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

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Date:   Mon, 26 Jun 2023 21:34:24 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Evan Green <evan@...osinc.com>
Cc:     Palmer Dabbelt <palmer@...osinc.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrew Jones <ajones@...tanamicro.com>,
        Anup Patel <apatel@...tanamicro.com>,
        Conor Dooley <conor.dooley@...rochip.com>,
        Heiko Stuebner <heiko.stuebner@...ll.eu>,
        Palmer Dabbelt <palmer@...belt.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Sunil V L <sunilvl@...tanamicro.com>,
        linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH] RISC-V: Show accurate per-hart isa in /proc/cpuinfo

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@...nel.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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