Re: [PATCH v4 05/16] riscv: Extend cpufeature.c to detect vendor extensions

Conor Dooley
2024-05-01T18:03:53Z

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On Wed, May 01, 2024 at 10:51:38AM -0700, Charlie Jenkins wrote:
> On Wed, May 01, 2024 at 09:44:15AM -0700, Evan Green wrote:
> > On Fri, Apr 26, 2024 at 2:29 PM Charlie Jenkins <charlie@rivosinc.com> wrote:
> > > + for (int i = 0; i < riscv_isa_vendor_ext_list_size; i++) {
> > > + const struct riscv_isa_vendor_ext_data_list *ext_list = riscv_isa_vendor_ext_list[i];
> > > +
> > > + if (bitmap_empty(ext_list->vendor_bitmap, ext_list->bitmap_size))
> > > + bitmap_copy(ext_list->vendor_bitmap,
> > > + ext_list->per_hart_vendor_bitmap[cpu].isa,
> > > + ext_list->bitmap_size);
> >
> > Could you get into trouble here if the set of vendor extensions
> > reduces to zero, and then becomes non-zero? To illustrate, consider
> > these masks:
> > cpu 0: 0x0000C000
> > cpu 1: 0x00000003 <<< vendor_bitmap ANDs out to 0
> > cpu 2: 0x00000010 <<< oops, we end up copying this into vendor_bitmap
> >
>
> Huh that's a good point. The standard extensions have that same bug too?
>
> if (bitmap_empty(riscv_isa, RISCV_ISA_EXT_MAX))
> bitmap_copy(riscv_isa, isainfo->isa, RISCV_ISA_EXT_MAX);
> else
> bitmap_and(riscv_isa, riscv_isa, isainfo->isa, RISCV_ISA_EXT_MAX);

I suppose it could in theory, but the boot hart needs ima to even get
this far. I think you'd only end up with this happening if there were
enabled harts that supported rvXXe, but I don't think we even add those
to the possible set of CPUs. I'll have to check.

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