linux-kernel - Re: [PATCH v2 4/6] RISC-V: Detect unaligned vector accesses supported.

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Date: Fri, 21 Jun 2024 19:02:53 +0100
From: Conor Dooley <conor@...nel.org>
To: Eric Biggers <ebiggers@...nel.org>
Cc: Charlie Jenkins <charlie@...osinc.com>,
	Conor Dooley <conor.dooley@...rochip.com>,
	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>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzk+dt@...nel.org>,
	Clément Léger <cleger@...osinc.com>,
	Evan Green <evan@...osinc.com>,
	Andrew Jones <ajones@...tanamicro.com>,
	Xiao Wang <xiao.w.wang@...el.com>, Andy Chiu <andy.chiu@...ive.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 v2 4/6] RISC-V: Detect unaligned vector accesses
 supported.

On Fri, Jun 21, 2024 at 10:58:16AM -0700, Eric Biggers wrote:
> On Fri, Jun 21, 2024 at 10:18:23AM -0700, Charlie Jenkins wrote:
> > > Additionally, what are we doing in the kernel if we detect that
> > > misaligned stuff isn't supported? Are we going to mandate that kernel
> > > code is aligned only, disable in-kernel vector or some other mechanism
> > > to make sure that things like crypto code don't have/introduce code
> > > that'll not run on these systems?
> > 
> > UNSUPPORTED will still be set by the quick probe so it would be possible
> > for the kernel/userspace to avoid running misaligned vector when it's
> > unsupported. Any kernel methods would probably want to always run
> > aligned vector unless misaligned support was determined to be FAST
> > anyway, I am doubtful that code will have different optimizations for
> > FAST, SLOW, and UNSUPPORTED but it is possible. 
> > 
> > I would prefer consistency between scalar and vector misaligned support,
> > but this is not a deal breaker for this patch. I am not convinced it is
> > the best choice, but I am okay with leaving this option in the kernel.
> > 
> 
> Note that most of the vector crypto code (in arch/riscv/crypto/) assumes that
> vector misaligned accesses are supported.  Many of the RISC-V vector crypto
> instructions require using SEW=32 or SEW=64, and as a result, loads and stores
> of data can be misaligned unless the code changes the SEW to 8 and back again,
> which would be inefficient and add extra complexity.  I don't anticipate
> workarounds for CPUs that couldn't be bothered to support misaligned accesses
> being added.

> So what we'll probably have to do is just disable the vector
> crypto algorithms if the CPU doesn't support misaligned accesses...

Right. I was thinking similarly, and that we should just disable all
in-kernel vector code if the platform doesn't support misaligned vector.

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