[PATCH v2 2/7] RISC-V: Detect AIA CSRs from ISA string

Wed Feb 8 06:57:23 PST 2023

On Wed, Feb 8, 2023 at 6:27 PM Conor Dooley <conor.dooley at microchip.com> wrote:
>
> Hey Anup!
>
> On Wed, Feb 08, 2023 at 09:24:28AM +0530, Anup Patel wrote:
> > On Wed, Feb 8, 2023 at 2:09 AM Conor Dooley <conor at kernel.org> wrote:
> > > On Tue, Feb 07, 2023 at 10:15:22AM -0800, Atish Patra wrote:
> > > > On Tue, Feb 7, 2023 at 10:05 AM Conor Dooley <conor at kernel.org> wrote:
> > > > > On Fri, Feb 03, 2023 at 05:31:01PM +0530, Anup Patel wrote:
> > > > > > On Fri, Feb 3, 2023 at 5:54 AM Palmer Dabbelt <palmer at dabbelt.com> wrote:
> > > > > > >
> > > > > > > On Fri, 27 Jan 2023 23:27:32 PST (-0800), apatel at ventanamicro.com wrote:
> > > > > > > > We have two extension names for AIA ISA support: Smaia (M-mode AIA CSRs)
> > > > > > > > and Ssaia (S-mode AIA CSRs).
> > > > > > >
> > > > > > > This has pretty much the same problem that we had with the other
> > > > > > > AIA-related ISA string patches, where there's that ambiguity with the
> > > > > > > non-ratified chapters.  IIRC when this came up in GCC the rough idea was
> > > > > > > to try and document that we're going to interpret the standard ISA
> > > > > > > strings that way, but now that we're doing custom ISA extensions it
> > > > > > > seems saner to just define on here that removes the ambiguity.
> > > > > > >
> > > > > > > I just sent
> > > > > > > <https://lore.kernel.org/r/20230203001201.14770-1-palmer@rivosinc.com/>
> > > > > > > which documents that.
> > > > > >
> > > > > > I am not sure why you say that these are custom extensions.
> > > > > >
> > > > > > Multiple folks have clarified that both Smaia and Ssaia are frozen
> > > > > > ISA extensions as-per RVI process. The individual chapters which
> > > > > > are in the draft state have nothing to do with Smaia and Ssaia CSRs.
> > > > > >
> > > > > > Please refer:
> > > > > > https://github.com/riscv/riscv-aia/pull/36
> > > > > > https://lists.riscv.org/g/tech-aia/message/336
> > > > > > https://lists.riscv.org/g/tech-aia/message/337
> > > > >
> > > > > All of these links seem to discuss the draft chapters somehow being
> > > > > incompatible with the non-draft ones. I would very expect that that,
> > > > > as pointed out in several places there, that the draft chapters
> > > > > finalisation would not lead to meaningful (and incompatible!) changes
> > > > > being made to the non-draft chapters.
> > > > >
> > > >
> > > > Here is the status of all RVI specs. It states that the Smaia, Ssaia
> > > > extensions are frozen (i.e. public review complete).
> > > > https://wiki.riscv.org/display/HOME/Specification+Status
> > > >
> > > > I have added stephano/Jeff to confirm the same.
> > > >
> > > > AFAIK, IOMMU spec is close to the public review phase and should be
> > > > frozen in this or next quarter.
> > > > IIRC, this chapter in AIA will be frozen along with IOMMU spec.
> > > >
> > > > Anup: Please correct me if that's not correct.
> > > >
> > > > > Maybe yourself and Palmer are looking at this from different
> > > > > perspectives? Looking at his patch from Friday:
> > > > > https://lore.kernel.org/linux-riscv/20230203001201.14770-1-palmer@rivosinc.com/
> > > > > He specifically mentioned this aspect, as opposed to the aspect that
> > > > > your links refer to.
> > > > >
> > > > > Surely a duo-plic, if that ever comes to be, could be detected from
> > > > > compatible strings in DT or w/e - but how do you intend differentiating
> > > > > between an implementation of S*aia that contains the IOMMU support in
> > > > > Chapter 9 in a finalised form, versus an implementation that may make
> > > > > "different decisions" when it comes to that chapter of the spec?
> > > >
> > > > We will most likely have an extension specific to iommu spec as well.
> > >
> > > Right, but unless I am misunderstanding you, that is an extension for the
> > > IOMMU spec, not for Chapter 9 of the AIA spec?
> > >
> > > I would say that it is likely that if you have AIA and IOMMU that you'd
> > > want to be implementing Chapter 9, but that would not appear sufficient to
> > > draw a conclusion from.
> > >
> > > Maybe the RVI lads that you've added (or Anup for that matter!) can
> > > clarify if there is a requirement that if you do AIA and IOMMU that you
> > > must do Chapter 9.
> > > If not, my prior question about a differentiation mechanism still applies
> > > I think!
> >
> > For the benefit of everyone, the AIA spec mainly defines three
> > modular components:
> > 1) Extended Local Interrupt CSRs (Smaia and Ssaia extensions)
> >     (ISA extension covered by: Chapter 2, Chapter 6, and Chapter 7)
> > 2) Incoming MSI Controller (IMSIC)
> >     (ISA and Non-ISA extension covered by: Chapter 3 and Chapter 8)
> > 3) Advanced PLIC (APLIC)
> >     (Non-ISA extension covered by: Chapter 4)
> >
> > Apart from above, we have Chapter 5 ("Duo-PLIC") and Chapter 9
> > ("IOMMU Support for MSIs to Virtual Machines") which are in draft
> > state.
> >
> > Currently, there are no RISC-V members who have expressed
> > interest in implementing Chapter 5 ("Duo-PLIC") so this chapter
> > will stay in draft state for a foreseeable future.
>
> Thanks for the clarifications :)
>
> > The Chapter 9 ("IOMMU Support for MSIs to Virtual Machines")
> > defines an optional feature of IOMMU which can be implemented
> > by a standard IOMMU (such as RISC-V IOMMU) or a vendor specific
> > IOMMU. A RISC-V platform can certainly support device pass-through
> > using IMSIC guest files and an IOMMU which does not implement
> > Chapter 9. Unfortunately, there is a limit on the maximum number
> > of per-HART IMSIC guest files which can further limit the number
> > of pass-through devices. The Chapter 9 allows RISC-V platforms
> > to support large number of pass-through devices by defining "MRIF
> > - memory resident interrupt files" for an IOMMU. Further, the MRIFs
> > defined by Chapter 9 are simply interrupt files located in main memory
> > and have nothing to do with AIA local interrupt CSRs (Smaia and Ssaia).
> >
> > The presence of S*aia in ISA string only implies that AIA extended
> > local interrupt CSRs are implemented by the underlying RISC-V
> > implementation.
>
> Would you mind linking to where this is documented & explaining in your
> commit message why it is okay operate on the basis of s*aia in the ISA
> string only mandates the presence of the CSRs and nothing more.
>
> I think when I was reading it last night, I saw some commentary in this
> vein in Section 1.6 of the rc2 spec. Although IIRC it noted changes in
> interrupt behaviour there too, so I'm not sure if that section is what you
> are referring to here.
>
> Perhaps this is all just a good argument for providing more information
> in commit messages ;)

