Limit BIOS bootloader's max_phys_addr to 4 GiB

When determining the maximum physical address for the BIOS bootloader's
identity mapping, we currently use the highest address in the E380
memory map, no matter how high it is. However, the bootloader runs in
protected mode, and therefore, it cannot address more than 4 GiB of
memory. We can save some time by not identity mapping addresses over 4
GiB, as the bootloader cannot address them anyway.

This commit changes the BIOS bootloader to skip addresses over 4 GiB
when determining the maximum physical address.

This is one of the changes described in issue rust-osdev#259.

Author: hawkw

src/bin/bios.rs

@@ -78,6 +78,7 @@ fn bootloader_main(
     use bootloader::binary::{
         bios::memory_descriptor::E820MemoryRegion, legacy_memory_region::LegacyFrameAllocator,
     };
+    const GIGABYTE: u64 = 4096 * 512 * 512;
 
     let e820_memory_map = {
         let ptr = usize_from(memory_map_addr.as_u64()) as *const E820MemoryRegion;
@@ -86,6 +87,10 @@ fn bootloader_main(
     let max_phys_addr = e820_memory_map
         .iter()
         .map(|r| r.start_addr + r.len)
+        // Don't consider addresses > 4GiB when determining the maximum
+        // physical address for the bootloader, as we are in  protected mode and
+        // cannot address more than 4 GiB of memory anyway.
+        .filter(|&addr| addr < GIGABYTE * 4)
         .max()
         .expect("no physical memory regions found");
 
@@ -106,7 +111,7 @@ fn bootloader_main(
     // identity-map remaining physical memory (first gigabyte is already identity-mapped)
     {
         let start_frame: PhysFrame<Size2MiB> =
-            PhysFrame::containing_address(PhysAddr::new(4096 * 512 * 512));
+            PhysFrame::containing_address(PhysAddr::new(GIGABYTE));
         let end_frame = PhysFrame::containing_address(PhysAddr::new(max_phys_addr - 1));
         for frame in PhysFrame::range_inclusive(start_frame, end_frame) {
             unsafe {