Add SOLID targets

[kawadakk]

This PR introduces new tier 3 targets for SOLID embedded development platform by Kyoto Microcomputer Co., Ltd.

Target name	target_arch	target_vendor	target_os
aarch64-kmc-solid_asp3	aarch64	kmc	solid_asp3
armv7a-kmc-solid_asp3-eabi	arm	kmc	solid_asp3
armv7a-kmc-solid_asp3-eabihf	arm	kmc	solid_asp3

Related PRs

• libc: Add SOLID target support libc#2227
• cc: Add SOLID target support cc-rs#609

Non-blocking Issues

• The target kernel can support Thread::unpark directly, but this property is not utilized because the underlying kernel feature is used to implement Condvar and it's unclear whether std should guarantee that parking tokens are not clobbered by other synchronization primitives.
• The rustc book: The page title "*-kmc-solid-*" shows up as "-kmc-solid-" in TOC

Tier 3 Target Policy

As tier 3 targets, the new targets are required to adhere to the tier 3 target policy requirements. This section quotes each requirement in entirety and describes how they are met.

• 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.)

See src/doc/rustc/src/platform-support/kmc-solid.md.

• 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 new target names follow this format: $ARCH-$VENDOR-$OS-$ABI, which is already adopted by most existing targets. $ARCH and $ABI follow the convention: aarch64-* for AArch64, armv7a-*-eabi for Armv7-A with EABI. $OS is used to distinguish multiple variations of the platform in a somewhat similar way to the Apple targets, though we are only adding one variation in this PR. $VENDOR denotes the platform vendor name similarly to the Apple, Solaris, SGX, and VxWorks targets.

$OS corresponds to the value of target_os and takes the format solid-$KERNEL. The inclusion of a hyphen prevents unique decomposition of target names, though the mapping between target names and target attributes isn't trivial in the first place, e.g., because of the Android targets.

More targets may be added later, as we support other base kernels (there are at least three at the point of writing) and are interested in supporting other processor architectures in the future.

• 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 Rust developers or users.
  • The target must not introduce license incompatibilities.
  • Anything added to the Rust repository must be under the standard Rust license (MIT OR Apache-2.0).
  • The target must not cause the Rust tools or libraries built for any other host (even when supporting cross-compilation to the target) to depend on any new dependency less permissive than the Rust licensing policy. This applies whether the dependency is a Rust crate that would require adding new license exceptions (as specified by the tidy tool in the rust-lang/rust repository), or whether the dependency is a native library or binary. In other words, the introduction of the target must not cause a user installing or running a version of Rust or the Rust tools to be subject to any new license requirements.
  • If the target supports building host tools (such as rustc or cargo), those host tools must not depend on proprietary (non-FOSS) libraries, other than ordinary runtime libraries supplied by the platform and commonly used by other binaries built for the target. For instance, rustc built for the target may depend on a common proprietary C runtime library or console output library, but must not depend on a proprietary code generation library or code optimization library. Rust's license permits such combinations, but the Rust project has no interest in maintaining such combinations within the scope of Rust itself, even at tier 3.
  • Targets should not require proprietary (non-FOSS) components to link a functional binary or library.
  • "onerous" here is an intentionally subjective term. At a minimum, "onerous" legal/licensing terms include but are not limited to: non-disclosure requirements, non-compete requirements, contributor license agreements (CLAs) or equivalent, "non-commercial"/"research-only"/etc terms, requirements conditional on the employer or employment of any particular Rust developers, revocable terms, any requirements that create liability for the Rust project or its developers or users, or any requirements that adversely affect the livelihood or prospects of the Rust project or its developers or users.

We intend to make the contribution fully available under the standard Rust license with no additional legal restrictions whatsoever. This PR does not introduce any new dependency less permissive than the Rust license policy, and we are willing to ensure this doesn't happen for future contributions regarding the new targets.

The new targets don't support building host tools.

Although the new targets use a platform-provided C compiler toolchain, it can be substituted by GNU Arm Embedded Toolchain for testing purposes.

• 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.

Most features are implemented. The following features are not implemented due to the lack of native support:

• fs::File::{file_attr, truncate, duplicate, set_permissions}
• fs::{symlink, link, canonicalize}
• Process creation
• Command-line arguments

Networking is not implemented yet, and we intend to add it as soon as it's ready.
Edit (2021-07-07): Networking is now implemented.

Backtrace generation is not really a good fit for embedded targets, so it's intentionally left unimplemented. Unwinding is functional, however.

• 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.

See src/doc/rustc/src/platform-support/kmc-solid.md. Running tests is not supported.

