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

[Palmer Dabbelt]

On Mon, 26 Jun 2023 13:34:24 PDT (-0700), Conor Dooley wrote:
> 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@xxxxxxxxxx> 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.

Maybe let's just add a pretty printed version of the hwprobe info to 
/proc/cpuinfo, and then leave the ISA string alone as a legacy 
interface?

Having something so poorly defined as uABI is a bit embarassing, but 
it's our mistake so we've got to live with it.

> > 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.