[RFC PATCH v4 1/5] RISC-V: Detect and Enable Svadu Extension Support

[Anup Patel]

On Mon, Jun 3, 2024 at 4:59 PM Alexandre Ghiti <alex at ghiti.fr> wrote:
>
> On 30/05/2024 11:24, Andrew Jones wrote:
> > On Thu, May 30, 2024 at 11:01:20AM GMT, Alexandre Ghiti wrote:
> >> Hi Andrew,
> >>
> >> On 30/05/2024 10:47, Andrew Jones wrote:
> >>> On Thu, May 30, 2024 at 10:19:12AM GMT, Alexandre Ghiti wrote:
> >>>> Hi Yong-Xuan,
> >>>>
> >>>> On 27/05/2024 18:25, Andrew Jones wrote:
> >>>>> On Fri, May 24, 2024 at 06:33:01PM GMT, Yong-Xuan Wang wrote:
> >>>>>> Svadu is a RISC-V extension for hardware updating of PTE A/D bits.
> >>>>>>
> >>>>>> In this patch we detect Svadu extension support from DTB and enable it
> >>>>>> with SBI FWFT extension. Also we add arch_has_hw_pte_young() to enable
> >>>>>> optimization in MGLRU and __wp_page_copy_user() if Svadu extension is
> >>>>>> available.
> >>>> So we talked about this yesterday during the linux-riscv patchwork meeting.
> >>>> We came to the conclusion that we should not wait for the SBI FWFT extension
> >>>> to enable Svadu but instead, it should be enabled by default by openSBI if
> >>>> the extension is present in the device tree. This is because we did not find
> >>>> any backward compatibility issues, meaning that enabling Svadu should not
> >>>> break any S-mode software.
> >>> Unfortunately I joined yesterday's patchwork call late and missed this
> >>> discussion. I'm still not sure how we avoid concerns with S-mode software
> >>> expecting exceptions by purposely not setting A/D bits, but then not
> >>> getting those exceptions.
> >>
> >> Most other architectures implement hardware A/D updates, so I don't see
> >> what's specific in riscv. In addition, if an OS really needs the exceptions,
> >> it can always play with the page table permissions to achieve such
> >> behaviour.
> > Hmm, yeah we're probably pretty safe since sorting this out is just one of
> > many things an OS will have to learn to manage when getting ported. Also,
> > handling both svade and svadu at boot is trivial since the OS simply needs
> > to set the A/D bits when creating the PTEs or have exception handlers
> > which do nothing but set the bits ready just in case.
> >
> >>
> >>>> This is what you did in your previous versions of
> >>>> this patchset so the changes should be easy. This behaviour must be added to
> >>>> the dtbinding description of the Svadu extension.
> >>>>
> >>>> Another thing that we discussed yesterday. There exist 2 schemes to manage
> >>>> the A/D bits updates, Svade and Svadu. If a platform supports both
> >>>> extensions and both are present in the device tree, it is M-mode firmware's
> >>>> responsibility to provide a "sane" device tree to the S-mode software,
> >>>> meaning the device tree can not contain both extensions. And because on such
> >>>> platforms, Svadu is more performant than Svade, Svadu should be enabled by
> >>>> the M-mode firmware and only Svadu should be present in the device tree.
> >>> I'm not sure firmware will be able to choose svadu when it's available.
> >>> For example, platforms which want to conform to the upcoming "Server
> >>> Platform" specification must also conform to the RVA23 profile, which
> >>> mandates Svade and lists Svadu as an optional extension. This implies to
> >>> me that S-mode should be boot with both svade and svadu in the DT and with
> >>> svade being the active one. Then, S-mode can choose to request switching
> >>> to svadu with FWFT.
> >>
> >> The problem is that FWFT is not there and won't be there for ~1y (according
> >> to Anup). So in the meantime, we prevent all uarchs that support Svadu to
> >> take advantage of this.
> > I think we should have documented behaviors for all four possibilities
> >
> >   1. Neither svade nor svadu in DT -- current behavior
> >   2. Only svade in DT -- current behavior
> >   3. Only svadu in DT -- expect hardware A/D updating
> >   4. Both svade and svadu in DT -- current behavior, but, if we have FWFT,
> >      then use it to switch to svadu. If we don't have FWFT, then, oh well...
> >
> > Platforms/firmwares that aren't concerned with the profiles can choose (3)
> > and Linux is fine. Those that do want to conform to the profile will
> > choose (4) but Linux won't get the benefit of svadu until it also gets
> > FWFT.
>
>
> I think this solution pleases everyone so I'd say we should go for it,
> thanks Andrew!

Yes, this looks good to me as well. The key aspect is documenting
the behaviour of these four possibilities.

Regards,
Anup

>
> @Yong-Xuan do you think you can prepare another spin with Andrew's
> proposal implemented?
>
> Thanks,
>
> Alex
>
>
> >
> > IOW, I think your proposal is fine except for wanting to document in the
> > DT bindings that only svade or svadu may be provided, since I think we'll
> > want both to be allowed eventually.
> >
> > Thanks,
> > drew
> >
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