169 lines
7.5 KiB
Markdown
169 lines
7.5 KiB
Markdown
# The memory map
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Before a kernel can manage memory, it has to *know what memory exists*: which
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physical address ranges are real RAM it may use, and which are firmware, hardware
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registers, or already occupied. That inventory is the **memory map**, and the
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firmware is the only thing that knows it. This page covers how danos gets that map
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from the firmware and hands it to the kernel — deliberately without dragging UEFI
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into the kernel.
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## Why not just pass UEFI's map through?
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UEFI hands the loader a perfectly good memory map. The tempting shortcut is to
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forward it to the kernel as-is. We don't, for two reasons:
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1. **It would tie the kernel to UEFI.** The kernel would compare against UEFI's
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memory-type numbers and walk the array using UEFI's variable descriptor stride.
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That's UEFI vocabulary bleeding across the handoff — and danos wants to boot on
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systems that have no UEFI at all (a Raspberry Pi describes its memory with a
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*device tree* instead). See [arch.md](arch.md) for the same "keep the kernel
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platform-agnostic" principle applied to CPU code.
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2. **We already established the better pattern.** The loader doesn't hand the
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kernel a raw UEFI GOP either — [`queryFramebuffer`](gop.md) converts it to
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danos's own `Framebuffer`. The memory map follows the same discipline.
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So the boundary is: **each boot path translates its native memory description into
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danos's own neutral format, and the kernel only ever sees that.**
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## The neutral format
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Defined in `src/root.zig`, the shared loader↔kernel contract:
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```zig
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pub const MemoryKind = enum(u32) {
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usable, // free RAM the kernel may allocate
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reserved, // firmware / kernel image / boot stack — real RAM, never hand out
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acpi_tables, // parse, then reclaim
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acpi_nvs, // preserve across sleep
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mmio, // device registers / reserved address space — not RAM at all
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};
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pub const MemoryRegion = extern struct {
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base: u64, // physical start
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pages: u64, // length in page_size (4 KiB) units
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kind: MemoryKind,
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_pad: u32 = 0,
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};
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pub const MemoryMap = extern struct {
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regions: usize, // pointer to a [len]MemoryRegion
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len: usize,
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};
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```
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`MemoryKind` is danos's *own* vocabulary — not UEFI's ~15 types, just the
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distinctions the kernel actually acts on. And because danos defines `MemoryRegion`
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itself, `@sizeOf` is authoritative: the kernel walks a plain `[]MemoryRegion` with
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no variable-stride subtlety (that stride problem is a UEFI-ism, and it stays in the
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loader).
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`BootInfo` carries it alongside the framebuffer:
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```zig
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pub const BootInfo = extern struct {
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framebuffer: Framebuffer,
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memory_map: MemoryMap,
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};
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```
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## The loader side (UEFI)
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Two functions in `src/efi.zig`, called from `exitBootServices`:
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- **`classify`** maps each UEFI descriptor to a `MemoryKind`:
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`conventional_memory` **and** `boot_services_code`/`boot_services_data → usable`;
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`acpi_reclaim_memory → acpi_tables`; `acpi_memory_nvs → acpi_nvs`; **everything
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else → reserved** (the safe default). Our own `loader_data` — the kernel image and
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these buffers — falls into `reserved`.
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Folding boot-services memory into `usable` is deliberate: we've already called
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ExitBootServices, so it's free RAM now, and doing the classification *here* (in
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the loader) means the kernel never learns about a UEFI-specific "reclaimable"
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state — it just sees usable RAM. The one catch is that our stack lives in
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boot-services memory and the kernel starts out running on it, so
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`convertMemoryMap` keeps the single region containing the current stack pointer
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`reserved`. All the boot-protocol knowledge stays on the loader side of the
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boundary; the kernel's frame allocator has no idea any of this happened.
