---
layout: post
title: 'Allocating Frames'
---
TODO
## Preparation
We still have a really tiny stack of 64 bytes, which won't suffice for this post. So we will increase it to 4096 (one page) in `boot.asm`:
```asm
section .bss
...
stack_bottom:
resb 4096
stack_top:
```
## 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]:
[calling convention of Linux]: https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI
> 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`):
```nasm
start:
mov esp, stack_top
mov edi, ebx ; Move Multiboot info pointer to edi
```
Now we can add the argument to our `rust_main`:
```rust
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.
[multiboot2-elf64]: https://github.com/phil-opp/multiboot2-elf64
So let's add a dependency on the git repository in the `Cargo.toml`:
```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:
```rust
let boot_info = unsafe{ multiboot2::load(multiboot_information_address) };
let memory_map_tag = boot_info.memory_map_tag().expect("Memory map tag required");
println!("memory areas:");
for area in emory_map_tag.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][Memory_map] for more information.
[Memory_map]: http://wiki.osdev.org/Memory_Map_(x86)
### Handling Panics
We used `expect` in the code above, which will panic if there is no memory map tag. But our current panic handler just loops without printing any error message. Of course we could replace `expect` by a `match`, but we should fix the panic handler nonetheless:
```rust
#[lang = "panic_fmt"]
extern fn panic_fmt() -> ! {
println!("PANIC");
loop{}
}
```
Now we get a `PANIC` message. But we can do even better. The `panic_fmt` function has actually some arguments:
```rust
#[lang = "panic_fmt"]
extern fn panic_fmt(fmt: core::fmt::Arguments, file: &str, line: u32) -> ! {
println!("\n\nPANIC in {} at line {}:", file, line);
println!(" {}", fmt);
loop{}
}
```
Be careful with these arguments as the compiler does not check arguments for `lang_items`.
You can try our new panic handler by inserting a `panic` somewhere. Now we get the panic message and the causing source line.
### Kernel ELF Sections
To read and print the sections of our kernel ELF file, we can use the _Elf-sections_ tag:
```rust
let elf_sections_tag = boot_info.elf_sections_tag()
.expect("Elf-sections tag required");
println!("kernel sections:");
for section in elf_sections_tag.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:
```rust
let kernel_start = elf_sections_tag.sections().map(|s| s.addr)
.min().unwrap();
let kernel_end = elf_sections_tag.sections().map(|s| s.addr + s.size)
.max().unwrap();
```
The other used memory area is the Multiboot Information structure:
```rust
let multiboot_start = multiboot_information_address;
let multiboot_end = multiboot_start + (boot_info.total_size as usize);
```
Printing these numbers gives us:
```
kernel_start: 0x100000, kernel_end: 0x11a168
multiboot_start: 0x11d400, multiboot_end: 0x11d9c8
```
So the kernel starts at 1MiB (like expected) and is about 105 KiB in size. The multiboot information structure was placed at `0x11d400` by GRUB and needs 1480 bytes. Of course your numbers could be a bit different due to different versions of Rust or GRUB.
## 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 information about memory areas to write such a frame allocator.
### A Memory Module
First we create a memory module with a `Frame` type (`src/memory/mod.rs`):
```rust
#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
pub struct Frame {
number: usize,
}
```
We use `usize` here since the number of frames depends on the memory size. The long `derive` line makes frames printable and comparable. (Don't forget to add the `mod memory` line to `src/lib.rs`.)
To make it easy to get the corresponding frame for a physical address, we add a `containing_address` method:
```rust
pub const PAGE_SIZE: usize = 4096;
impl Frame {
fn containing_address(address: usize) -> Frame {
Frame{ number: address / PAGE_SIZE }
}
}
```
We also add a `FrameAllocator` trait:
```rust
pub trait FrameAllocator {
fn allocate_frame(&mut self) -> Option;
fn deallocate_frame(&mut self, frame: Frame);
}
```
This allows us to create another, more advanced frame allocator in the future.
### The Allocator
Now we can put everything together and create the actual frame allocator. It looks like this:
```rust
use memory::{Frame, FrameAllocator};
use multiboot2::{MemoryAreaIter, MemoryArea};
pub struct AreaFrameAllocator {
next_free_frame: Frame,
current_area: Option<&'static MemoryArea>,
areas: MemoryAreaIter,
kernel_start: Frame,
kernel_end: Frame,
multiboot_start: Frame,
multiboot_end: Frame,
}
```
The `next_free_frame` field is a simple counter that is increased every time we return a frame. The `current_area` field holds the memory area that contains `next_free_frame`. If `next_free_frame` leaves this area, we will look for the next one in `areas`. When there are no areas left, all frames are used and `current_area` becomes `None`. The `{kernel, multiboot}_{start, end}` fields are used to avoid returning already used fields.
To implement the `FrameAllocator` trait, we need to implement the `allocate_frame` and the `deallocate_frame` methods. The former looks like this:
```rust
fn allocate_frame(&mut self) -> Option {
if let Some(area) = self.current_area {
let frame = self.next_free_frame;
// the last frame of the current area
let current_area_last_frame = {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize)
};
if frame > current_area_last_frame {
// all frames of current area are used, switch to next area
self.choose_next_area();
} else if frame >= self.kernel_start && frame <= self.kernel_end {
// `frame` is used by the kernel
self.next_free_frame = Frame {
number: self.kernel_end.number + 1
};
} else if frame >= self.multiboot_start && frame <= self.multiboot_end {
// `frame` is used by the multiboot information structure
self.next_free_frame = Frame {
number: self.multiboot_end.number + 1
};
} else {
// frame is unused, increment `next_free_frame` and return it
self.next_free_frame.number += 1;
return Some(frame);
}
// `frame` was not valid, try it again with the updated `next_free_frame`
self.allocate_frame()
} else {
None // no free frames left
}
}
```
The `choose_next_area` method isn't part of the trait and thus goes into an `impl AreaFrameAllocator` block:
```rust
fn choose_next_area(&mut self) {
self.current_area = self.areas.clone().filter(|area| {
let address = area.base_addr + area.length - 1;
Frame::containing_address(address as usize) >= self.next_free_frame
}).min_by(|area| area.base_addr);
}
```
This function chooses the area with the minimal base address that still has free frames, i.e. `next_free_frame` is smaller than its last frame. Note that we need to clone the iterator because the order of areas in the memory map isn't specified.
We don't have a data structure to store free frames, so we can't implement `deallocate_frame` reasonably. Thus we use the `unimplemented` macro, which just panics when called:
```rust
fn deallocate_frame(&mut self, _frame: Frame) {
unimplemented!()
}
```
Now we only need a constructor function:
```rust
pub fn new(kernel_start: usize, kernel_end: usize,
multiboot_start: usize, multiboot_end: usize,
memory_areas: MemoryAreaIter) -> AreaFrameAllocator
{
let mut allocator = AreaFrameAllocator {
next_free_frame: Frame::containing_address(0),
current_area: None,
areas: memory_areas,
kernel_start: Frame::containing_address(kernel_start),
kernel_end: Frame::containing_address(kernel_end),
multiboot_start: Frame::containing_address(multiboot_start),
multiboot_end: Frame::containing_address(multiboot_end),
};
allocator.choose_next_area();
allocator
}
```
Note that we call `choose_next_area` manually here because `allocate_frame` returns `None` as soon as `current_area` is `None`.
## Remapping the Kernel Sections
We can use the ELF section tag to write a skeleton that remaps the kernel correctly:
```rust
for section in multiboot.elf_tag().sections() {
for page in start_page..end_page {
// TODO identity_map(page, section.writable(), section.executable())
}
}
```
TODO