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

[Alexandre Ghiti]

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!

@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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