Add riscv64gc-unknown-hermit Tier 3 target

[simonschoening]

This pull request adds the riscv64gc-unknown-hermit target and is based on existing targets (hermit, riscv64gc).

I understand that Tier 3 targets must meet certain requirements. Adding this target introduces no new dependencies
or changes affecting other targets.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @nagisa (or someone else) soon.

Please see the contribution instructions for more information.

[rust-highfive]

⚠️ Warning ⚠️

• These commits modify compiler targets. (See the Target Tier Policy.)

[nagisa]

Why is this TLS model used by default? Does the target not support InitialExec?

[simonschoening]

I don't think that InitialExec is needed since the target is not used to build shared libraries.

[nagisa]

Please quote the corresponding requirements verbatim from the tier 3 target tier policy to explain how the target meets those requirements (just a comment here is fine).

Adding a page to src/doc/rustc/src/platform-support/ would also be nice, though not required.

[simonschoening]

• A tier 3 target must have a designated developer or developers (the "target
  maintainers") on record to be CCed when issues arise regarding the target.
  (The mechanism to track and CC such developers may evolve over time.)

Yes, that's me. I am working on the riscv64 port of RustyHermit.

• Targets must use naming consistent with any existing targets; for instance, a
  target for the same CPU or OS as an existing Rust target should use the same
  name for that CPU or OS. Targets should normally use the same names and
  naming conventions as used elsewhere in the broader ecosystem beyond Rust
  (such as in other toolchains), unless they have a very good reason to
  diverge. Changing the name of a target can be highly disruptive, especially
  once the target reaches a higher tier, so getting the name right is important
  even for a tier 3 target.
  • Target names should not introduce undue confusion or ambiguity unless
    absolutely necessary to maintain ecosystem compatibility. For example, if
    the name of the target makes people extremely likely to form incorrect
    beliefs about what it targets, the name should be changed or augmented to
    disambiguate it.

The naming riscv64gc-unknown-hermit is consistent with already existing targets like x86_64-unknown-hermit and riscv64gc-unknown-linux-gnu.

• Tier 3 targets may have unusual requirements to build or use, but must not
  create legal issues or impose onerous legal terms for the Rust project or for
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    developers or users.

This is not a problem. The changes for the new target are very small and do not include any new dependencies.

• Neither this policy nor any decisions made regarding targets shall create any
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I understand.

• Tier 3 targets should attempt to implement as much of the standard libraries
  as possible and appropriate (core for most targets, alloc for targets
  that can support dynamic memory allocation, std for targets with an
  operating system or equivalent layer of system-provided functionality), but
  may leave some code unimplemented (either unavailable or stubbed out as
  appropriate), whether because the target makes it impossible to implement or
  challenging to implement. The authors of pull requests are not obligated to
  avoid calling any portions of the standard library on the basis of a tier 3
  target not implementing those portions.

Not all parts of std are implemented.

• The target must provide documentation for the Rust community explaining how
  to build for the target, using cross-compilation if possible. If the target
  supports running tests (even if they do not pass), the documentation must
  explain how to run tests for the target, using emulation if possible or
  dedicated hardware if necessary.

This is still very work-in-progress. RustyHermit will be usable on RISC-V like on x86-64.
A small documentation on using RustyHermit is given in the README.md of the rusty-hermit repository).

The riscv64 port is available here. Building the programs in the example/ folder already works using the new target triple and disabling the default features. Supported features are smp and smoltcp.

The kernel can be used with the hypervisor uhyve (but only on Linux with KVM support running on RISC-V cores supporting the Hypervisor extension).

With rusty-loader located in loader/, RustyHermit can be run on qemu with the OpenSBI firmware and possibly real hardware with a SBI compatible firmware.
A simple driver is used to support the NIC of qemu machine sifive_u.

• Tier 3 targets must not impose burden on the authors of pull requests, or
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I understand.

• Patches adding or updating tier 3 targets must not break any existing tier 2
  or tier 1 target, and must not knowingly break another tier 3 target without
  approval of either the compiler team or the maintainers of the other tier 3
  target.
  • In particular, this may come up when working on closely related targets,
    such as variations of the same architecture with different features. Avoid
    introducing unconditional uses of features that another variation of the
    target may not have; use conditional compilation or runtime detection, as
    appropriate, to let each target run code supported by that target.

There are no changes affecting other targets.

[joshtriplett]

Can you please add the platform documentation as a markdown file linked from the target's entry in platform-support?

[nagisa]

As requested by Josh above, please add an entry to src/doc/rustc/src/platform-support (I know I said its not required in my previous comment, but since its been asked for twice, it seems like adding one would make this go more smoothly).

r=me afterwards.

[JohnCSimon]

triage:
Looks like the author did the requested comments but didn't set to review.

@rustbot label: -S-waiting-on-author +S-waiting-on-review

[bors]

☔ The latest upstream changes (presumably #91284) made this pull request unmergeable. Please resolve the merge conflicts.

[nagisa]

Looks like the author did the requested comments but didn't set to review.

That is not the case. What @joshtriplett was requesting is a file under src/doc/rustc/src/platform-support/ with a target description, much like e.g. https://github.com/rust-lang/rust/blob/master/src/doc/rustc/src/platform-support/x86_64-unknown-none.md. @simonschoening, can you make sure such a file is added?

In addition to this for this to be mergeable, you will need to rebase your PR branch, rather than merging in the master. See https://rustc-dev-guide.rust-lang.org/git.html#rebasing-and-conflicts

[simonschoening]

@stlankes will revise and re-open the pull request.