Re: [PATCH 14/19] riscv: hwprobe: Disambiguate vector and xtheadvector in hwprobe

[Charlie Jenkins]

On Fri, Apr 12, 2024 at 10:04:42AM -0700, Evan Green wrote:
> On Fri, Apr 12, 2024 at 4:35 AM Conor Dooley <conor.dooley@xxxxxxxxxxxxx> wrote:
> >
> > On Thu, Apr 11, 2024 at 09:11:20PM -0700, Charlie Jenkins wrote:
> > > Ensure that hwprobe does not flag "v" when xtheadvector is present.
> > >
> > > Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> > > ---
> > >  arch/riscv/kernel/sys_hwprobe.c | 4 ++--
> > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/arch/riscv/kernel/sys_hwprobe.c b/arch/riscv/kernel/sys_hwprobe.c
> > > index 8cae41a502dd..e0a42c851511 100644
> > > --- a/arch/riscv/kernel/sys_hwprobe.c
> > > +++ b/arch/riscv/kernel/sys_hwprobe.c
> > > @@ -69,7 +69,7 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
> > >       if (riscv_isa_extension_available(NULL, c))
> > >               pair->value |= RISCV_HWPROBE_IMA_C;
> > >
> > > -     if (has_vector())
> > > +     if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR))
> >
> > Hmm, I think this is "dangerous". has_vector() is used across the kernel
> > now in several places for the in-kernel vector. I don't think that
> > has_vector() should return true for the T-Head stuff given that &
> > has_vector() should represent the ratified spec. I'll have to think
> > about this one and how nasty this makes any of the save/restore code
> > etc.
> 
> Yeah, my nose crinkled here as well. If you're having to do a
> vendorish thing in this generic spot, then others may too, suggesting
> perhaps this isn't the cleanest way to go about it. Ideally extensions
> are all additive, rather than subtractive, I guess?

This was the "easiest" way to support this but I agree this is not
ideal. The vector code is naturally coupled with having support for
"v" and I wanted to leverage that. The other concern is all of the
ifdefs for having V enabled. I can make all of those V or XTHEADVECTOR;
that will increase the surface area of xtheadvector but it is probably
the right(?) way to go.

- Charlie

> 
> 
> >
> > >               pair->value |= RISCV_HWPROBE_IMA_V;
> > >
> > >       /*
> > > @@ -112,7 +112,7 @@ static void hwprobe_isa_ext0(struct riscv_hwprobe *pair,
> > >               EXT_KEY(ZACAS);
> > >               EXT_KEY(ZICOND);
> > >
> > > -             if (has_vector()) {
> > > +             if (has_vector() && !riscv_has_vendor_extension_unlikely(RISCV_ISA_VENDOR_EXT_XTHEADVECTOR)) {
> > >                       EXT_KEY(ZVBB);
> > >                       EXT_KEY(ZVBC);
> > >                       EXT_KEY(ZVKB);
> > >
> > > --
> > > 2.44.0
> > >