pvh: Move note definition/declaration to Rust

The structure of the ELF note can be done with pure Rust code. We can
definie the Name and Desc types and use a static struct to hold the
note.

Due to Rust's limitations on "pointer-to-integer cast", we have to have
Desc have a function pointer type, which means that field is now 8 bytes
long instead of 4. However, this doesn't seem to be an issue. The binary
still works w/ PVH Boot on QEMU and CH.

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

Author: josephlr

src/asm/mod.rs

@@ -1,4 +1,3 @@
-global_asm!(include_str!("note.s"));
 global_asm!(include_str!("ram32.s"));
 global_asm!(include_str!("ram64.s"));
 global_asm!(include_str!("gdt64.s"));

src/asm/note.s

@@ -1,3 +1,5 @@
+use core::mem::size_of;
+
 use crate::{
     boot::{E820Entry, Info},
     common,
@@ -53,3 +55,35 @@ impl Info for StartInfo {
         }
     }
 }
+
+// The PVH Boot Protocol starts at the 32-bit entrypoint to our firmware.
+extern "C" {
+    fn ram32_start();
+}
+
+// The kind/name/desc of the PHV ELF Note are from xen/include/public/elfnote.h.
+// This is the "Physical entry point into the kernel".
+const XEN_ELFNOTE_PHYS32_ENTRY: u32 = 18;
+type Name = [u8; 4];
+type Desc = unsafe extern "C" fn();
+
+#[repr(C, packed(4))]
+struct Note {
+    name_size: u32,
+    desc_size: u32,
+    kind: u32,
+    name: Name,
+    desc: Desc,
+}
+
+// This is: ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .quad ram32_start)
+#[cfg(not(test))]
+#[link_section = ".note"]
+#[used]
+static PVH_NOTE: Note = Note {
+    name_size: size_of::<Name>() as u32,
+    desc_size: size_of::<Desc>() as u32,
+    kind: XEN_ELFNOTE_PHYS32_ENTRY,
+    name: *b"Xen\0",
+    desc: ram32_start,
+};