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One subtlety: **a region that isn't writeback-cacheable (the descriptor's `wb`
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attribute) is classified `mmio` regardless of type.** UEFI overloads
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`reserved_memory_type` for both reserved RAM *and* reserved address-space windows
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(PCIe config space, device BARs); the cache attribute is what actually tells them
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apart, since only real RAM is writeback-cacheable. Without this, a QEMU q35 guest
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reports ~12 GiB of "reserved" that is really a PCIe address hole near the 1 TB
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mark — not memory at all.
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- **`convertMemoryMap`** walks the UEFI descriptors (striding by
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`descriptor_size`, *not* `@sizeOf`), classifies each, and writes danos
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`MemoryRegion`s into an output buffer, coalescing adjacent same-kind regions.
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### The ordering that makes it correct
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This is the fiddly part, dictated by two UEFI rules: you can only allocate memory
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*before* `ExitBootServices`, and the memory map is only final *at* the moment you
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exit (its "key" proves you've seen the latest state). So `exitBootServices` does,
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per attempt:
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1. `getMemoryMapInfo` to size things, then `allocatePool` **two** LoaderData
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buffers — one for the raw UEFI map, one for the converted regions. Allocating
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now, before exit, is mandatory.
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2. `getMemoryMap` then `exitBootServices(key)`. If either fails (allocating can
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perturb the map and invalidate the key), free both buffers and retry.
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3. **After** the exit succeeds, convert. Conversion is pure computation on memory
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we already hold — no boot-services calls — so it's safe once services are gone.
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Both buffers are `LoaderData`, which survives `ExitBootServices`, so the converted
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array the kernel is pointed at stays valid. (The raw UEFI buffer is just scratch
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for the conversion.)
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## The kernel side
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The kernel receives a plain array and reads it with zero UEFI knowledge:
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```zig
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const mm = boot_info.memory_map;
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const regions = @as([*]const danos.MemoryRegion, @ptrFromInt(mm.regions))[0..mm.len];
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for (regions) |r| {
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if (r.kind == .usable) usable_pages += r.pages;
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}
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```
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`kmain` summarises the map to prove the handoff works. Booted in QEMU with
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128 MiB, it reports:
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```
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danos: physical memory
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total RAM : 0.12 GiB (127 MiB) - RAM the firmware reported
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usable : 121 MiB - free RAM (incl. reclaimed boot-services memory)
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reserved : 6 MiB - kernel image, boot stack, ACPI, runtime services
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regions : 28 - entries in the firmware memory map
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```
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`usable` is ~121 of ~127 MiB because the loader already folded the boot-services
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memory into it — so the frame allocator gets it all with no special step. The ~6 MiB
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`reserved` is the kernel image, the boot stack's region, ACPI, and runtime services.
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`total` counts only writeback-cacheable RAM, so the ~12 GiB PCIe address hole is
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excluded (it's `mmio`), and the RAM categories summing back to the firmware's total
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is the sanity check that nothing was dropped.
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## How Raspberry Pi will fit
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No UEFI there, but the boundary is unchanged. The Pi's firmware jumps into the
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kernel with a **device-tree blob**; the AArch64 entry code will parse its
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`/memory` and `/reserved-memory` nodes and produce the *same* `MemoryRegion`
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array. The kernel's memory code — the frame allocator and everything above it —
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never knows the difference.
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## What's next
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This page is plumbing plus classification only. The map's first consumer, the
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**physical frame allocator**, is built directly on the `usable` regions here — which
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already include the reclaimed boot-services memory the loader folded in (see
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[frame-allocator.md](frame-allocator.md)). Still to come:
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- Freeing the `reserved` `loader_data` (these boot-time buffers) once the kernel is
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done reading the map.
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- Capturing the ACPI RSDP from the UEFI configuration table before exit (the same
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"grab it before ExitBootServices" pattern), for when ACPI parsing arrives.
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See the roadmap in [efi.md](efi.md) for where this sits in the boot flow.
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