• Neither this policy nor any decisions made regarding targets shall create any binding agreement or estoppel by any party. If any member of an approving Rust team serves as one of the maintainers of a target, or has any legal or employment requirement (explicit or implicit) that might affect their decisions regarding a target, they must recuse themselves from any approval decisions regarding the target's tier status, though they may otherwise participate in discussions.
  • This requirement does not prevent part or all of this policy from being cited in an explicit contract or work agreement (e.g. to implement or maintain support for a target). This requirement exists to ensure that a developer or team responsible for reviewing and approving a target does not face any legal threats or obligations that would prevent them from freely exercising their judgment in such approval, even if such judgment involves subjective matters or goes beyond the letter of these requirements.
• Tier 3 targets must not impose burden on the authors of pull requests, or other developers in the community, to maintain the target. In particular, do not post comments (automated or manual) on a PR that derail or suggest a block on the PR based on a tier 3 target. Do not send automated messages or notifications (via any medium, including via @) to a PR author or others involved with a PR regarding a tier 3 target, unless they have opted into such messages.
  • Backlinks such as those generated by the issue/PR tracker when linking to an issue or PR are not considered a violation of this policy, within reason. However, such messages (even on a separate repository) must not generate notifications to anyone involved with a PR who has not requested such notifications.
• 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.

We acknowledge these requirements and intend to ensure they are met.

There are no closely related targets at the moment.

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[nagisa]

cc @rust-lang/libs do we want to add more targets that have a large portion of the libstd stubbed out?

So far there is some precedent in wasm doing the same, but I'm not clear if that's something we want to continue adding.

[nagisa]

Minor nit (fine to not address it as, I think, these do not end up in the public API of the libstd): Here and for the other doc-comments in this file, the summary line should be a proper sentence (i.e. end with a period.)

As a less minor nit, it would be nice if the types that do not end up being exposed in the public API of the libstd had a crate-level visibility, rather than pub.

[CDirkx]

Almost everything in sys is pub right now though; the module itself is private.

[nagisa]

Almost everything in sys is pub right now though; the module itself is private.

Yeah, the reason to use crate visibility, especially for new code is because it makes reviewing easier and visibility mistakes very unlikely to impossible.

[joshtriplett]

On Sat, Jun 12, 2021 at 07:36:14AM -0700, Simonas Kazlauskas wrote: cc @rust-lang/libs do we want to add more targets that have a large portion of the libstd stubbed out?

No, I don't think we should. I understand why it happened for wasm32-unknown-unknown, but I don't think we should repeat that. If a target can't implement std it should just provide core/alloc. I *don't* think crates should have to cope with subsets of std, especially not before we have any kind of feature/subsetting mechanism. Perhaps this can be better supported once we have build-std.

[Amanieu]

I don't think the same logic applies here: this target runs under an RTOS, and has most of the functionality of an OS available. In particular, it has a filesystem, threads and synchronization primitives.

On the other hand wasm32-unknown-unknown has none of these and really should have been a no_std-only target.

[joshtriplett]

@Amanieu That certainly sounds more reasonable. I'd based that comment on the description of "a large portion of the libstd stubbed out".

That said, reading the list of missing functionality, I still don't think we want to stub those functions out, which would make programs calling them experience unexpected behavior at runtime. I would prefer to see those functions simply not provided at all, so that code calling them doesn't compile.

[bjorn3]

I would prefer to see those functions simply not provided at all, so that code calling them doesn't compile.

That would require cfging the user facing functions away, for which there is no precedent.

[nagisa]

Alternatively, the target could provide the portions of the standard library it supports as an ecosystem crate. There's another good reason to do so – right now this PR implements a number of synchronization primitives… AFAICT from mostly scratch. It isn't clear to me if libstd is the best place for implementations like these.

[joshtriplett]

@bjorn3 That's true, but I think it's a possibility we should discuss, for this and future targets. I think it's preferable to omit functions rather than stub them out, based on the general principle that Rust tries to catch issues at compile time whenever possible rather than allowing them to become runtime errors or unexpected behavior.

@nagisa That seems like a reasonable possibility as well. We can always add an implementation of std at a later time.

[kawadakk]

right now this PR implements a number of synchronization primitives… AFAICT from mostly scratch. It isn't clear to me if libstd is the best place for implementations like these.

There are two to be exact: Condvar and Thread::join. I think it's unavoidable (and reasonable) for libstd to supplement some synchronization primitives, since it has to expose a uniform set of primitives for every platform despite the fact that some of them might not be supported directly. E.g., Condvar in Windows XP and earlier, Thread::unpark in all platforms, and everything but Thread::join in HermitCore.

[yaahc]

Regarding the unimplemented stubs vs cfg disabled portions of std, is there any reason why we couldn't start with stabilizing this with unimplemented APIs then defer finding a way to turn those panicking interfaces into compiler errors in the future? My feeling is that we shouldn't consider that a breaking change since these functions would already not really be available on these platforms in practice.

Though, thinking about it I can think of a situation where this might still be an issue. Where some of the code calls those panicking methods, but those interfaces end up being unused on this target, and removing the std apis would turn a runtime error that never occurs into a compiler error that always does.

I ask because I'm hesitant to block this PR on answering all the questions around conditional compilation for std.

[bors]

📌 Commit da9ca41 has been approved by nagisa,estebank,m-ou-se,

[bors]

⌛ Testing commit da9ca41 with merge 1d71ba8...

[bors]

☀️ Test successful - checks-actions
Approved by: nagisa,estebank,m-ou-se,
Pushing 1d71ba8 to master...

[rust-timer]

Finished benchmarking commit (1d71ba8): comparison url.

Summary: This benchmark run did not return any relevant changes.

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression