Re: [PATCH 14/19] riscv: hwprobe: Disambiguate vector and xtheadvector in hwprobe

From: Evan Green <evan@rivosinc.com>
To: Charlie Jenkins <charlie@rivosinc.com>
Cc: "Conor Dooley" <conor.dooley@microchip.com>,
	"Conor Dooley" <conor@kernel.org>,
	"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>,
	"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 14/19] riscv: hwprobe: Disambiguate vector and xtheadvector in hwprobe
Date: Fri, 12 Apr 2024 15:08:31 -0700	[thread overview]
Message-ID: <CALs-HsuL8A87mCRU2+1F2sEPYmZWTz_MFwAys0h=1uKhb7EYqg@mail.gmail.com> (raw)
In-Reply-To: <Zhl717rsQ7XLBe2F@ghost>

On Fri, Apr 12, 2024 at 11:22 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
>
> On Fri, Apr 12, 2024 at 10:04:42AM -0700, Evan Green wrote:
> > On Fri, Apr 12, 2024 at 4:35 AM Conor Dooley <conor.dooley@microchip.com> wrote:
> > >
> > > On Thu, Apr 11, 2024 at 09:11:20PM -0700, Charlie Jenkins wrote:
> > > > Ensure that hwprobe does not flag "v" when xtheadvector is present.
> > > >
> > > > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > > > ---
> > > >  arch/riscv/kernel/sys_hwprobe.c | 4 ++--
> > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > >
> > > > diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> > > > index 8cae41a502dd..e0a42c851511 100644
> > > > --- a/arch/riscv/kernel/sys_hwprobe.c
> > > > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > > > @@ -69,7 +69,7 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
> > > >       if (riscv_isa_extension_available(NULL, c))
> > > >               pair->value |= RISCV_HWPROBE_IMA_C;
> > > >
> > > > -     if (has_vector())
> > > > +     if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR))
> > >
> > > Hmm, I think this is "dangerous". has_vector() is used across the kernel
> > > now in several places for the in-kernel vector. I don't think that
> > > has_vector() should return true for the T-Head stuff given that &
> > > has_vector() should represent the ratified spec. I'll have to think
> > > about this one and how nasty this makes any of the save/restore code
> > > etc.
> >
> > Yeah, my nose crinkled here as well. If you're having to do a
> > vendorish thing in this generic spot, then others may too, suggesting
> > perhaps this isn't the cleanest way to go about it. Ideally extensions
> > are all additive, rather than subtractive, I guess?
>
> This was the "easiest" way to support this but I agree this is not
> ideal. The vector code is naturally coupled with having support for
> "v" and I wanted to leverage that. The other concern is all of the
> ifdefs for having V enabled. I can make all of those V or XTHEADVECTOR;
> that will increase the surface area of xtheadvector but it is probably
> the right(?) way to go.

For the ifdefs, if you've got a Kconfig somewhere for THEAD_VECTOR,
can't that just depend on the V config? We'd end up with the
limitation that you can't add V 0.7 support without also dragging in
V1.0 support, but that's probably fine, right?

-Evan

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