Tier 3 target proposal: x86_64-unknown-none (freestanding/bare-metal x86-64)

[joshtriplett]

This compiler MCP proposes a new target, x86_64-unknown-none, for standalone
or "freestanding" x86-64 binaries with no operating system, using
ELF as the
object format. This target is intended for firmware, kernels, modules, and
other software running without an operating system.

This target will not provide an implementation of std, since that would
depend on an operating system. This target will not provide a default
allocator, but can provide an implementation of alloc if the user of the
target supplies an allocator.

The x86_64-unknown-none target will default to not allowing the use of any
vector or floating-point registers, because kernels, modules, and similar
software needs special care to use such registers. For instance, the Linux
kernel normally optimizes the entry and exit of the kernel by not saving and
restoring such registers; kernel code must explicitly request the use of such
registers before running code that uses them, and once requested, the kernel
must pay the additional overhead of saving the userspace registers to make way
for the kernel usage. Users of the x86_64-unknown-none target can still opt
into the use of such registers via the target-feature mechanism, either for
an entire program or for individual functions.

This target has a lot in common with x86_64-unknown-none-linuxkernel, but is
intended to be general enough to work for any standalone/freestanding software
without requiring a specific target for each such environment.

I plan to work to get this target to tier 2, but the target needs to start out
at tier 3 and work its way up.

Checking off the target tier policy requirements for tier 3:

• 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.)

Harald Hoyer harald@profian.com, https://github.com/haraldh
Mike Leany, https://github.com/mikeleany

• 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.

Standalone targets have had a variety of names in different compilers. Some
compilers and embedded toolchains use the name x86_64-unknown-elf for such
targets; in other toolchains, however, -elf may imply some minimal amount of
C library support. Rust has several standalone targets for various
architectures; RISC-V standalone targets use none-elf (e.g.
riscv64gc-unknown-none-elf), while most other standalone targets use just
none (e.g. aarch64-unknown-none, mipsel-unknown-none) or none with an
unrelated suffix for the ABI (e.g. armv7r-none-eabi, armv7a-none-eabi).
There is also precedent in the form of the x86_64-unknown-none-linuxkernel
and x86_64-unknown-none-hermitkernel targets.

This target proposal chooses to use x86_64-unknown-none, to avoid any
connotations of library support, for consistency with more existing Rust
targets, and for consistency with the existing Rust x86_64 kernel-specific
targets.

• 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 x86_64-unknown-none target will not introduce any legal issues or new
legal terms.

• The target must not introduce license incompatibilities.

✅ The x86_64-unknown-none target will not introduce any new dependencies or
licenses at all.

• Anything added to the Rust repository must be under the standard Rust
  license (MIT OR Apache-2.0).

✅ The pull requests adding and maintaining this target will of course be under
the standard Rust license.

• 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.

✅ This target adds no new dependencies of any kind.

• 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.

✅ The target will not support host tools (nor does the target have any
proprietary libraries, or any libraries at all).

• Targets should not require proprietary (non-FOSS) components to link a
  functional binary or library.

✅ The target will use a normal toolchain.

• "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.

✅ Understood.

• 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.

✅

I'm being paid to work on this target. I am not part of any approval decisions
regarding the tier status of the target.

• 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.

✅ The x86_64-unknown-none target will implement core, and will support
users using alloc with their own memory allocator. The x86_64-unknown-none
target will not support std, and will not provide a default memory allocator.

• 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.

✅ Building for the target does not require any special considerations.
However, for completeness, the PR adding the target will provide documentation
to that effect.

The target does not support running tests.

• 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.

✅ Understood.

• 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.

✅ This target has some common functionality with the
x86_64-unknown-none-linuxkernel target. After adding the
x86_64-unknown-none target, a follow-up PR may coordinate with the
maintainers of the Linux kernel target to share this functionality. The
addition of x86_64-unknown-none will not knowingly break any other target.

Work sponsored by Profian.

Process

The main points of the Major Change Process are as follows:

• File an issue describing the proposal.
• A compiler team member or contributor who is knowledgeable in the area can second by writing @rustbot second.
  • Finding a "second" suffices for internal changes. If however, you are proposing a new public-facing feature, such as a -C flag, then full team check-off is required.
  • Compiler team members can initiate a check-off via @rfcbot fcp merge on either the MCP or the PR.
• Once an MCP is seconded, the Final Comment Period begins. If no objections are raised after 10 days, the MCP is considered approved.

You can read more about Major Change Proposals on forge.

Comments

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

[rustbot]

This issue is not meant to be used for technical discussion. There is a Zulip stream for that. Use this issue to leave procedural comments, such as volunteering to review, indicating that you second the proposal (or third, etc), or raising a concern that you would like to be addressed.

cc @rust-lang/compiler @rust-lang/compiler-contributors

[joshtriplett]

(Fixed the issue title, and the Zulip stream link.)

[wesleywiser]

@rustbot second

[apiraino]

@rustbot label -final-comment-period +major-change-accepted