blog_os/posts/DRAFT-remapping-the-kernel.md

layout, title

layout	title
post	Remapping the Kernel

TODO

The Multiboot Information Structure

When a Multiboot compliant bootloader loads a kernel, it passes a pointer to a boot information structure in the ebx register. We can use it to get information about available memory and loaded kernel sections.

First, we need to pass this pointer to our kernel as an argument to rust_main. To find out how arguments are passed to functions, we can look at the calling convention of Linux:

The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9

So to pass the pointer to our kernel, we need to move it to rdi before calling the kernel. Since we're not using the rdi/edi register in our bootstrap code right now, we can simply set the edi register right after booting (in boot.asm):

start:
    mov esp, stack_top
    mov edi, ebx       ; Move Multiboot info pointer to edi

Now we can add the argument to our rust_main:

pub extern fn rust_main(multiboot_information_address: usize) { ... }

Now we can use the multiboot2-elf64 crate to query get some information about mapped kernel sections and available memory. I just wrote it for this blog post since I could not find any other Multiboot 2 crate. It's really ugly and incomplete, but it does its job.

So let's add a dependency on the git repository in the Cargo.toml:

...
[dependencies.multiboot2]
git = "https://github.com/phil-opp/multiboot2-elf64"

Now we can add extern crate multiboot2 and use it to print available memory areas.

Available Memory

The boot information structure consists of various tags. The memory map tag contains a list of all areas of available RAM. Special areas such as the VGA text buffer at 0xb8000 are not available. Note that some of the available memory is already used by our kernel and by the multiboot information structure itself.

To print available memory areas, we can use the multiboot2 crate in our rust_main as follows:

let boot_info = unsafe{ multiboot2::load(multiboot_information_address) };

println!("memory areas:");
for area in boot_info.memory_map_tag().unwrap().memory_areas() {
    println!("    start: 0x{:x}, length: 0x{:x}", area.base_addr, area.length);
}

The load function is unsafe because it relies on a valid address. Since the memory tag is not required, the memory_map_tag() function returns an Option. The memory_areas() function returns the desired memory area iterator.

The output looks like this:

Hello World!
memory areas:
    start: 0x0, length: 0x9fc00
    start: 0x100000, length: 0x7ee0000

So we have one area from 0x0 to 0x9fc00, which is a bit below the 1MiB mark. The second, bigger area starts at 1MiB and contains the rest of available memory. The area from 0x9fc00 to 1MiB is not available. For example the VGA text buffer at 0xb8000 is in that area. This is the reason for putting our kernel at 1MiB and not at e.g. 0x0.

If you give QEMU more than 4GiB of memory by passing -m 5G, you get another unusable area below the 4GiB mark. This memory is normally mapped to some hardware devices. See the OSDev Wiki for more information.

Kernel ELF Sections

To read and print the sections of our kernel ELF file, we can use the Elf-sections tag:

println!("kernel sections:");
for section in boot_info.elf_sections_tag().unwrap().sections() {
    println!("    addr: 0x{:x}, size: 0x{:x}, flags: 0x{:x}",
        section.addr, section.size, section.flags);
}

This should print out the start address and size of all kernel sections. If the section is writable, the 0x1 is set in flags. The 0x4 bit marks an executable section and the 0x2 indicates that the section was loaded in memory. For example, the .text section is executable but not writable and the .data section just the opposite.

But when we execute it, tons of really small sections are printed. We can use the objdump -h build/kernel-x86_64.bin command to list the sections with name. There seem to be over 200 sections and many of them start with .text.* or .data.rel.ro.local.*. The Rust compiler puts each function in an own .text subsection. To merge these subsections, we can update our linker script:

SECTIONS {
    . = 1M;

    .boot :
    {
        KEEP(*(.multiboot_header))
    }

    .text :
    {
        *(.text .text.*)
    }

    .rodata : {
        *(.rodata .rodata.*)
    }

    .data.rel.ro : {
        *(.data.rel.ro.local*) *(.data.rel.ro .data.rel.ro.*)
    }
}

These lines are taken from the default linker script of ld, which can be obtained through ld ‑verbose. Now there are only 12 sections left and we get a much more useful output:

Start and End of Kernel

We can now use the ELF section tag to calculate the start and end address of our loaded kernel:

TODO

A frame allocator

When we create a paging module in the next post, we will need to map virtual pages to free physical frames. So we will need some kind of allocator that keeps track of physical frames and gives us a free one when needed. We can use the memory tag to write such a frame allocator.

The allocator struct looks like this:

struct AreaFrameAllocator {
    first_used_frame: Frame,
    last_used_frame: Frame,
    current_area: Option<MemoryArea>,
    areas: MemoryAreaIter,
}

TODO

To allocate a frame we try to find one in the current area and update the first/last used bounds. If we can't find one, we look for the new area with the minimal start address, that still contains free frames. If the current area is None, there are no free frames left.

TODO

Unit Tests

TODO

Remapping the Kernel Sections

We can use the ELF section tag to write a skeleton that remaps the kernel correctly:

for section in multiboot.elf_tag().sections() {
    for page in start_page..end_page {
        // TODO identity_map(page, section.writable(), section.executable())
    }
}

TODO