Rollup merge of #127490 - ferrocene:hoverbear/add-riscv64gc-unknown-linux-target-page, r=pietroalbini

Add target page for riscv64gc-unknown-linux-gnu

I was reading https://github.com/rust-lang/rust/issues/113739 and realized I knew most of the information necessary to create the `riscv64gc-unknown-linux-gnu` target page.

src/doc/rustc/src/SUMMARY.md

@@ -65,6 +65,7 @@
     - [riscv32im-risc0-zkvm-elf](platform-support/riscv32im-risc0-zkvm-elf.md)
     - [riscv32imac-unknown-xous-elf](platform-support/riscv32imac-unknown-xous-elf.md)
     - [riscv32*-unknown-none-elf](platform-support/riscv32-unknown-none-elf.md)
+    - [riscv64gc-unknown-linux-gnu](platform-support/riscv64gc-unknown-linux-gnu.md)
     - [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)

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

@@ -97,7 +97,7 @@ target | notes
 `powerpc-unknown-linux-gnu` | PowerPC Linux (kernel 3.2, glibc 2.17)
 `powerpc64-unknown-linux-gnu` | PPC64 Linux (kernel 3.2, glibc 2.17)
 `powerpc64le-unknown-linux-gnu` | PPC64LE Linux (kernel 3.10, glibc 2.17)
-`riscv64gc-unknown-linux-gnu` | RISC-V Linux (kernel 4.20, glibc 2.29)
+[`riscv64gc-unknown-linux-gnu`](platform-support/riscv64gc-unknown-linux-gnu.md) | RISC-V Linux (kernel 4.20, glibc 2.29)
 `s390x-unknown-linux-gnu` | S390x Linux (kernel 3.2, glibc 2.17)
 `x86_64-unknown-freebsd` | 64-bit FreeBSD
 `x86_64-unknown-illumos` | illumos

src/doc/rustc/src/platform-support/riscv64gc-unknown-linux-gnu.md

@@ -0,0 +1,129 @@
+# `riscv64gc-unknown-linux-gnu`
+
+**Tier: 2 (with Host Tools)**
+
+RISC-V targets using the *RV64I* base instruction set with the *G* collection of extensions, as well as the *C* extension.
+
+
+## Target maintainers
+
+- Kito Cheng, <kito.cheng@gmail.com>, [@kito-cheng](https://github.com/kito-cheng)
+- Michael Maitland, <michaeltmaitland@gmail.com>, [@michaelmaitland](https://github.com/michaelmaitland)
+- Robin Randhawa, <robin.randhawa@sifive.com>, [@robin-randhawa-sifive](https://github.com/robin-randhawa-sifive)
+- Craig Topper, <craig.topper@sifive.com>, [@topperc](https://github.com/topperc)
+
+## Requirements
+
+This target requires:
+
+* Linux Kernel version 4.20 or later
+* glibc 2.17 or later
+
+
+## Building the target
+
+These targets are distributed through `rustup`, and otherwise require no
+special configuration.
+
+If you need to build your own Rust for some reason though, the targets can be
+enabled in `config.toml`. For example:
+
+```toml
+[build]
+target = ["riscv64gc-unknown-linux-gnu"]
+```
+
+
+## Building Rust programs
+
+
+On a RISC-V host, the `riscv64gc-unknown-linux-gnu` target should be automatically
+installed and used by default.
+
+On a non-RISC-V host, add the target:
+
+```bash
+rustup target add riscv64gc-unknown-linux-gnu
+```
+
+Then cross compile crates with:
+
+```bash
+cargo build --target riscv64gc-unknown-linux-gnu
+```
+
+
+## Testing
+
+There are no special requirements for testing and running the targets.
+For testing cross builds on the host, please refer to the "Cross-compilation
+toolchains and C code"
+section below.
+
+
+## Cross-compilation toolchains and C code
+
+A RISC-V toolchain can be obtained for Windows/Mac/Linux from the
+[`riscv-gnu-toolchain`](https://github.com/riscv-collab/riscv-gnu-toolchain)
+repostory. Binaries are available via
+[embecosm](https://www.embecosm.com/resources/tool-chain-downloads/#riscv-linux),
+and may also be available from your OS's package manager.
+
+On Ubuntu, a RISC-V toolchain can be installed with:
+
+```bash
+apt install gcc-riscv64-linux-gnu g++-riscv64-linux-gnu libc6-dev-riscv64-cross
+```
+
+Depending on your system, you may need to configure the target to use the GNU
+GCC linker. To use it, add the following to your `.cargo/config.toml`:
+
+```toml
+[target.riscv64gc-unknown-linux-gnu]
+linker = "riscv64-linux-gnu-gcc"
+```
+
+If your `riscv64-linux-gnu-*` toolchain is not in your `PATH` you may need to
+configure additional settings:
+
+```toml
+[target.riscv64gc-unknown-linux-gnu]
+# Adjust the paths to point at your toolchain
+cc = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-gcc"
+cxx = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-g++"
+ar = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-ar"
+ranlib = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-ranlib"
+linker = "/TOOLCHAIN_PATH/bin/riscv64-linux-gnu-gcc"
+```
+
+To test cross compiled binaries on a non-RISCV-V host, you can use
+[`qemu`](https://www.qemu.org/docs/master/system/target-riscv.html).
+On Ubuntu, a RISC-V emulator can be obtained with:
+
+```bash
+apt install qemu-system-riscv64
+```
+
+Then, in `.cargo/config.toml` set the `runner`:
+
+```toml
+[target.riscv64gc-unknown-linux-gnu]
+runner = "qemu-riscv64-static -L /usr/riscv64-linux-gnu -cpu rv64"
+```
+
+On Mac and Linux, it's also possible to use
+[`lima`](https://github.com/lima-vm/lima) to emulate RISC-V in a similar way to
+how WSL2 works on Windows:
+
+```bash
+limactl start template://riscv
+limactl shell riscv
+```
+
+Using [Docker (with BuildKit)](https://docs.docker.com/build/buildkit/) the
+[`riscv64/ubuntu`](https://hub.docker.com/r/riscv64/ubuntu) image can be used
+to buiild or run `riscv64gc-unknown-linux-gnu` binaries.
+
+```bash
+docker run --platform linux/riscv64 -ti --rm --mount "type=bind,src=$(pwd),dst=/checkout" riscv64/ubuntu bash
+```