arch: Support loading initramfs with PVH boot protocol

Fill and write to guest memory the necessary boot module
structure to allow a guest using the PVH boot protocol
to load an initramfs image.

Signed-off-by: Alejandro Jimenez <alejandro.j.jimenez@oracle.com>

arch/src/lib.rs

@@ -43,6 +43,8 @@ pub enum Error {
     StartInfoSetup,
     /// Failed to compute initramfs address.
     InitramfsAddress,
+    /// Error writing module entry to guest memory.
+    ModlistSetup(vm_memory::GuestMemoryError),
 }
 pub type Result<T> = result::Result<T, Error>;
 

arch/src/x86_64/layout.rs

@@ -31,6 +31,10 @@ pub const BOOT_IDT_START: GuestAddress = GuestAddress(0x520);
 /// Address for the hvm_start_info struct used in PVH boot
 pub const PVH_INFO_START: GuestAddress = GuestAddress(0x6000);
 
+/// Starting address of array of modules of hvm_modlist_entry type.
+/// Used to enable initrd support using the PVH boot ABI.
+pub const MODLIST_START: GuestAddress = GuestAddress(0x6040);
+
 /// Address of memory map table used in PVH boot. Can overlap
 /// with the zero page address since they are mutually exclusive.
 pub const MEMMAP_START: GuestAddress = GuestAddress(0x7000);

arch/src/x86_64/mod.rs

@@ -16,7 +16,9 @@ pub mod regs;
 use crate::InitramfsConfig;
 use crate::RegionType;
 use linux_loader::loader::bootparam::{boot_params, setup_header};
-use linux_loader::loader::elf::start_info::{hvm_memmap_table_entry, hvm_start_info};
+use linux_loader::loader::elf::start_info::{
+    hvm_memmap_table_entry, hvm_modlist_entry, hvm_start_info,
+};
 use std::mem;
 use vm_memory::{
     Address, ByteValued, Bytes, GuestAddress, GuestMemory, GuestMemoryMmap, GuestMemoryRegion,
@@ -68,6 +70,11 @@ struct MemmapTableEntryWrapper(hvm_memmap_table_entry);
 
 unsafe impl ByteValued for MemmapTableEntryWrapper {}
 
+#[derive(Copy, Clone, Default)]
+struct ModlistEntryWrapper(hvm_modlist_entry);
+
+unsafe impl ByteValued for ModlistEntryWrapper {}
+
 // This is a workaround to the Rust enforcement specifying that any implementation of a foreign
 // trait (in this case `DataInit`) where:
 // *    the type that is implementing the trait is foreign or
@@ -164,7 +171,7 @@ pub fn configure_system(
 
     match boot_prot {
         BootProtocol::PvhBoot => {
-            configure_pvh(guest_mem, cmdline_addr, rsdp_addr)?;
+            configure_pvh(guest_mem, cmdline_addr, initramfs, rsdp_addr)?;
         }
         BootProtocol::LinuxBoot => {
             configure_64bit_boot(
@@ -184,6 +191,7 @@ pub fn configure_system(
 fn configure_pvh(
     guest_mem: &GuestMemoryMmap,
     cmdline_addr: GuestAddress,
+    initramfs: &Option<InitramfsConfig>,
     rsdp_addr: Option<GuestAddress>,
 ) -> super::Result<()> {
     const XEN_HVM_START_MAGIC_VALUE: u32 = 0x336ec578;
@@ -200,6 +208,24 @@ fn configure_pvh(
         start_info.0.rsdp_paddr = rsdp_addr.0;
     }
 
+    if let Some(initramfs_config) = initramfs {
+        // The initramfs has been written to guest memory already, here we just need to
+        // create the module structure that describes it.
+        let ramdisk_mod: ModlistEntryWrapper = ModlistEntryWrapper(hvm_modlist_entry {
+            paddr: initramfs_config.address.raw_value(),
+            size: initramfs_config.size as u64,
+            ..Default::default()
+        });
+
+        start_info.0.nr_modules += 1;
+        start_info.0.modlist_paddr = layout::MODLIST_START.raw_value();
+
+        // Write the modlist struct to guest memory.
+        guest_mem
+            .write_obj(ramdisk_mod, layout::MODLIST_START)
+            .map_err(super::Error::ModlistSetup)?;
+    }
+
     // Vector to hold the memory maps which needs to be written to guest memory
     // at MEMMAP_START after all of the mappings are recorded.
     let mut memmap: Vec<hvm_memmap_table_entry> = Vec::new();