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

Charlie Jenkins
2024-04-12T21:32:06Z

On Fri, Apr 12, 2024 at 10:27:47PM +0100, Conor Dooley wrote:
> On Fri, Apr 12, 2024 at 01:48:46PM -0700, Charlie Jenkins wrote:
> > On Fri, Apr 12, 2024 at 07:47:48PM +0100, Conor Dooley wrote:
> > > On Fri, Apr 12, 2024 at 10:12:20AM -0700, Charlie Jenkins wrote:
> > > > On Fri, Apr 12, 2024 at 11:25:47AM +0100, Conor Dooley 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.
> > > >
> > > > Yes, the DT those shipped with will not have the property in the DT so
> > > > will fall back on the boot hart. The addition of the DT properties allow
> > > > future heterogenous systems to be able to function.
> > >
> > > I think you've kinda missed the point about what the original code was
> > > actually doing here. Really the kernel should not be doing validation of
> > > the devicetree at all, but I was trying to avoid people shooting
> > > themselves in the foot by doing something simple that would work for
> > > their (incorrect) vendor dtbs.
> > > Future heterogenous systems should be using riscv,isa-extensions, which
> > > is totally unaffected by this codepath (and setting actual values for
> > > mimpid/marchid too ideally!).
> > >
> >
> > I am on the same page with you about that.
> >
> > > > > 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.
> > > >
> > > > Yeah that is a minor optimization that can I can apply.
> > > >
> > > > >
> > > > > 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.
> > > >
> > > > They have an mvendorid of 0x5b7.
> > >
> > > That was a braino, clearly I should have typed "mimpid".
> > >
> > > > This is already falling back on the boot CPU, but that is not a solution
> > > > that scales. Even though all systems currently have homogenous
> > > > marchid/mvendorid I am hesitant to assert that all systems are
> > > > homogenous without providing an option to override this.
> > >
> > > There are already is an option. Use the non-deprecated property in your
> > > new system for describing what extesions you support. We don't need to
> > > add any more properties (for now at least).
> >
> > The issue is that it is not possible to know which vendor extensions are
> > associated with a vendor. That requires a global namespace where each
> > extension can be looked up in a table. I have opted to have a
> > vendor-specific namespace so that vendors don't have to worry about
> > stepping on other vendor's toes (or the other way around). In order to
> > support that, the vendorid of the hart needs to be known prior.
>
> Nah, I think you're mixing up something like hwprobe and having
> namespaces there with needing namespacing on the devicetree probing side
> too. You don't need any vendor namespacing, it's perfectly fine (IMO)
> for a vendor to implement someone else's extension and I think we should
> allow probing any vendors extension on any CPU.

I am not mixing it up. Sure a vendor can implement somebody else's
extension, they just need to add it to their namespace too.

- Charlie

>
> > I know a rebuttal here is that this is taking away from the point of
> > the original patch. I can split this patch up if so. The goal here is to
> > allow vendor extensions to play nicely with the rest of the system.
> > There are two uses of the mvendorid DT value, this fix, and the patch
> > that adds vendor extension support. I felt that it was applicable to
> > wrap the mvendorid DT value into this patch, but if you would prefer
> > that to live separate of this fix then that is fine too.
> >
> > - Charlie
> >
> > >
> > > > The overhead is
> > > > looking for a field in the DT which does not seem to be impactful enough
> > > > to prevent the addition of this option.
> > > >
> > > > >
> > > > > > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > > > >
> > > > > > @@ -514,12 +521,23 @@ static void __init riscv_fill_hwcap_from_isa_string(unsigned long *isa2hwcap)
> > > > > > pr_warn("Unable to find \"riscv,isa\" devicetree entry\n");
> > > > > > continue;
> > > > > > }
> > > > > > + if (of_property_read_u64(node, "riscv,vendorid", &this_vendorid) < 0) {
> > > > > > + pr_warn("Unable to find \"riscv,vendorid\" devicetree entry, using boot hart mvendorid instead\n");
> > > > >
> > > > > This should 100% not be a warning, it's not a required property in the
> > > > > binding.
> > > >
> > > > Yes definitely, thank you.
> > > >
> > > > - Charlie
> > > >
> > > > >
> > > > > Cheers,
> > > > > Conor.
> > > > >
> > > > > > + this_vendorid = boot_vendorid;
> > > > > > + }
> > > > >
> > > >
> > > >