[PATCH] RISC-V: Allow the used to downgrade to sv48 when HW supports sv57

[Anup Patel]

On Mon, Apr 25, 2022 at 6:12 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> Hi Palmer,.
>
> On Fri, Apr 22, 2022 at 11:42 PM Palmer Dabbelt <palmer at rivosinc.com> wrote:
> > Similar to the previous patch, this allows a dt-selected downgrade to
> > sv48 on systems that support sv57 in case users don't need the extra VA
> > bits and want to save memory or improve performance.
> >
> > Signed-off-by: Palmer Dabbelt <palmer at rivosinc.com>
> > ---
> > This is on top of the patches from Alex's set that I dropped.
>
> You mean "[PATCH v3 13/13] riscv: Allow user to downgrade to sv39
> when hw supports sv48 if !KASAN"?
> 20211206104657.433304-14-alexandre.ghiti at canonical.com
>
> For both: "DT describes hardware, not software policy"?

It is possible that HW is designed to support both Sv48 and Sv39 but
there is some errata due to which Sv48 does not work correctly ?

We should allow users to downgrade the MMU mode, due to
their own reasons. In fact, users can also disable an extension
by not showing it in the DT ISA string.

Regards,
Anup

>
> > --- a/arch/riscv/mm/init.c
> > +++ b/arch/riscv/mm/init.c
> > @@ -740,6 +740,11 @@ static __init void set_satp_mode(uintptr_t dtb_pa)
> >                         if (!mmu_type)
> >                                 continue;
> >
> > +                       if (!strcmp(mmu_type, "riscv,sv48")) {
> > +                               disable_pgtable_l5();
> > +                               return;
> > +                       }
> > +
> >                         if (!strcmp(mmu_type, "riscv,sv39")) {
> >                                 disable_pgtable_l5();
> >                                 disable_pgtable_l4();
>
> Gr{oetje,eeting}s,
>
>                         Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at linux-m68k.org
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds