danos/src/root.zig

//! Shared definitions that form the contract between a bootloader
//! (src/boot/, e.g. efi.zig built as BOOTX64.efi) and the kernel (src/kernel/main.zig).
//!
//! Both binaries import this as the "danos" module, so the handoff layout is
//! defined in exactly one place.

const std = @import("std");

/// Calling convention for the bootloader→kernel jump. Pinned to SysV so it does
/// not depend on each binary's target default: the UEFI bootloader's C
/// convention is Microsoft x64 (first arg in RCX), the freestanding kernel's is
/// SysV (first arg in RDI). Both reference this to agree on where `*BootInfo`
/// is passed.
pub const kernel_abi: std.builtin.CallingConvention = .{ .x86_64_sysv = .{} };

/// Pixel byte order of the linear framebuffer the firmware handed us.
pub const PixelFormat = enum(u32) {
    /// Byte 0 = Red, 1 = Green, 2 = Blue, 3 = reserved.
    rgbx,
    /// Byte 0 = Blue, 1 = Green, 2 = Red, 3 = reserved.
    bgrx,
};

/// A linear framebuffer: `width`x`height` pixels, each a 32-bit value, with
/// `pitch` bytes between the start of one row and the next (which may be larger
/// than `width * 4` due to hardware padding).
///
/// A `base` of 0 means **no framebuffer** — the firmware exposed no Graphics
/// Output Protocol (a headless server, say). The kernel must treat on-screen
/// output as optional and never assume a framebuffer exists.
pub const Framebuffer = extern struct {
    base: usize,            // the memory address where pixel data starts (0 = none)
    width: u32,             // visible pixels per row (e.g. 1920)
    height: u32,            // visible rows (e.g. 1080)
    pitch: u32,             // bytes from the start of one row to the start of the next
    format: PixelFormat,

    /// Whether a usable framebuffer was handed over.
    pub fn present(self: Framebuffer) bool {
        return self.base != 0 and self.width != 0 and self.height != 0;
    }
};

/// Page size the memory map is measured in. 4 KiB on every architecture danos
/// targets so far.
pub const page_size = 4096;

/// danos's own classification of a span of physical memory — deliberately not
/// UEFI's vocabulary. Each boot path (UEFI now, device tree later) translates its
/// native memory description into these kinds, so the kernel never learns what
/// booted it. [[arch]] keeps the same discipline for CPU code.
pub const MemoryKind = enum(u32) {
    /// Free RAM the kernel may allocate. Each boot path folds its own transient
    /// memory into this once it's genuinely free (e.g. the UEFI loader classifies
    /// boot-services memory as usable after ExitBootServices), so the kernel never
    /// has to know about boot-protocol-specific "reclaimable" states.
    usable,
    /// Firmware, MMIO, the kernel image, our own boot buffers, the boot stack —
    /// never hand out.
    reserved,
    /// ACPI tables: parse, then reclaim.
    acpi_tables,
    /// ACPI non-volatile storage: preserve across sleep, do not allocate.
    acpi_nvs,
    /// Not backed by RAM: memory-mapped device registers or a reserved
    /// address-space window (e.g. PCIe config space). Kept distinct from
    /// `reserved` so RAM accounting doesn't count device address space.
    mmio,
};

/// One contiguous span of physical memory. Because danos defines this layout
/// itself (unlike the UEFI descriptor it's built from), `@sizeOf` is
/// authoritative — the kernel walks a plain `[]MemoryRegion`, with none of the
/// firmware's variable descriptor-stride to worry about.
pub const MemoryRegion = extern struct {
    base: u64, // physical start address
    pages: u64, // length in `page_size` units
    kind: MemoryKind,
    _pad: u32 = 0,
};

/// The physical memory layout handed to the kernel: a pointer to an array of
/// `len` `MemoryRegion`s, in a buffer that outlives the loader.
pub const MemoryMap = extern struct {
    regions: usize, // address of a `[len]MemoryRegion`
    len: usize,
};

/// One PT_LOAD segment of the kernel image, so the kernel can re-map itself with
/// correct permissions (code R+X, rodata R, data R+W+NX). `flags` are raw ELF
/// segment flags: PF_X=1, PF_W=2, PF_R=4.
pub const KernelSegment = extern struct {
    virt: u64,
    pages: u64,
    flags: u32,
    _pad: u32 = 0,
};

/// Handoff structure the bootloader fills in and passes to the kernel's
/// `_start` in RDI (the first argument under the SysV AMD64 C ABI).
pub const BootInfo = extern struct {
    framebuffer: Framebuffer,
    memory_map: MemoryMap,
    /// The kernel's own PT_LOAD segments (it has three: text, rodata, data).
    kernel_segments: [8]KernelSegment,
    kernel_segment_count: u32,
    /// Physical address of the ACPI RSDP the firmware exposed, or 0 if none. The
    /// kernel's device layer parses the ACPI tables from here to discover hardware.
    /// A device-tree boot path leaves this 0 and (later) fills a `device_tree_blob`
    /// field instead, so the kernel discovers devices without knowing what booted it.
    acpi_rsdp: u64 = 0,
};