Re: Supporting RISC-V Vendor Extensions in the GNU Toolchain

["Christoph Müllner" <cmuellner@gcc.gnu.org>]

On Wed, May 11, 2022 at 2:02 AM Palmer Dabbelt <palmer@dabbelt.com> wrote:
>
> [Sorry for cross-posting to a bunch of lists, I figured it'd be best to
> have all the discussions in one thread.]
>
> We currently only support what is defined by official RISC-V
> specifications in the various GNU toolchain projects.  There's certainly
> some grey areas there, but in general that means not taking code that
> relies on drafts or vendor defined extensions, even if that would result
> in higher performance or more featured systems for users.
>
> The original goal of these policies were to steer RISC-V implementers
> towards a common set of specifications, but over the last year or so
> it's become abundantly clear that this is causing more harm that good.
> All extant RISC-V systems rely on behaviors defined outside the official
> specifications, and while that's technically always been the case we've
> gotten to the point where trying to ignore that fact is impacting real
> users on real systems.  There's been consistent feedback from users that
> we're not meeting their needs, which can clearly be seen in the many out
> of tree patch sets in common use.
>
> There's been a handful of discussions about this, but we've yet to have
> a proper discussion on the mailing lists.  From the various discussions
> I've had it seems that folks are broadly in favor of supporting vendor
> extensions, but the devil's always in the details with this sort of
> thing so I thought it'd be best to write something up so we can have a
> concrete discussion.
>
> The idea is to start taking code that depends on vendor-defined behavior
> into the core GNU toolchain ports, as long as it meets the following
> criteria:
>
> * An ISA manual is available that can be redistributed/archived, defines
>   the behaviors in question as one or more vendor-specific extensions,
>   and is clearly versioned.  The RISC-V foundation is setting various
>   guidelines around how vendor-defined extensions and instructions
>   should be named, we strongly suggest that vendors follow those
>   conventions whenever possible (this is all new, though, so exactly
>   what's necessary from vendor specifications will likely evolve as we
>   learn).
> * There is a substantial user base that depends on the behavior in
>   question, which probably means there is hardware in the wild that
>   implements the extensions and users that require those extensions in
>   order for that hardware to be useful for common applications.  This is
>   always going to be a grey area, but it's essentially the same spot
>   everyone else is in.
> * There is a mechanism for testing the code in question without direct
>   access to hardware, which in practice means a QEMU port (or whatever
>   simulator is relevant in the space and that folks use for testing) or
>   some community commitment to long-term availability of the hardware
>   for testing (something like the GCC compile farm, for example).
> * It is possible to produce binaries that are compatible with all
>   upstream vendors' implementations.  That means we'll need mechanisms
>   to allow extensions from multiple vendors to be linked together and
>   then probed at runtime.  That's not to say that all binaries will be
>   compatible, as users are always free to skip the compatibility code
>   and there will be conflicting definitions of instruction encodings,
>   but we can at least provide users with the option of compatibility.
>
> These are pretty loosely written on purpose, both because this is all
> new and because each project has its own set of contribution
> requirements so it's going to be all but impossible to have a single
> concrete set of rules that applies everywhere -- that's nothing specific
> to the vendor extensions (or even RISC-V), it's just life.  Specifically
> a major goal here is to balance the needs of users, both in the short
> term (ie, getting new hardware to work) and the long term (ie, the long
> term stability of their software).  We're not talking about taking code
> that can't be tested, hasn't been reviewed, isn't going to be supported
> long-term, or doesn't have a stable ABI; just dropping the specific
> requirement that a specification must be furnished by the RISC-V
> foundation in order to accept code.
>
> Nothing is decided yet, so happy to hear any thought folks have.  This
> is certainly a very different development methodology than what we've
> done in the past and isn't something that should be entreated into
> lightly, so any comments are welcome.

I'd like to add two points to this topic and raise two questions.

1) Accepting vendor extensions = avoidance of fragmentation

RISC-V implementors are actively encouraged to implement their
own ISA extensions. To avoid fragmentation in the SW ecosystem
(every vendor maintains a fork of tools, distros and binaries) there
needs to be a principle acceptance to get vendor extension support
upstream.

2) Leading upstream maintainers already agreed on supporting vendor-extensions

We have discussed the topic of vendor extensions in many forums last year.
This topic was also part of the discussions at the Linux Plumbers conference.
Further, there exists a place for documenting toolchain conventions of
the RISC-V
ecosystem ([1]), which everyone in the RISC-V ecosystem is aware about.

As a result of the discussions last year, a PR ([2]) has been crafted to clarify
the rules for upstreaming vendor extension support. These RISC-V
toolchain conventions recently added a section for vendor extensions
which covers important aspects like:

* naming schemes
* assembly mnemonic prefixes
* links to the documentation and version information

The PR even includes an explicit rule to clarify that maintainers decide on
upstream inclusion:
"""
Open source toolchain maintainer has final say on accepting vendor extension,
comply with this conventions isn't guarantee upstream will accept.
"""

A lot of people (maintainers and active developers) were notified
(including you)
and many also actively contributed to the PR. In the end there was an agreement
of how to document vendor extensions (as a requirement for upstreaming).

I believe that your set of rules is compatible with what is specified there.
Note however, that you could have mentioned them during the PRs review
process as you were notified when the PR was created.

My questions are now the following:

* Where to document the requirements?

  Most RISC-V upstream maintainers are accepting the riscv-toolchains-convention
  repository. Where if not there should we document requirements for the tool's
  conventions? Why not accept what has already been agreed upon?

* Where to track the status?

  You mentioned testing requirements (e.g. QEMU support).
  I've created a wiki page to show the adoption status of all the
RISC-V extensions ([3])
  last year. As the chair of the Toolchains SIG, I'm willing to create
and maintain one for
  vendor extensions as well. This allows users to see which projects
support which
  extensions upstream. However, a wiki is a joint effort. So
maintainers that merge
  changes upstream need to update the page. Will you support this?
  If not what is your proposal to track the status of the upstream
extension support?

BR
Christoph

[1] https://github.com/riscv-non-isa/riscv-toolchain-conventions#conventions-for-vendor-extension
[2] https://github.com/riscv-non-isa/riscv-toolchain-conventions/pull/17
[3] https://wiki.riscv.org/display/HOME/RISC-V+extension+and+feature+support+in+the+Open+Source+SW+Ecosystem