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

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Date: Thu, 9 May 2024 09:25:25 +0100
From: Conor Dooley <conor.dooley@...rochip.com>
To: Andy Chiu <andy.chiu@...ive.com>
CC: Conor Dooley <conor@...nel.org>, 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>
Subject: Re: [PATCH v4 7/9] riscv: vector: adjust minimum Vector requirement
 to ZVE32X

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.

Thanks,
Conor.

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