Add example kernel that directly exits QEMU for testing on CI

Author: phil-opp

example-kernel/.gitignore

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+/target/
+**/*.rs.bk

example-kernel/Cargo.lock

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+[package]
+name = "example-kernel"
+version = "0.1.0"
+authors = ["Philipp Oppermann <[email protected]>"]
+edition = "2018"
+
+[dependencies]
+x86_64 = "0.3.4"

example-kernel/Cargo.toml

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+#![no_std] // don't link the Rust standard library
+#![no_main] // disable all Rust-level entry points
+
+use core::panic::PanicInfo;
+
+/// This function is called on panic.
+#[panic_handler]
+fn panic(_info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[no_mangle] // don't mangle the name of this function
+pub extern "C" fn _start() -> ! {
+    // this function is the entry point, since the linker looks for a function
+    // named `_start` by default
+
+    // exit QEMU (see https://os.phil-opp.com/integration-tests/#shutting-down-qemu)
+    unsafe { exit_qemu(); }
+
+    loop {}
+}
+
+pub unsafe fn exit_qemu() {
+    use x86_64::instructions::port::Port;
+
+    let mut port = Port::<u32>::new(0xf4);
+    port.write(61); // exit code is (61 << 1) | 1 = 123
+}

example-kernel/src/main.rs

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+{
+    "llvm-target": "x86_64-unknown-none",
+    "data-layout": "e-m:e-i64:64-f80:128-n8:16:32:64-S128",
+    "arch": "x86_64",
+    "target-endian": "little",
+    "target-pointer-width": "64",
+    "target-c-int-width": "32",
+    "os": "none",
+    "executables": true,
+    "linker-flavor": "ld.lld",
+    "linker": "rust-lld",
+    "panic-strategy": "abort",
+    "disable-redzone": true,
+    "features": "-mmx,-sse,+soft-float"
+  }