don't switch to new page tables in bootloader

We don't need to do this anymore because we no longer modify the
bootloader's page tables. This also finally makes it possible to access
all memory mapped in by UEFI and not just the memory accessible through
the first PML4 entry. Some UEFI implementations map the frame buffer
into ranges not accessible through the first PML4 entry.

Author: Freax13

bios/stage-4/src/main.rs

@@ -221,13 +221,6 @@ fn create_page_tables(frame_allocator: &mut impl FrameAllocator<Size4KiB>) -> Pa
     // We identity-mapped all memory, so the offset between physical and virtual addresses is 0
     let phys_offset = VirtAddr::new(0);
 
-    // copy the currently active level 4 page table, because it might be read-only
-    let bootloader_page_table = {
-        let frame = x86_64::registers::control::Cr3::read().0;
-        let table: *mut PageTable = (phys_offset + frame.start_address().as_u64()).as_mut_ptr();
-        unsafe { OffsetPageTable::new(&mut *table, phys_offset) }
-    };
-
     // create a new page table hierarchy for the kernel
     let (kernel_page_table, kernel_level_4_frame) = {
         // get an unused frame for new level 4 page table
@@ -246,7 +239,6 @@ fn create_page_tables(frame_allocator: &mut impl FrameAllocator<Size4KiB>) -> Pa
     };
 
     PageTables {
-        bootloader: bootloader_page_table,
         kernel: kernel_page_table,
         kernel_level_4_frame,
     }

common/src/lib.rs

@@ -605,8 +605,6 @@ pub fn switch_to_kernel(page_tables: PageTables, mappings: Mappings, boot_info:
 
 /// Provides access to the page tables of the bootloader and kernel address space.
 pub struct PageTables {
-    /// Provides access to the page tables of the bootloader address space.
-    pub bootloader: OffsetPageTable<'static>,
     /// Provides access to the page tables of the kernel address space (not active).
     pub kernel: OffsetPageTable<'static>,
     /// The physical frame where the level 4 page table of the kernel address space is stored.

uefi/src/main.rs

@@ -389,41 +389,6 @@ fn create_page_tables(
     // UEFI identity-maps all memory, so the offset between physical and virtual addresses is 0
     let phys_offset = VirtAddr::new(0);
 
-    // copy the currently active level 4 page table, because it might be read-only
-    log::trace!("switching to new level 4 table");
-    let bootloader_page_table = {
-        let old_table = {
-            let frame = x86_64::registers::control::Cr3::read().0;
-            let ptr: *const PageTable = (phys_offset + frame.start_address().as_u64()).as_ptr();
-            unsafe { &*ptr }
-        };
-        let new_frame = frame_allocator
-            .allocate_frame()
-            .expect("Failed to allocate frame for new level 4 table");
-        let new_table: &mut PageTable = {
-            let ptr: *mut PageTable =
-                (phys_offset + new_frame.start_address().as_u64()).as_mut_ptr();
-            // create a new, empty page table
-            unsafe {
-                ptr.write(PageTable::new());
-                &mut *ptr
-            }
-        };
-
-        // copy the first entry (we don't need to access more than 512 GiB; also, some UEFI
-        // implementations seem to create an level 4 table entry 0 in all slots)
-        new_table[0] = old_table[0].clone();
-
-        // the first level 4 table entry is now identical, so we can just load the new one
-        unsafe {
-            x86_64::registers::control::Cr3::write(
-                new_frame,
-                x86_64::registers::control::Cr3Flags::empty(),
-            );
-            OffsetPageTable::new(&mut *new_table, phys_offset)
-        }
-    };
-
     // create a new page table hierarchy for the kernel
     let (kernel_page_table, kernel_level_4_frame) = {
         // get an unused frame for new level 4 page table
@@ -442,7 +407,6 @@ fn create_page_tables(
     };
 
     bootloader_x86_64_common::PageTables {
-        bootloader: bootloader_page_table,
         kernel: kernel_page_table,
         kernel_level_4_frame,
     }