x86_64-unknown-none: Add target documentation

In particular, document the default properties and assumptions of code
built for the target.

(Work on this target sponsored by Profian.)

src/doc/rustc/src/platform-support.md

@@ -285,7 +285,7 @@ target | std | host | notes
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku
 `x86_64-unknown-hermit` | ? |  |
 `x86_64-unknown-l4re-uclibc` | ? |  |
-`x86_64-unknown-none` | * |  | Freestanding/bare-metal x86_64, softfloat
+[`x86_64-unknown-none`](platform-support/x86_64-unknown-none.md) | * |  | Freestanding/bare-metal x86_64, softfloat
 `x86_64-unknown-none-hermitkernel` | ? |  | HermitCore kernel
 `x86_64-unknown-none-linuxkernel` | * |  | Linux kernel modules
 `x86_64-unknown-openbsd` | ✓ | ✓ | 64-bit OpenBSD

src/doc/rustc/src/platform-support/x86_64-unknown-none.md

@@ -0,0 +1,76 @@
+# `x86_64-unknown-none`
+
+**Tier: 3**
+
+Freestanding/bare-metal x86-64 binaries in ELF format: firmware, kernels, etc.
+
+## Target maintainers
+
+Harald Hoyer <harald@profian.com>, https://github.com/haraldh
+Mike Leany, https://github.com/mikeleany
+
+## Requirements
+
+This target is cross-compiled. There is no support for `std`. There is no
+default allocator, but it's possible to use `alloc` by supplying an allocator.
+
+By default, Rust code generated for this target does not use any vector or
+floating-point registers (e.g. SSE, AVX). This allows the generated code to run
+in environments, such as kernels, which may need to avoid the use of such
+registers or which may have special considerations about the use of such
+registers (e.g. saving and restoring them to avoid breaking userspace code
+using the same registers). You can change code generation to use additional CPU
+features via the `-C target-feature=` codegen options to rustc, or via the
+`#[target_feature]` mechanism within Rust code.
+
+By default, code generated with this target should run on any `x86_64`
+hardware; enabling additional target features may raise this baseline.
+
+Code generated with this target will use the `kernel` code model by default.
+You can change this using the `-C code-model=` option to rustc.
+
+On `x86_64-unknown-none`, `extern "C"` uses the [standard System V calling
+convention](https://gitlab.com/x86-psABIs/x86-64-ABI), without red zones.
+
+This target generated binaries in the ELF format. Any alternate formats or
+special considerations for binary layout will require linker options or linker
+scripts.
+
+## Building the target
+
+You can build Rust with support for the target by adding it to the `target`
+list in `config.toml`:
+
+```toml
+[build]
+build-stage = 1
+target = ["x86_64-unknown-none"]
+```
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for this target. To compile for
+this target, you will either need to build Rust with the target enabled (see
+"Building the target" above), or build your own copy of `core` by using
+`build-std` or similar.
+
+## Testing
+
+As `x86_64-unknown-none` supports a variety of different environments and does
+not support `std`, this target does not support running the Rust testsuite.
+
+## Cross-compilation toolchains and C code
+
+If you want to compile C code along with Rust (such as for Rust crates with C
+dependencies), you will need an appropriate `x86_64` toolchain.
+
+Rust *may* be able to use an `x86_64-linux-gnu-` toolchain with appropriate
+standalone flags to build for this toolchain (depending on the assumptions of
+that toolchain, see below), or you may wish to use a separate
+`x86_64-unknown-none` (or `x86_64-elf-`) toolchain.
+
+On some `x86_64` hosts that use ELF binaries, you *may* be able to use the host
+C toolchain, if it does not introduce assumptions about the host environment
+that don't match the expectations of a standalone environment. Otherwise, you
+may need a separate toolchain for standalone/freestanding development, just as
+when cross-compiling from a non-`x86_64` platform.