Sure, I am anyway going to send v3 after rebase so I will cite the
Section 1.6 of AIA spec in the commit description.

>
> > I confirm that it is certainly not mandatory for a RISC-V platform to
> > implement Chapter 9 of the AIA specification if the RISC-V platform
> > already implements AIA and IOMMU.
>
> Cool, thanks.
> By what mechanism, since you say that "s*aia" in the ISA string only
> implies presence of the CSRs [sic], are we meant to discover the
> presence of Chapter 9?
> A property of the IOMMU node seems the most logical I suppose, or
> perhaps inferred from the presence/config of other properties of said
> node?

The discovery of AIA Chapter 9 features is IOMMU specific.

The upcoming RISC-V IOMMU spec defines "capabilities" MMIO register
which has read-only bits MSI_MRIF and MSI_FLAT for discovering features
defined in AIA Chapter 9. Due to this dependency, the AIA Chapter 9 is
going to be ratified along with RISC-V IOMMU ratification.

>
> > > > > I thought that would be handled by extension versions, but I am told
> > > > > that those are not a thing any more.
> > > > > If that's not true, and there'll be a version number that we can pull in
> > > > > from a DT and parse which will distinguish between the two, then please
> > > > > correct my misunderstanding here!
>
> Thanks again Anup,
> Conor.
>

Regards,
Anup