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

[Anup Patel]

On Mon, Apr 25, 2022 at 7:06 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
>
> Hi Anup,
>
> On Mon, Apr 25, 2022 at 3:14 PM Anup Patel <apatel at ventanamicro.com> wrote:
> > On Mon, Apr 25, 2022 at 6:12 PM Geert Uytterhoeven <geert at linux-m68k.org> wrote:
> > > 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 ?
>
> In that case, I assume the software has to disable Sv48 on its own?
> Fixed hardware should use a different compatible value, so software
> will know when the issue is fixed, and the feature can be used.
> How else is DTB backwards-compatibility supposed to work?

Usually, HW vendors will use different names for incrementally
improving implementations so they will tend to create separate
dts/dtsi files for newer implementations with some sharing via
common dtsi files.

>
> > 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.
>
> That sounds like a software policy, too.
> What is wrong with a kernel command line option?

The MMU modes are detected very early and even before the kernel
command-line is parsed.

> If you want it in your DTB, you can add it to chosen/bootargs.

If HW vendor describe complete details in DT and disables few
things in chosen/bootargs then it means there is some issue with
those things disabled via chosen/bootargs.

A HW vendor might never want to advertise broken extensions in
their implementation so the ISA string and various HART features
in DT will be set based on working functionality on real hardware.

Regards,
Anup

>
> 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