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

Conor Dooley
2024-05-01T18:09:35Z

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On Wed, May 01, 2024 at 07:03:46PM +0100, Conor Dooley wrote:
> 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.

Ye, you don't get marked possible if you don't have ima, so I don't
think this is possible to have happen. Maybe a comment here is
sufficient, explaining why this cannot reduce to zeros?

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