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

Evan Green
2024-04-12T17:05:12Z

On Fri, Apr 12, 2024 at 3:26 AM Conor Dooley <conor.dooley@microchip.com> wrote:
>
> On Thu, Apr 11, 2024 at 09:11:08PM -0700, Charlie Jenkins wrote:
> > The riscv_cpuinfo struct that contains mvendorid and marchid is not
> > populated until all harts are booted which happens after the DT parsing.
> > Use the vendorid/archid values from the DT if available or assume all
> > harts have the same values as the boot hart as a fallback.
> >
> > Fixes: d82f32202e0d ("RISC-V: Ignore V from the riscv,isa DT property on older T-Head CPUs")
>
> If this is our only use case for getting the mvendorid/marchid stuff
> from dt, then I don't think we should add it. None of the devicetrees
> that the commit you're fixing here addresses will have these properties
> and if they did have them, they'd then also be new enough to hopefully
> not have "v" either - the issue is they're using whatever crap the
> vendor shipped.
> If we're gonna get the information from DT, we already have something
> that we can look at to perform the disable as the cpu compatibles give
> us enough information to make the decision.
>
> I also think that we could just cache the boot CPU's marchid/mvendorid,
> since we already have to look at it in riscv_fill_cpu_mfr_info(), avoid
> repeating these ecalls on all systems.
>
> Perhaps for now we could just look at the boot CPU alone? To my
> knowledge the systems that this targets all have homogeneous
> marchid/mvendorid values of 0x0.

It's possible I'm misinterpreting, but is the suggestion to apply the
marchid/mvendorid we find on the boot CPU and assume it's the same on
all other CPUs? Since we're reporting the marchid/mvendorid/mimpid to
usermode in a per-hart way, it would be better IMO if we really do
query marchid/mvendorid/mimpid on each hart. The problem with applying
the boot CPU's value everywhere is if we're ever wrong in the future
(ie that assumption doesn't hold on some machine), we'll only find out
about it after the fact. Since we reported the wrong information to
usermode via hwprobe, it'll be an ugly userspace ABI issue to clean
up.

-Evan