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356 lines
16 KiB
Markdown
356 lines
16 KiB
Markdown
---
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layout: post
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title: 'Allocating Frames'
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---
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In this post we create an allocator that provides free physical frames for a future paging module. To get the required information about available and used memory we use the Multiboot information structure. Additionally, we improve the `panic` handler to print the corresponding message and source line.
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## Preparation
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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`:
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```asm
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section .bss
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...
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stack_bottom:
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resb 4096
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stack_top:
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```
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## The Multiboot Information Structure
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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.
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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]:
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[calling convention of Linux]: https://en.wikipedia.org/wiki/X86_calling_conventions#System_V_AMD64_ABI
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> The first six integer or pointer arguments are passed in registers RDI, RSI, RDX, RCX, R8, and R9
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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`):
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```nasm
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start:
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mov esp, stack_top
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mov edi, ebx ; Move Multiboot info pointer to edi
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```
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Now we can add the argument to our `rust_main`:
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```rust
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pub extern fn rust_main(multiboot_information_address: usize) { ... }
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```
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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.
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[multiboot2-elf64]: https://github.com/phil-opp/multiboot2-elf64
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So let's add a dependency on the git repository in the `Cargo.toml`:
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```toml
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...
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[dependencies.multiboot2]
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git = "https://github.com/phil-opp/multiboot2-elf64"
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```
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Now we can add `extern crate multiboot2` and use it to print available memory areas.
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### Available Memory
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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.
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To print available memory areas, we can use the `multiboot2` crate in our `rust_main` as follows:
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```rust
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let boot_info = unsafe{ multiboot2::load(multiboot_information_address) };
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let memory_map_tag = boot_info.memory_map_tag().expect("Memory map tag required");
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println!("memory areas:");
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for area in emory_map_tag.memory_areas() {
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println!(" start: 0x{:x}, length: 0x{:x}", area.base_addr, area.length);
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}
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```
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The `load` function is `unsafe` because it relies on a valid address. Since the memory tag is not required by the Multiboot specification, the `memory_map_tag()` function returns an `Option`. The `memory_areas()` function returns the desired memory area iterator.
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The output looks like this:
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```
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Hello World!
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memory areas:
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start: 0x0, length: 0x9fc00
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start: 0x100000, length: 0x7ee0000
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```
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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 since it contains for example the VGA text buffer at `0xb8000`. This is the reason for putting our kernel at 1MiB and not at e.g. `0x0`.
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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.
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[Memory_map]: http://wiki.osdev.org/Memory_Map_(x86)
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### Handling Panics
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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:
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```rust
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#[lang = "panic_fmt"]
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extern fn panic_fmt() -> ! {
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println!("PANIC");
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loop{}
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}
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```
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Now we get a `PANIC` message. But we can do even better. The `panic_fmt` function has actually some arguments:
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```rust
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#[lang = "panic_fmt"]
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extern fn panic_fmt(fmt: core::fmt::Arguments, file: &str, line: u32) -> ! {
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println!("\n\nPANIC in {} at line {}:", file, line);
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println!(" {}", fmt);
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loop{}
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}
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```
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Be careful with these arguments as the compiler does not check the function signature for `lang_items`.
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You can try our new panic handler by inserting a `panic` somewhere. Now we get the panic message and the causing source line.
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### Kernel ELF Sections
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To read and print the sections of our kernel ELF file, we can use the _Elf-sections_ tag:
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```rust
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let elf_sections_tag = boot_info.elf_sections_tag()
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.expect("Elf-sections tag required");
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println!("kernel sections:");
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for section in elf_sections_tag.sections() {
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println!(" addr: 0x{:x}, size: 0x{:x}, flags: 0x{:x}",
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section.addr, section.size, section.flags);
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}
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```
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This should print out the start address and size of all kernel sections. If the section is writable, the `0x1` bit is set in `flags`. The `0x4` bit marks an executable section and the `0x2` bit 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.
