Re: [PATCH] RISC-V: Show accurate per-hart isa in /proc/cpuinfo

From: Conor Dooley <conor@kernel.org>
To: Evan Green <evan@rivosinc.com>
Cc: Anup Patel <apatel@ventanamicro.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	linux-kernel@vger.kernel.org,
	Palmer Dabbelt <palmer@rivosinc.com>,
	Conor Dooley <conor.dooley@microchip.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	linux-riscv@lists.infradead.org,
	Heiko Stuebner <heiko.stuebner@vrull.eu>,
	Andrew Jones <ajones@ventanamicro.com>
Subject: Re: [PATCH] RISC-V: Show accurate per-hart isa in /proc/cpuinfo
Date: Mon, 26 Jun 2023 21:34:24 +0100	[thread overview]
Message-ID: <20230626-nature-seventh-6102e17bb4be@spud> (raw)
In-Reply-To: <CALs-HstvXcwBa+h0u4mz_Pjm-bNd=9DX49Xy-=Fq0t3ECmAi2w@mail.gmail.com>

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On Mon, Jun 26, 2023 at 12:25:42PM -0700, Evan Green wrote:
> On Fri, Jun 23, 2023 at 5:12 PM Conor Dooley <conor@kernel.org> wrote:
> > On Fri, Jun 23, 2023 at 03:23:53PM -0700, Evan Green wrote:
> > > In /proc/cpuinfo, most of the information we show for each processor is
> > > specific to that hart: marchid, mvendorid, mimpid, processor, hart,
> > > compatible, and the mmu size. But the ISA string gets filtered through a
> > > lowest common denominator mask, so that if one CPU is missing an ISA
> > > extension, no CPUs will show it.
> > >
> > > Now that we track the ISA extensions for each hart, let's report ISA
> > > extension info accurately per-hart in /proc/cpuinfo.
> >
> > No, you can't do this as it breaks the assumptions of userspace that
> > this shows the set supported across all harts.
> > Sorry, but NAK.

> My hope was that we were still early enough that no production systems
> existed (yet) that actually had different ISA extensions in the set we
> track, and therefore usermode would have been unable to make those
> assumptions at this point. If such a system exists, and I don't know
> if it does or not, then I agree it's too late to make a change like
> this.

You should put this information into your commit messages & not just
hope that people understand your intent.
Userspace does actually make these assumptions already, see for example
this Google "cpu features" repo:
https://github.com/google/cpu_features/tree/main
To be quite honest, I really dislike the fragility of what they have
implemented - with only one of the reasons being they made the mistake
of assuming homogeneity.

There's got to be a line somewhere for what constitutes buggy userspace
and what's a regression. Up to Palmer I suppose as to what constitutes
which.

> I thought I'd put this out here and see if someone could point at such
> a system; but if not it'd be great to keep /proc/cpuinfo accurate and
> consistent with hwprobe (which does return accurate per-hart ISA
> extension info).

Just another nail in the coffin for a bad interface :)
There are apparently some mixed c906 chips that support vector on one
core and not the other - although it is thead vector which is not
supported upstream yet...

Other than that, SiFive stuff technically can be mixed - rv64imac &
rv64imafdc on a bunch of the older stuff. I don't think anyone actually
runs those sort of configurations on them though.

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