linux-kernel - Re: [PATCH 02/19] riscv: cpufeature: Fix thead vector hwcap removal

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Message-ID: <20240412-employer-crier-c201704d22e3@spud>
Date: Fri, 12 Apr 2024 19:38:04 +0100
From: Conor Dooley <conor@...nel.org>
To: Evan Green <evan@...osinc.com>
Cc: Conor Dooley <conor.dooley@...rochip.com>,
	Charlie Jenkins <charlie@...osinc.com>,
	Rob Herring <robh@...nel.org>,
	Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Guo Ren <guoren@...nel.org>,
	Conor Dooley <conor+dt@...nel.org>, Chen-Yu Tsai <wens@...e.org>,
	Jernej Skrabec <jernej.skrabec@...il.com>,
	Samuel Holland <samuel@...lland.org>,
	Clément Léger <cleger@...osinc.com>,
	Jonathan Corbet <corbet@....net>, Shuah Khan <shuah@...nel.org>,
	linux-riscv@...ts.infradead.org, devicetree@...r.kernel.org,
	linux-kernel@...r.kernel.org, Palmer Dabbelt <palmer@...osinc.com>,
	linux-arm-kernel@...ts.infradead.org, linux-sunxi@...ts.linux.dev,
	linux-doc@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 02/19] riscv: cpufeature: Fix thead vector hwcap removal

On Fri, Apr 12, 2024 at 10:04:17AM -0700, Evan Green wrote:
> On Fri, Apr 12, 2024 at 3:26 AM Conor Dooley <conor.dooley@...rochip.com> wrote:
> >
> > On Thu, Apr 11, 2024 at 09:11:08PM -0700, Charlie Jenkins wrote:
> > > The riscv_cpuinfo struct that contains mvendorid and marchid is not
> > > populated until all harts are booted which happens after the DT parsing.
> > > Use the vendorid/archid values from the DT if available or assume all
> > > harts have the same values as the boot hart as a fallback.
> > >
> > > Fixes: d82f32202e0d ("RISC-V: Ignore V from the riscv,isa DT property on older T-Head CPUs")
> >
> > If this is our only use case for getting the mvendorid/marchid stuff
> > from dt, then I don't think we should add it. None of the devicetrees
> > that the commit you're fixing here addresses will have these properties
> > and if they did have them, they'd then also be new enough to hopefully
> > not have "v" either - the issue is they're using whatever crap the
> > vendor shipped.
> > If we're gonna get the information from DT, we already have something
> > that we can look at to perform the disable as the cpu compatibles give
> > us enough information to make the decision.
> >
> > I also think that we could just cache the boot CPU's marchid/mvendorid,
> > since we already have to look at it in riscv_fill_cpu_mfr_info(), avoid
> > repeating these ecalls on all systems.
> >
> > Perhaps for now we could just look at the boot CPU alone? To my
> > knowledge the systems that this targets all have homogeneous
> > marchid/mvendorid values of 0x0.
> 
> It's possible I'm misinterpreting, but is the suggestion to apply the
> marchid/mvendorid we find on the boot CPU and assume it's the same on
> all other CPUs? Since we're reporting the marchid/mvendorid/mimpid to
> usermode in a per-hart way, it would be better IMO if we really do
> query marchid/mvendorid/mimpid on each hart. The problem with applying
> the boot CPU's value everywhere is if we're ever wrong in the future
> (ie that assumption doesn't hold on some machine), we'll only find out
> about it after the fact. Since we reported the wrong information to
> usermode via hwprobe, it'll be an ugly userspace ABI issue to clean
> up.

You're misinterpreting, we do get the values on all individually as
they're brought online. This is only used by the code that throws a bone
to people with crappy vendor dtbs that put "v" in riscv,isa when they
support the unratified version.

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