rust64_start: Read kernel boot params from %rsi

This works because Option<&T> has the same FFI layout as *const T
except that a null pointer is None.

Signed-off-by: Joe Richey <joerichey@google.com>

Author: josephlr

src/asm/ram64.s

@@ -11,4 +11,5 @@ ram64_start:
     # Setup the stack (at the end of our RAM region)
     movq $ram_max, %rsp
 
+    # BootParams are in %rsi, the second paramter of the System V ABI.
     jmp rust64_start

src/main.rs

@@ -153,9 +153,16 @@ fn boot_from_device(device: &mut block::VirtioBlockDevice) -> bool {
     true
 }
 
-#[cfg_attr(not(test), no_mangle)]
-pub extern "C" fn rust64_start() -> ! {
-    log!("\nStarting..");
+#[no_mangle]
+pub extern "C" fn rust64_start(_rdi: *const (), rsi: Option<&boot::Params>) -> ! {
+    if let Some(boot_params) = rsi {
+        log!("\nBooting via Linux Boot Protocol");
+        run(boot_params)
+    }
+    panic!("Unable to determine boot protocol")
+}
+
+fn run(info: &dyn boot::Info) -> ! {
     enable_sse();
     paging::setup();