -0400, Jesse Taube wrote:
> > > On 6/6/24 19:13, Charlie Jenkins wrote:
> > > > On Thu, Jun 06, 2024 at 02:29:23PM -0700, Charlie Jenkins wrote:
> > > > > On Thu, Jun 06, 2024 at 02:32:14PM -0400, Jesse Taube wrote:
> 
> > > > > Please use the exising UNKNOWN terminology instead of renaming to
> > > > > SUPPORTED. Any option that is not UNSUPPORTED implies that unaligned
> > > > > accesses are supported.
> > > 
> > > Conor didnt like using UNKNOWN a proxy for "SUPPORTED"
> 
> I did say this, but in the context of wanting you to actually add the
> performance probing (and potentially the other infrastructure that
> Charlie added for scalar).
> 
> > > Having SUPPORTED is better then assuing the speed to be slow.
> > 
> > The HWPROBE key is about misaligned access performance. UNKNOWN means
> > that the performance is unknown.
> 
> Right. I also don't think that assuming "slow" is even problematic -
> seemingly all extant hardware doesn't even support misaligned access.
> But really, just whack in the probing, it shouldn't be too bad, right?
>

Yeah that's a good point, slow is a reasonable default.

> > The scalar and vector names need to
> > match up.
> 
> That's definitely not the case. A different hwprobe key is allowed to
> behave differently, but...

It of course can behave differently in purely technical sense, I said
"need" because it would not be a very intuitive interface to have a
different name for vector and scalar versions of the same thing.

> 
> > UNKNOWN was already merged and is supported by linux so if you
> > want to use SUPPORTED here then you need to add a scalar SUPPORTED key
> > that is an alias of the UNKNOWN key.
> 
> ...this suggestion of a scalar change I disagree with anyway, so it's
> moot. Unknown should be a state that we only have internally when we
> actually do not know, and not something that userspace should ever see,
> unless there's a bug in the probing code IMO. Unknown gives userspace no
> actionable information anyways.
> 

I agree, returning slow is probably always be more useful than unknown.

- Charlie

> > I would rather keep UNKNOWN as it
> > is, but that's up to you.