pvh: Support booting via PVH ELFNOTE

This PR adds support for booting via the Xen HVM direct boot ABI.
  See: https://xenbits.xen.org/docs/4.12-testing/misc/pvh.html

This uses a 32-bit unpaged entry point, so we just point it at
ram32_start. This allows our firmware to be used with QEMU's -kernel
option.

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

Author: josephlr

layout.ld

@@ -2,6 +2,7 @@ ENTRY(ram64_start)
 
 PHDRS
 {
+  notes  PT_NOTE               ;
   rodata PT_LOAD FILEHDR PHDRS ;
   data   PT_LOAD               ;
   text   PT_LOAD               ;
@@ -23,6 +24,7 @@ SECTIONS
 	. = ram_min;
 	. += SIZEOF_HEADERS;
 
+	.notes  : { *(.note .note.* .notes)         } :rodata :notes
 	.rodata : { *(.rodata .rodata.*)            } :rodata
 	.data   : { *(.data .data.*) *(.bss .bss.*) } :data
 	.text   : {

src/asm/notes.s

@@ -0,0 +1,20 @@
+.section .notes, "a"
+
+# From xen/include/public/elfnote.h, "Physical entry point into the kernel."
+XEN_ELFNOTE_PHYS32_ENTRY = 18
+
+# We don't bother defining an ELFNOTE macro, as we only have one note.
+# This is equialent to the kernel's:
+# ELFNOTE(Xen, XEN_ELFNOTE_PHYS32_ENTRY, .long ram32_start)
+.align 4
+    .long name_end - name_start    # namesz
+    .long desc_end - desc_start    # descsz
+    .long XEN_ELFNOTE_PHYS32_ENTRY # type
+name_start:
+    .asciz "Xen"
+name_end:
+.align 4
+desc_start:
+    .long ram32_start
+desc_end:
+.align 4

src/main.rs

@@ -41,6 +41,8 @@ mod virtio;
 #[cfg(all(not(test), feature = "rom"))]
 global_asm!(include_str!("asm/rom.s"));
 #[cfg(not(test))]
+global_asm!(include_str!("asm/notes.s"));
+#[cfg(not(test))]
 global_asm!(include_str!("asm/ram32.s"));
 #[cfg(not(test))]
 global_asm!(include_str!("asm/ram64.s"));