Use aarch64-rt in bare metal AP section and exercise (#2751)

Also use `global_asm!` rather than `cc::Build` to build assembly files.
This avoids a dependency and some build scripts.

Author: qwandor

src/SUMMARY.md

@@ -361,6 +361,7 @@
   - [Logging](bare-metal/aps/logging.md)
     - [Using It](bare-metal/aps/logging/using.md)
   - [Exceptions](bare-metal/aps/exceptions.md)
+  - [aarch64-rt](bare-metal/aps/aarch64-rt.md)
   - [Other Projects](bare-metal/aps/other-projects.md)
 - [Useful Crates](bare-metal/useful-crates.md)
   - [`zerocopy`](bare-metal/useful-crates/zerocopy.md)

src/bare-metal/aps/aarch64-rt.md

@@ -0,0 +1,15 @@
+# aarch64-rt
+
+The `aarch64-rt` crate provides the assembly entry point and exception vector
+that we implemented before. We just need to mark our main function with the
+`entry!` macro.
+
+It also provides the `initial_pagetable!` macro to let us define an initial
+static pagetable in Rust, rather than in assembly code like we did before.
+
+We can also use the UART driver from the `arm-pl011-uart` crate rather than
+writing our own.
+
+```rust,editable,compile_fail
+{{#include examples/src/main_rt.rs:main}}
+```

src/bare-metal/aps/entry-point.md

@@ -3,12 +3,12 @@
 Before we can start running Rust code, we need to do some initialisation.
 
 ```armasm
-{{#include examples/entry.S:entry}}
+{{#include examples/src/entry.S:entry}}
 ```
 
 <details>
 
-This code is in `src/bare-metal/aps/examples/entry.S`. It's not necessary to
+This code is in `src/bare-metal/aps/examples/src/entry.S`. It's not necessary to
 understand this in detail -- the takeaway is that typically some low-level setup
 is needed to meet Rust's expectations of the system.
 

src/bare-metal/aps/examples/Cargo.lock

@@ -7,14 +7,14 @@ edition = "2021"
 publish = false
 
 [dependencies]
+aarch64-paging = { version = "0.9.1", default-features = false }
+aarch64-rt = "0.1.3"
+arm-pl011-uart = "0.3.1"
 bitflags = "2.9.0"
 log = "0.4.27"
 smccc = "0.2.0"
 spin = "0.10.0"
 
-[build-dependencies]
-cc = "1.2.20"
-
 [[bin]]
 name = "improved"
 path = "src/main_improved.rs"
@@ -27,6 +27,10 @@ path = "src/main_logger.rs"
 name = "minimal"
 path = "src/main_minimal.rs"
 
+[[bin]]
+name = "rt"
+path = "src/main_rt.rs"
+
 [[bin]]
 name = "psci"
 path = "src/main_psci.rs"

src/bare-metal/aps/examples/Cargo.toml

@@ -1,4 +1,4 @@
-// Copyright 2023 Google LLC
+// Copyright 2025 Google LLC
 //
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
@@ -12,16 +12,8 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-use cc::Build;
-use std::env;
+use core::arch::global_asm;
 
-fn main() {
-    env::set_var("CROSS_COMPILE", "aarch64-none-elf");
-    env::set_var("CC", "clang");
-
-    Build::new()
-        .file("entry.S")
-        .file("exceptions.S")
-        .file("idmap.S")
-        .compile("empty")
-}
+global_asm!(include_str!("entry.S"));
+global_asm!(include_str!("exceptions.S"));
+global_asm!(include_str!("idmap.S"));