set type_of_loader after measurements

QEMU assumes all SEV virtual machines use OVMF and EFI stub for booting, and leaves type_of_loader as 0. In stage0, after reading from fw_cfg, the type_of_loader is overwritten to 0. However, the guest kernel needs a non-zero value for direct kernel boot. This patch adds a setter function for type_of_loader, and set it to 0xFF after measurement calculation.

Bug: b/343736249
Change-Id: If388eced6de4e28c5784eca878584bc671008050

stage0/src/lib.rs

@@ -306,6 +306,11 @@ pub fn rust64_start(encrypted: u64) -> ! {
             })
             .unwrap_or_default();
 
+    // QEMU assumes all SEV VMs use OVMF, and leaves type_of_loader 0x00. This
+    // would make the kernel unable to load stage1. We need to fix it to make
+    // stage1 loads smoothly.
+    zero_page.set_type_of_loader(zero_page::BOOT_LOADER_TYPE_UNDEFINED);
+
     let memory_map_sha2_256_digest = measure_byte_slice(zero_page.e820_table().as_bytes());
 
     // Generate Stage0 Event Log data.

stage0/src/zero_page.rs

@@ -44,14 +44,16 @@ impl Default for ZeroPage {
     }
 }
 
+pub const BOOT_LOADER_TYPE_UNDEFINED: u8 = 0xFF;
+
 impl ZeroPage {
     /// Constructs a empty zero page, filling in some magic values needed by the
     /// kernel.
     pub fn new() -> Self {
         let mut zero_page = BootParams::zeroed();
         // Magic constants.
-        // See https://www.kernel.org/doc/html/latest/x86/boot.html#the-real-mode-kernel-header for more details.
-        zero_page.hdr.type_of_loader = 0xFF; // loader type undefined
+        // See https://www.kernel.org/doc/html/latest/arch/x86/boot.html#the-real-mode-kernel-header for more details.
+        zero_page.hdr.type_of_loader = BOOT_LOADER_TYPE_UNDEFINED; // loader type undefined
         zero_page.hdr.boot_flag = 0xAA55; // magic number
         zero_page.hdr.header = 0x53726448; // Magic "HdrS" string
         zero_page.hdr.kernel_alignment = 0x1000000; // Magic number from crosvm source.
@@ -345,6 +347,11 @@ impl ZeroPage {
         // maximum of 1GiB of RAM.
         self.inner.hdr.ramdisk_size = ram_disk.len() as u32;
     }
+
+    /// Sets the type of loader for direct kernel boot
+    pub fn set_type_of_loader(&mut self, loader_type: u8) {
+        self.inner.hdr.type_of_loader = loader_type;
+    }
 }
 
 /// Builds an E820 table by reading the low and high memory amount from CMOS.