Rework: LegacyFrameAllocator::construct_memory_map

The new implementation takes in a list of slices that are used by the
bootloader. It then goes through all regions and checks if they overlap
any of those slices. If so, the region is split and this process is
started recursively until none of the usable regions overlap the memory
used by the bootloader.

bios/common/src/lib.rs

@@ -5,6 +5,7 @@ pub mod racy_cell;
 #[cfg_attr(feature = "debug", derive(Debug))]
 #[repr(C)]
 pub struct BiosInfo {
+    pub stage_3: Region,
     pub stage_4: Region,
     pub kernel: Region,
     pub ramdisk: Region,

bios/stage-2/src/main.rs

@@ -145,6 +145,10 @@ fn start(disk_number: u16, partition_table_start: *const u8) -> ! {
     vesa_mode.enable().unwrap();
 
     let mut info = BiosInfo {
+        stage_3: Region {
+            start: STAGE_3_DST as u64,
+            len: stage_3_len,
+        },
         stage_4: Region {
             start: stage_4_dst as u64,
             len: stage_4_len,

bios/stage-4/src/main.rs

@@ -4,7 +4,8 @@
 use crate::memory_descriptor::MemoryRegion;
 use bootloader_api::info::{FrameBufferInfo, PixelFormat};
 use bootloader_boot_config::{BootConfig, LevelFilter};
-use bootloader_x86_64_bios_common::{BiosFramebufferInfo, BiosInfo, E820MemoryRegion};
+use bootloader_x86_64_bios_common::{BiosFramebufferInfo, BiosInfo, E820MemoryRegion, Region};
+use bootloader_x86_64_common::legacy_memory_region::UsedMemorySlice;
 use bootloader_x86_64_common::RawFrameBufferInfo;
 use bootloader_x86_64_common::{
     legacy_memory_region::LegacyFrameAllocator, load_and_switch_to_kernel, Kernel, PageTables,
@@ -55,9 +56,10 @@ pub extern "C" fn _start(info: &mut BiosInfo) -> ! {
     };
     let kernel_size = info.kernel.len;
     let next_free_frame = PhysFrame::containing_address(PhysAddr::new(info.last_used_addr)) + 1;
-    let mut frame_allocator = LegacyFrameAllocator::new_starting_at(
+    let mut frame_allocator = LegacyFrameAllocator::new_with_used_slices(
         next_free_frame,
         memory_map.iter().copied().map(MemoryRegion),
+        used_memory_slices(info),
     );
 
     // We identity-mapped all memory, so the offset between physical and virtual addresses is 0
@@ -216,6 +218,17 @@ fn init_logger(
     framebuffer_info
 }
 
+fn used_memory_slices(info: &BiosInfo) -> impl Iterator<Item = UsedMemorySlice> + Clone {
+    // skip kernel and ramdisk because they are handled individually by the
+    // uefi/bios common code
+    [info.stage_3, info.stage_4, info.config_file]
+        .into_iter()
+        .map(|region| UsedMemorySlice {
+            start: PhysAddr::new(region.start).as_u64(),
+            len: region.len,
+        })
+}
+
 /// Creates page table abstraction types for both the bootloader and kernel page tables.
 fn create_page_tables(frame_allocator: &mut impl FrameAllocator<Size4KiB>) -> PageTables {
     // We identity-mapped all memory, so the offset between physical and virtual addresses is 0