linux-kernel - Re: [PATCH v4 7/9] riscv: vector: adjust minimum Vector requirement to ZVE32X

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Message-ID: <20240509-irritant-morphing-a25428a4f750@spud>
Date: Thu, 9 May 2024 23:22:42 +0100
From: Conor Dooley <conor@...nel.org>
To: Conor Dooley <conor.dooley@...rochip.com>
Cc: Andy Chiu <andy.chiu@...ive.com>, Eric Biggers <ebiggers@...nel.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Heiko Stuebner <heiko@...ech.de>,
	Guo Ren <guoren@...nel.org>, Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Jonathan Corbet <corbet@....net>, Evan Green <evan@...osinc.com>,
	Clément Léger <cleger@...osinc.com>,
	Shuah Khan <shuah@...nel.org>, linux-riscv@...ts.infradead.org,
	linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...osinc.com>,
	Vincent Chen <vincent.chen@...ive.com>,
	Greentime Hu <greentime.hu@...ive.com>, devicetree@...r.kernel.org,
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org,
	Joel Granados <j.granados@...sung.com>,
	Jerry Shih <jerry.shih@...ive.com>,
	Charlie Jenkins <charlie@...osinc.com>
Subject: Re: [PATCH v4 7/9] riscv: vector: adjust minimum Vector requirement
 to ZVE32X

On Thu, May 09, 2024 at 09:25:25AM +0100, Conor Dooley wrote:
> On Thu, May 09, 2024 at 08:48:09AM +0100, Conor Dooley wrote:
> > On Thu, May 09, 2024 at 02:56:30PM +0800, Andy Chiu wrote:
> > > Hi Conor,
> > > 
> > > Should we check if "v" presents for vector crypto extensions in
> > > riscv_isa_extension_check()? We are not checking this for now. So a
> > > kernel compiled with RISCV_ISA_V still has a problem if its isa-string
> > > includes any of vector crypto ("zvbb, zvkg, etc") but not "v".
> > 
> > 
> > Yeah, one of the things I took away from this discussion is that we need
> > to improve the implementation of both the methods we have at the moment
> > for drivers etc to check if extensions are present and usable.
> > In general, I don't think checks like that are "safe" to do in
> > riscv_isa_extension_check(), because the dependencies may not all have
> > been resolved when we probe an extension (Clement's current Zca etc
> > series improves the situation though by only calling the checks after
> > we probe all extensions).
> > 
> > The simple V cases are all fine though - the DT binding and ACPI rules
> > for compatible strings all mandate that single-letter extensions must
> > come before multi-letter ones. For riscv,isa-extensions we control the
> > probe ordering and probe V before any multi-letter stuff. Additionally,
> > we should make it a requirement for V to be present if things that
> > depend on it are.
> > 
> > That said, is it permitted by the specs to have any of the extensions
> > you mention without the full V extension, but with one of the cut-down
> > variants you mention here? If not, I'd be more interested in figuring
> > out the non-extension dependencies: whether or not the kernel itself
> > supports vector and if the kernel has opted to disable vector due to
> > detecting that harts have mismatching vector lengths.
> > 
> > TL;DR: I think we should add some checks in riscv_isa_extension_check().
> 
> Also, unless this only becomes a problem with this series that adds the
> cut-down forms of vector, I think this is a separate problem to solve
> and I can send some patches for it (along with some other cleanup I'd like
> to do as a result of Eric's comments) and you can just submit the v2 you
> were planning to without it. I can't, off the top of my head, think of
> why this particular series would break the vector crypto stuff though,
> the problems with enabling extensions seem underlying.

Here's something buggy that I chucked together as an idea of what I
meant:
https://git.kernel.org/pub/scm/linux/kernel/git/conor/linux.git/commit/?h=riscv-check_vector
Beware, it is entirely untested :)
It's based on both this series and patches 2 & 3 of Charlie's series doing
the T-Head vector stuff. It really needs Clement's extension_check()
rework that I mentioned 2 mails ago to function correctly for any of these
vector subsets. Without Clement's stuff, it'll have "random" behaviour
depending on probe order for riscv,isa and a determinate, but incorrect,
behaviour otherwise.

Cheers,
Conor.

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