pvh: Add code to read PVH Boot Protocol stucts

Note that this also requires zeroing out %rdi in the Linux Boot
Protocol path, so that the rdi parameter will have a valid repr.

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

Author: josephlr

layout.ld

@@ -1,4 +1,4 @@
-ENTRY(ram64_start)
+ENTRY(linux64_start)
 
 PHDRS
 {

src/asm/ram64.s

@@ -1,10 +1,15 @@
 .section .text, "ax"
-.global ram64_start
+.global linux64_start
 .code64
 
+linux64_start:
+    # Zero out %rdi, its value is unspecificed in the Linux Boot Protocol.
+    xorq %rdi, %rdi
+
 ram64_start:
     # Setup the stack (at the end of our RAM region)
     movq $ram_max, %rsp
 
+    # PVH start_info is in %rdi, the first paramter of the System V ABI.
     # BootParams are in %rsi, the second paramter of the System V ABI.
     jmp rust64_start

src/main.rs

@@ -140,7 +140,11 @@ fn boot_from_device(device: &mut block::VirtioBlockDevice, info: &dyn boot::Info
 }
 
 #[no_mangle]
-pub extern "C" fn rust64_start(_rdi: *const (), rsi: Option<&boot::Params>) -> ! {
+pub extern "C" fn rust64_start(rdi: Option<&pvh::StartInfo>, rsi: Option<&boot::Params>) -> ! {
+    if let Some(start_info) = rdi {
+        log!("\nBooting via PVH Boot Protocol");
+        run(start_info)
+    }
     if let Some(boot_params) = rsi {
         log!("\nBooting via Linux Boot Protocol");
         run(boot_params)