Use x86_64-unknown-uefi target

Rust now supports uefi as a build target: rust-lang/rust#56769

This change mainly eliminates references to the custom JSON target
file. It also requires that the entry point's name be changed to
`efi_main`.

Note that the "C" abi is now correct for EFI applications, but will
still be incorrect when uefi-rs is brought in as an external dependancy
for a different target. This means the entry point should always be
`extern "C"` while the table of function pointers should be `extern
"win64"`.

Author: josephlr

BUILDING.md

@@ -2,8 +2,8 @@
 
 UEFI applications are simple COFF (Windows) executables, with the special
 `EFI_Application` subsystem, and some limitations (such as no dynamic linking).
-
-The `x86_64-uefi.json` file describes a custom target for building UEFI apps.
+[Rust supports building UEFI applications](https://github.com/rust-lang/rust/pull/56769)
+though the `x86_64-unknown-uefi` target.
 
 ## Prerequisites
 
@@ -19,15 +19,12 @@ The following steps allow you to build a simple UEFI app.
 
 ```rust
 #[no_mangle]
-pub extern "win64" fn uefi_start(handle: Handle, system_table: SystemTable<Boot>) -> Status;
+pub extern "C" fn efi_main(handle: Handle, system_table: SystemTable<Boot>) -> Status;
 ```
 
-- Copy the `uefi-test-runner/x86_64-uefi.json` target file to your project's root.
-  You can customize it.
-
-- Build using `cargo xbuild --target x86_64-uefi`.
+- Build using `cargo xbuild --target x86_64-unknown-uefi`.
 
-- The `target` directory will contain a `x86_64-uefi` subdirectory,
+- The `target` directory will contain a `x86_64-unknown-uefi` subdirectory,
   where you will find the `uefi_app.efi` file - a normal UEFI executable.
 
 - To run this on a real computer:

README.md

@@ -8,7 +8,9 @@
 OS loaders, hypervisors and other low-level applications. While it started out
 as x86-specific, it has been adopted on other platforms, such as ARM.
 
-This crates makes it easy to write UEFI applications in Rust.
+This crate makes it easy to both:
+  - Write UEFI applications in Rust (via the [`x86_64-unknown-uefi`][rustc-uefi] target)
+  - Call UEFI functions from an OS (usually built with a [custom target][rustc-custom])
 
 The objective is to provide **safe** and **performant** wrappers for UEFI interfaces,
 and allow developers to write idiomatic Rust code.
@@ -20,6 +22,8 @@ and has been tested _only_ with **64-bit** UEFI.
 
 [UEFI]: https://en.wikipedia.org/wiki/Unified_Extensible_Firmware_Interface
 [gm-blog]: https://medium.com/@gil0mendes/an-efi-app-a-bit-rusty-82c36b745f49
+[rustc-uefi]: https://github.com/rust-lang/rust/pull/56769
+[rustc-custom]: https://doc.rust-lang.org/rustc/targets/custom.html
 
 ![uefi-rs running in QEMU](https://imgur.com/SFPSVuO.png)
 
@@ -49,7 +53,7 @@ This project contains multiple sub-crates:
 
 ## Building kernels which use UEFI
 
-This crate makes it easy to start buildimg simple applications with UEFI.
+This crate makes it easy to start building simple applications with UEFI.
 However, there are some limitations you should be aware of:
 
 - The global logger / allocator **can only be set once** per binary.
@@ -92,8 +96,6 @@ An example UEFI app is built in the `uefi-test-runner` directory.
 
 Check out the testing [README.md](uefi-test-runner/README.md) for instructions on how to run the crate's tests.
 
-This repo also contains a `x86_64-uefi.json` file, which is a custom Rust target for 64-bit UEFI applications.
-
 ## Building UEFI programs
 
 For instructions on how to create your own UEFI apps, see the [BUILDING.md](BUILDING.md) file.

uefi-test-runner/build.py

@@ -23,7 +23,7 @@
     # Run QEMU without showing GUI
     'headless': False,
     # Target to build for.
-    'target': 'x86_64-uefi',
+    'target': 'x86_64-unknown-uefi',
     # Configuration to build.
     'config': 'debug',
     # QEMU executable to use

uefi-test-runner/src/main.rs

@@ -19,7 +19,7 @@ mod boot;
 mod proto;
 
 #[no_mangle]
-pub extern "win64" fn uefi_start(image: uefi::Handle, st: SystemTable<Boot>) -> Status {
+pub extern "C" fn efi_main(image: uefi::Handle, st: SystemTable<Boot>) -> Status {
     // Initialize utilities (logging, memory allocation...)
     uefi_services::init(&st).expect_success("Failed to initialize utilities");