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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.*`. This is because the Rust compiler puts e.g. each function in an own `.text` subsection. To merge these subsections, we can update our linker script:
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```
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SECTIONS {
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. = 1M;
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.boot :
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{
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KEEP(*(.multiboot_header))
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}
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.text :
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{
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*(.text .text.*)
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}
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.rodata : {
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*(.rodata .rodata.*)
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}
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.data.rel.ro : {
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*(.data.rel.ro.local*) *(.data.rel.ro .data.rel.ro.*)
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}
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}
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```
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These lines are taken from the default linker script of `ld`, which can be obtained through `ld ‑verbose`. The `.text` _output_ section contains now all `.text.*` _input_ sections of the static library (and the same applies for the `.rodata` and `.data.rel.ro` sections).
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Now there are only 12 sections left and we get a much more useful output:
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If you like, you can compare this output to the `objdump -h build/kernel-x86_64.bin` output. You will see that the start addresses and sizes match exactly for each section. The sections with flags `0x0` are mostly debug sections, so they don't need to be loaded. And the last few sections of the QEMU output aren't in the `objdump` output because they are special sections such as string tables.
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### Start and End of Kernel
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We can now use the ELF section tag to calculate the start and end address of our loaded kernel:
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```rust
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let kernel_start = elf_sections_tag.sections().map(|s| s.addr)
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.min().unwrap();
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let kernel_end = elf_sections_tag.sections().map(|s| s.addr + s.size)
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.max().unwrap();
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```
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The other used memory area is the Multiboot Information structure:
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```rust
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let multiboot_start = multiboot_information_address;
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let multiboot_end = multiboot_start + (boot_info.total_size as usize);
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```
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Printing these numbers gives us:
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```
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kernel_start: 0x100000, kernel_end: 0x11a168
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multiboot_start: 0x11d400, multiboot_end: 0x11d9c8
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```
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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 (or some differences in the source code).
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## A frame allocator
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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.
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### A Memory Module
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First we create a memory module with a `Frame` type (`src/memory/mod.rs`):
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```rust
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#[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
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pub struct Frame {
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number: usize,
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}
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```
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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`.)
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To make it easy to get the corresponding frame for a physical address, we add a `containing_address` method:
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```rust
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pub const PAGE_SIZE: usize = 4096;
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impl Frame {
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fn containing_address(address: usize) -> Frame {
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Frame{ number: address / PAGE_SIZE }
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}
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}
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```
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We also add a `FrameAllocator` trait:
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```rust
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pub trait FrameAllocator {
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fn allocate_frame(&mut self) -> Option<Frame>;
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fn deallocate_frame(&mut self, frame: Frame);
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}
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```
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This allows us to create another, more advanced frame allocator in the future.
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### The Allocator
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Now we can put everything together and create the actual frame allocator. Therefor we create a `src/memory/area_frame_allocator.rs` submodule. The allocator struct looks like this:
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```rust
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use memory::{Frame, FrameAllocator};
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use multiboot2::{MemoryAreaIter, MemoryArea};
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pub struct AreaFrameAllocator {
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next_free_frame: Frame,
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current_area: Option<&'static MemoryArea>,
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areas: MemoryAreaIter,
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kernel_start: Frame,
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kernel_end: Frame,
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multiboot_start: Frame,
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multiboot_end: Frame,
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}
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```
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The `next_free_frame` field is a simple counter that is increased every time we return a frame. It's initialized to `0` and every frame below it counts as used. 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.
