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

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Apr 12, 2024 at 07:38:04PM +0100, Conor Dooley wrote:
> On Fri, Apr 12, 2024 at 10:04:17AM -0700, Evan Green wrote:
> > 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.
> 
> You're misinterpreting, we do get the values on all individually as
> they're brought online. This is only used by the code that throws a bone
> to people with crappy vendor dtbs that put "v" in riscv,isa when they
> support the unratified version.

Not quite, the alternatives are patched before the other cpus are
booted, so the alternatives will have false positives resulting in
broken kernels.

- Charlie