Add test that lower memory is actually writable

The "lower_mem_free" test will now also try to write to all usable
regions below 1GB. This way we ensure that regions below 1Mb are
writable and that bios and uefi bootloader correctly mark regions
as used.

Author: Wasabi375

tests/test_kernels/lower_memory_free/src/bin/lower_memory_free.rs

@@ -1,17 +1,27 @@
 #![no_std] // don't link the Rust standard library
 #![no_main] // disable all Rust-level entry points
 
-use bootloader_api::{entry_point, info::MemoryRegionKind, BootInfo};
+use bootloader_api::{
+    config::Mapping, entry_point, info::MemoryRegionKind, BootInfo, BootloaderConfig,
+};
 use test_kernel_lower_memory_free::{exit_qemu, QemuExitCode};
 
 const LOWER_MEMORY_END_PAGE: u64 = 0x0010_0000;
 
-entry_point!(kernel_main);
+pub const BOOTLOADER_CONFIG: BootloaderConfig = {
+    let mut config = BootloaderConfig::new_default();
+    config.mappings.physical_memory = Some(Mapping::FixedAddress(0x0000_4000_0000_0000));
+    config
+};
+
+entry_point!(kernel_main, config = &BOOTLOADER_CONFIG);
 
 fn kernel_main(boot_info: &'static mut BootInfo) -> ! {
     use core::fmt::Write;
     use test_kernel_lower_memory_free::serial;
 
+    let phys_mem_offset = boot_info.physical_memory_offset.into_option().unwrap();
+
     let mut count = 0;
     for region in boot_info.memory_regions.iter() {
         writeln!(
@@ -24,6 +34,14 @@ fn kernel_main(boot_info: &'static mut BootInfo) -> ! {
         .unwrap();
         if region.kind == MemoryRegionKind::Usable && region.start < LOWER_MEMORY_END_PAGE {
             let end = core::cmp::min(region.end, LOWER_MEMORY_END_PAGE);
+
+            // ensure region is actually writable
+            let addr = phys_mem_offset + region.start;
+            let size = end - region.start;
+            unsafe {
+                core::ptr::write_bytes(addr as *mut u8, 0xff, size as usize);
+            }
+
             let pages = (end - region.start) / 4096;
             count += pages;
         }