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To implement the `FrameAllocator` trait, we need to implement the `allocate_frame` and the `deallocate_frame` methods. The former looks like this:
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```rust
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fn allocate_frame(&mut self) -> Option<Frame> {
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if let Some(area) = self.current_area {
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let frame = self.next_free_frame;
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// the last frame of the current area
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let current_area_last_frame = {
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let address = area.base_addr + area.length - 1;
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Frame::containing_address(address as usize)
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};
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if frame > current_area_last_frame {
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// all frames of current area are used, switch to next area
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self.choose_next_area();
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} else if frame >= self.kernel_start && frame <= self.kernel_end {
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// `frame` is used by the kernel
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self.next_free_frame = Frame {
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number: self.kernel_end.number + 1
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};
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} else if frame >= self.multiboot_start && frame <= self.multiboot_end {
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// `frame` is used by the multiboot information structure
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self.next_free_frame = Frame {
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number: self.multiboot_end.number + 1
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};
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} else {
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// frame is unused, increment `next_free_frame` and return it
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self.next_free_frame.number += 1;
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return Some(frame);
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}
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// `frame` was not valid, try it again with the updated `next_free_frame`
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self.allocate_frame()
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} else {
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None // no free frames left
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}
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}
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```
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The `choose_next_area` method isn't part of the trait and thus goes into an `impl AreaFrameAllocator` block:
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```rust
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fn choose_next_area(&mut self) {
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self.current_area = self.areas.clone().filter(|area| {
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let address = area.base_addr + area.length - 1;
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Frame::containing_address(address as usize) >= self.next_free_frame
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}).min_by(|area| area.base_addr);
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}
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```
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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. If there are no areas with free frames left, `min_by` automatically returns the desired `None`.
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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 the method is called:
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```rust
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fn deallocate_frame(&mut self, _frame: Frame) {
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unimplemented!()
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}
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```
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Now we only need a constructor function to make it usable:
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```rust
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pub fn new(kernel_start: usize, kernel_end: usize,
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multiboot_start: usize, multiboot_end: usize,
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memory_areas: MemoryAreaIter) -> AreaFrameAllocator
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{
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let mut allocator = AreaFrameAllocator {
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next_free_frame: Frame::containing_address(0),
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current_area: None,
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areas: memory_areas,
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kernel_start: Frame::containing_address(kernel_start),
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kernel_end: Frame::containing_address(kernel_end),
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multiboot_start: Frame::containing_address(multiboot_start),
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multiboot_end: Frame::containing_address(multiboot_end),
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};
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allocator.choose_next_area();
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allocator
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}
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```
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Note that we call `choose_next_area` manually here because `allocate_frame` returns `None` as soon as `current_area` is `None`. So by calling `choose_next_area` we initialize it to the area with the minimal base address.
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### Testing it
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Now we can test it in main. Therefor we need to [re-export] the `AreaFrameAllocator` in the `memory` module. Then we can create a new allocator:
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[re-export]: https://doc.rust-lang.org/book/crates-and-modules.html#re-exporting-with-pub-use
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```rust
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let mut frame_allocator = memory::AreaFrameAllocator::new(
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kernel_start as usize, kernel_end as usize, multiboot_start,
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multiboot_end, memory_map_tag.memory_areas());
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```
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Now you can test it by adding some test allocations:
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```rust
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println!("{:?}", frame_allocator.allocate_frame())
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```
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You will see that frame number starts at `0` and increases steadily, but the kernel and Multiboot frames are left out (you need to allocate many frames to see this since the kernel starts at frame 256).
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The following `for` loop allocates all frames and prints out the total number of allocated frames:
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```rust
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for i in 0.. {
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if let None = frame_allocator.allocate_frame() {
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println!("allocated {} frames", i);
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break;
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}
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}
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```
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You can try different amounts of memory by passing e.g. `-m 500M` to QEMU. To compare these numbers, [WolframAlpha] can be very helpful.
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[WolframAlpha]: http://www.wolframalpha.com/input/?i=%2832698+*+4096%29+bytes+in+MiB
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## What's next?
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The next post will be about paging again. But this time we will use the frame allocator to create a safe rust module that allows us to switch page tables and map pages. Then we will use this module and the information from the ELF sections tag to remap the kernel correctly.
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## Recommended Posts
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Eric Kidd started the [Bare Metal Rust] series last week. Like this post, it builds upon the code from [Printing to Screen], but tries to support keyboard input instead of wrestling through memory management details ;).
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[Bare Metal Rust]: http://www.randomhacks.net/bare-metal-rust/
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[Printing to Screen]: {% post_url 2015-10-23-printing-to-screen %}
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