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

From: Charlie Jenkins <charlie@rivosinc.com>
To: Conor Dooley <conor@kernel.org>
Cc: "Conor Dooley" <conor.dooley@microchip.com>,
	"Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzysztof.kozlowski+dt@linaro.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Guo Ren" <guoren@kernel.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Chen-Yu Tsai" <wens@csie.org>,
	"Jernej Skrabec" <jernej.skrabec@gmail.com>,
	"Samuel Holland" <samuel@sholland.org>,
	"Evan Green" <evan@rivosinc.com>,
	"Clément Léger" <cleger@rivosinc.com>,
	"Jonathan Corbet" <corbet@lwn.net>,
	"Shuah Khan" <shuah@kernel.org>,
	linux-riscv@lists.infradead.org, devicetree@vger.kernel.org,
	linux-kernel@vger.kernel.org,
	"Palmer Dabbelt" <palmer@rivosinc.com>,
	linux-arm-kernel@lists.infradead.org,
	linux-sunxi@lists.linux.dev, linux-doc@vger.kernel.org,
	linux-kselftest@vger.kernel.org
Subject: Re: [PATCH 02/19] riscv: cpufeature: Fix thead vector hwcap removal
Date: Mon, 15 Apr 2024 20:34:05 -0700	[thread overview]
Message-ID: <Zh3xrTfjjk3b4GHb@ghost> (raw)
In-Reply-To: <20240413-sharper-unlivable-5a65660b19e2@spud>

On Sat, Apr 13, 2024 at 12:40:26AM +0100, Conor Dooley wrote:
> On Fri, Apr 12, 2024 at 02:31:42PM -0700, Charlie Jenkins wrote:
> > On Fri, Apr 12, 2024 at 10:27:47PM +0100, Conor Dooley wrote:
> > > On Fri, Apr 12, 2024 at 01:48:46PM -0700, Charlie Jenkins wrote:
> > > > On Fri, Apr 12, 2024 at 07:47:48PM +0100, Conor Dooley wrote:
> > > > > On Fri, Apr 12, 2024 at 10:12:20AM -0700, Charlie Jenkins wrote:
> 
> > > > > > This is already falling back on the boot CPU, but that is not a solution
> > > > > > that scales. Even though all systems currently have homogenous
> > > > > > marchid/mvendorid I am hesitant to assert that all systems are
> > > > > > homogenous without providing an option to override this.
> > > > > 
> > > > > There are already is an option. Use the non-deprecated property in your
> > > > > new system for describing what extesions you support. We don't need to
> > > > > add any more properties (for now at least).
> > > > 
> > > > The issue is that it is not possible to know which vendor extensions are
> > > > associated with a vendor. That requires a global namespace where each
> > > > extension can be looked up in a table. I have opted to have a
> > > > vendor-specific namespace so that vendors don't have to worry about
> > > > stepping on other vendor's toes (or the other way around). In order to
> > > > support that, the vendorid of the hart needs to be known prior.
> > > 
> > > Nah, I think you're mixing up something like hwprobe and having
> > > namespaces there with needing namespacing on the devicetree probing side
> > > too. You don't need any vendor namespacing, it's perfectly fine (IMO)
> > > for a vendor to implement someone else's extension and I think we should
> > > allow probing any vendors extension on any CPU.
> > 
> > I am not mixing it up. Sure a vendor can implement somebody else's
> > extension, they just need to add it to their namespace too.
> 
> I didn't mean that you were mixing up how your implementation worked, my
> point was that you're mixing up the hwprobe stuff which may need
> namespacing for $a{b,p}i_reason and probing from DT which does not.
> I don't think that the kernel should need to be changed at all if
> someone shows up and implements another vendor's extension - we already
> have far too many kernel changes required to display support for
> extensions and I don't welcome potential for more.

Yes I understand where you are coming from. We do not want it to require
very many changes to add an extension. With this framework, there are
the same number of changes to add a vendor extension as there is to add
a standard extension. There is the upfront cost of creating the struct
for the first vendor extension from a vendor, but after that the
extension only needs to be added to the associated vendor's file (I am
extracting this out to a vendor file in the next version). This is also
a very easy task since the fields from a different vendor can be copied
and adapted.

> 
> Another thing I just thought of was systems where the SoC vendor
> implements some extension that gets communicated in the ISA string but
> is not the vendor in mvendorid in their various CPUs. I wouldn't want to
> see several different entries in structs (or several different hwprobe
> keys, but that's another story) for this situation because you're only
> allowing probing what's in the struct matching the vendorid.

Since the isa string is a per-hart field, the vendor associated with the
hart will be used.

- Charlie

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