Add target description for riscv32im-risc0-zkvm-elf

Co-authored-by: Frank Laub <github@frank.laub.io>

src/doc/rustc/src/SUMMARY.md

@@ -47,6 +47,7 @@
     - [mipsisa\*r6\*-unknown-linux-gnu\*](platform-support/mips-release-6.md)
     - [nvptx64-nvidia-cuda](platform-support/nvptx64-nvidia-cuda.md)
     - [powerpc64-ibm-aix](platform-support/aix.md)
+    - [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/riscv32imac-unknown-none-elf.md)
     - [sparc-unknown-none-elf](./platform-support/sparc-unknown-none-elf.md)

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

@@ -338,6 +338,7 @@ target | std | host | notes
 [`powerpc64-ibm-aix`](platform-support/aix.md) | ? |  | 64-bit AIX (7.2 and newer)
 `riscv32gc-unknown-linux-gnu` |   |   | RISC-V Linux (kernel 5.4, glibc 2.33)
 `riscv32gc-unknown-linux-musl` |   |   | RISC-V Linux (kernel 5.4, musl + RISCV32 support patches)
+[`riscv32im-risc0-zkvm-elf`](platform-support/riscv32im-risc0-zkvm-elf.md) | ? |  | RISC Zero's zero-knowledge Virtual Machine (RV32IM ISA)
 [`riscv32imac-unknown-xous-elf`](platform-support/riscv32imac-unknown-xous-elf.md) | ? |  | RISC-V Xous (RV32IMAC ISA)
 [`riscv32imc-esp-espidf`](platform-support/esp-idf.md) | ✓ |  | RISC-V ESP-IDF
 [`riscv32imac-esp-espidf`](platform-support/esp-idf.md) | ✓ |  | RISC-V ESP-IDF

src/doc/rustc/src/platform-support/riscv32im-risc0-zkvm-elf.md

@@ -0,0 +1,86 @@
+# `riscv32im-risc0-zkvm-elf`
+
+**Tier: 3**
+
+RISC Zero's Zero Knowledge Virtual Machine (zkVM) implementing the RV32IM instruction set.
+
+## Target maintainers
+
+- Frank Laub, `frank@risczero.com`, https://github.com/flaub
+- Jeremy Bruestle, `jeremy@risczero.com`, https://github.com/jbruestle
+- Erik Kaneda, `erik@risczero.com`, https://github.com/SchmErik
+
+## Background
+
+This target is an execution environment to produce a proof of execution of
+a RISC-V ELF binary and any output that the developer of the binary wishes to
+display publicly. In order to do this, the target will execute the ELF to
+generate a receipt containing the output of the computation along with a
+cryptographic seal. This receipt can be verified to ensure the integrity of the
+computation and its result. This target is implemented as software only; it has
+no hardware implementation.
+
+We have a cargo extension called [cargo-risczero] that allow users to generate
+project templates, install tools for improved user experience, build the binary
+using a docker environment and test programs.
+
+## Requirements
+
+The target only supports cross compilation and no host tools. The target
+supports `alloc` with a default allocator and has experimental support for
+`std`. The target expects the binaries to be in ELF.
+
+The target's execution environment is single threaded, non-preemptive, and does
+not support any privileged instructions, nor unaligned accesses. At the time of
+writing the VM has 192 MB of memory and text/data, heap, and stack need to be
+with in the address range `0x400` - `0x0C000000`. The binaries themselves expect
+no operating system and can be thought of as running on bare-metal. The target
+does not use `#[target_feature(...)]` or `-C target-feature=` values.
+
+Calling `extern "C"` on the target uses the C calling convention outlined in the
+[RISC-V specification].
+
+## Building for the zkVM
+
+Programs for the zkVM could be built by adding it to the `target` list in
+`config.toml`. However, we recommend building programs in our starter template
+generated by the [cargo-risczero] utility and the [risc0-build] crate. This
+crate calls `rustc` with `-C "link-arg=-Ttext=` so that it maps the text in the
+appropriate location as well as generating variables that represent the ELF and
+a unique ID associated with the ELF. The starter template provides developers
+with system calls that are useful to zero knowledge computing such as writing to
+the public output, hashing using sha256, and multiply big integers.
+
+## 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). We do not recommend using `build-std` as we have
+run into issues building core in the past on our starter template. An alternate
+solution is to download the risc0 tool chain by running `cargo risczero install`.
+
+## Testing
+
+Note: the target is implemented as a software emulator called the zkVM and there
+is no hardware implementation of the target.
+
+The most practical way to test the target program is to use our starter template
+that can be generated by using the `cargo risczero new` command. The template
+generates a sample "host" and "guest" code. The guest code compiled to the
+target (which is RV32IM) whereas the "host" code is compiled to run on the
+programmer's machine running either a Linux distribution or macOS. The host
+program is responsible for running the guest binary on the zkVM and retrieving
+its public output.
+
+The target currently does not support running the Rust test suite.
+
+## Cross-compilation toolchains and C code
+
+Compatible C code can be built for this target on any compiler that has a RV32IM
+target.  On clang and ld.lld linker, it can be generated using the
+`-march=rv32im`, `-mabi=ilp32` with llvm features flag `features=+m` and llvm
+target `riscv32-unknown-none`.
+
+[RISC-V specification]: https://riscv.org/wp-content/uploads/2015/01/riscv-calling.pdf
+[cargo-risczero]: https://docs.rs/cargo-risczero/latest/cargo_risczero/
+[risc0-build]: https://crates.io/crates/risc0-build