compiler: Add *-unikraft-linux-musl documentation

Signed-off-by: Martin Kröning <martin.kroening@eonerc.rwth-aachen.de>

Author: mkroening

src/doc/rustc/src/SUMMARY.md

@@ -42,6 +42,7 @@
     - [sparc-unknown-none-elf](./platform-support/sparc-unknown-none-elf.md)
     - [*-pc-windows-gnullvm](platform-support/pc-windows-gnullvm.md)
     - [\*-nto-qnx-\*](platform-support/nto-qnx.md)
+    - [*-unikraft-linux-musl](platform-support/unikraft-linux-musl.md)
     - [*-unknown-hermit](platform-support/hermit.md)
     - [\*-unknown-netbsd\*](platform-support/netbsd.md)
     - [*-unknown-openbsd](platform-support/openbsd.md)

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

@@ -326,6 +326,7 @@ target | std | host | notes
 [`x86_64-pc-windows-gnullvm`](platform-support/pc-windows-gnullvm.md) | ✓ | ✓ |
 `x86_64-pc-windows-msvc` | * |  | 64-bit Windows XP support
 `x86_64-sun-solaris` | ? |  | Deprecated target for 64-bit Solaris 10/11, illumos
+[`x86_64-unikraft-linux-musl`](platform-support/unikraft-linux-musl.md) | ✓ |   | 64-bit Unikraft with musl
 `x86_64-unknown-dragonfly` | ✓ | ✓ | 64-bit DragonFlyBSD
 `x86_64-unknown-haiku` | ✓ | ✓ | 64-bit Haiku
 [`x86_64-unknown-hermit`](platform-support/hermit.md) | ✓ |  | x86_64 Hermit

src/doc/rustc/src/platform-support/unikraft-linux-musl.md

@@ -0,0 +1,67 @@
+# `*-unikraft-linux-musl`
+
+**Tier: 3**
+
+Targets for the [Unikraft] Unikernel Development Kit (with musl).
+
+[Unikraft]: https://unikraft.org/
+
+Target triplets available so far:
+
+- `x86_64-unikraft-linux-musl`
+
+## Target maintainers
+
+- Martin Kröning ([@mkroening](https://github.com/mkroening))
+
+## Requirements
+
+These targets only support cross-compilation.
+The targets do support std.
+
+Unikraft pretends to behave exactly like Linux.
+How much of that functionality is available depends on the individual unikernel configuration.
+For example, the basic Unikraft + musl config does not support `poll` or networking out of the box.
+That functionality requires enabling [`LIBPOSIX_EVENT`] or [lwIP] respectively.
+
+[`LIBPOSIX_EVENT`]: https://github.com/unikraft/unikraft/blob/RELEASE-0.13.1/lib/posix-event/Config.uk
+[lwIP]: https://github.com/unikraft/lib-lwip
+
+The Unikraft targets follow Linux's `extern "C"` calling convention.
+
+For these targets, `rustc` does not perform the final linking step.
+Instead, the Unikraft build system will produce the final Unikernel image for the selected platform (e.g., KVM, Linux user space, and Xen).
+
+## Building the targets
+
+You can build Rust with support for the targets by adding it to the `target` list in `config.toml`:
+
+```toml
+[build]
+build-stage = 1
+target = [ "x86_64-unikraft-linux-musl" ]
+```
+
+## Building Rust programs
+
+Rust does not yet ship pre-compiled artifacts for these targets.
+To compile for these targets, you will either need to build Rust with the targets enabled
+(see “Building the targets” above), or build your own copy of `core` by using `build-std` or similar.
+
+Linking requires a [KraftKit] shim.
+See [unikraft/kraftkit#612] for more information.
+
+[KraftKit]: https://github.com/unikraft/kraftkit
+[unikraft/kraftkit#612]: https://github.com/unikraft/kraftkit/issues/612
+
+## Testing
+
+The targets do support running binaries in the form of unikernel images.
+How the unikernel image is run depends on the specific platform (e.g., KVM, Linux user space, and Xen).
+The targets do not support running the Rust test suite.
+
+## Cross-compilation toolchains and C code
+
+The targets do support C code.
+To build compatible C code, you have to use the same compiler and flags as does the Unikraft build system for your specific configuration.
+The easiest way to achieve that, is to build the C code with the Unikraft build system when building your unikernel image.