reclaiming uefi memory

docs/frame-allocator.md

@@ -100,12 +100,25 @@ The `free frames` MiB agreeing with the memory map's `usable RAM`, the three
 distinct consecutive addresses, and the count returning to its start after freeing
 are the three signals that init, alloc and free are all correct.
 
+## Boot-services memory comes pre-reclaimed
+
+The UEFI boot-services memory (~44 MiB) is defunct and free once
+`ExitBootServices` runs, taking usable RAM from ~76 MiB up to ~121 MiB. The frame
+allocator does **nothing special** to get it: the loader already classified it as
+`usable` (see [memory-map.md](memory-map.md)), so it's just part of the `usable`
+regions `init` frees. Keeping that boot-protocol knowledge on the loader side is
+deliberate — the kernel has no notion of "reclaimable" or of UEFI at all.
+
+The one live piece in that memory is the boot stack the kernel starts on; the loader
+leaves the single region containing it `reserved`, so `init` won't hand it out. A
+later step will move task 0 onto a kernel-owned stack, freeing that last ~1 MiB
+region too (and giving user mode the clean stack it wants).
+
 ## What's next (not done here)
 
 - **Contiguous allocation** — scan for N consecutive free bits — for callers that
   need physically adjacent frames.
-- **Consumers**: the virtual memory manager / page tables and then the kernel heap
+- **A kernel stack for task 0**, so the boot stack's region can be freed too (and
-  will be the first real users, each asking `alloc()` for frames.
+  for the clean stack user mode wants).
-- **Reclaiming `reclaimable`** (UEFI boot-services) memory, and eventually the
+- **Freeing the `reserved` `loader_data`** (the boot-time map buffers) once the
-  `reserved` `loader_data` (kernel image, boot buffers) once nothing needs it —
+  kernel is done reading the memory map.
-  see the deferred list in [memory-map.md](memory-map.md).

docs/memory-map.md

@@ -32,8 +32,7 @@ Defined in `src/root.zig`, the shared loader↔kernel contract:
 ```zig
 pub const MemoryKind = enum(u32) {
     usable,       // free RAM the kernel may allocate
-    reserved,     // firmware / kernel image — real RAM, but never hand out
+    reserved,     // firmware / kernel image / boot stack — real RAM, never hand out
-    reclaimable,  // usable once boot-time structures are done with
     acpi_tables,  // parse, then reclaim
     acpi_nvs,     // preserve across sleep
     mmio,         // device registers / reserved address space — not RAM at all
@@ -72,11 +71,19 @@ pub const BootInfo = extern struct {
 Two functions in `src/efi.zig`, called from `exitBootServices`:
 
 - **`classify`** maps each UEFI descriptor to a `MemoryKind`:
-  `conventional_memory → usable`; `boot_services_code`/`boot_services_data →
+  `conventional_memory` **and** `boot_services_code`/`boot_services_data → usable`;
-  reclaimable` (free once we've exited); `acpi_reclaim_memory → acpi_tables`;
+  `acpi_reclaim_memory → acpi_tables`; `acpi_memory_nvs → acpi_nvs`; **everything
-  `acpi_memory_nvs → acpi_nvs`; **everything else → reserved** (the safe default).
+  else → reserved** (the safe default). Our own `loader_data` — the kernel image and
-  Our own `loader_data` — the kernel image and these buffers — falls into
+  these buffers — falls into `reserved`.
-  `reserved`, so it won't be handed out until the kernel deliberately reclaims it.
+
+  Folding boot-services memory into `usable` is deliberate: we've already called
+  ExitBootServices, so it's free RAM now, and doing the classification *here* (in
+  the loader) means the kernel never learns about a UEFI-specific "reclaimable"
+  state — it just sees usable RAM. The one catch is that our stack lives in
+  boot-services memory and the kernel starts out running on it, so
+  `convertMemoryMap` keeps the single region containing the current stack pointer
+  `reserved`. All the boot-protocol knowledge stays on the loader side of the
+  boundary; the kernel's frame allocator has no idea any of this happened.
 
   One subtlety: **a region that isn't writeback-cacheable (the descriptor's `wb`
   attribute) is classified `mmio` regardless of type.** UEFI overloads
@@ -126,18 +133,17 @@ for (regions) |r| {
 ```
 danos: physical memory
   total RAM  : 0.12 GiB (127 MiB) - RAM the firmware reported
-  usable     : 77 MiB - free now; owned by the frame allocator
+  usable     : 121 MiB - free RAM (incl. reclaimed boot-services memory)
-  reclaimable: 44 MiB - UEFI boot-services memory, free after exit
+  reserved   : 6 MiB - kernel image, boot stack, ACPI, runtime services
-  reserved   : 6 MiB - kernel image, ACPI, runtime services
+  regions    : 28 - entries in the firmware memory map
-  regions    : 35 - entries in the firmware memory map
 ```
 
-The `usable` figure is only ~77 of ~127 MiB because most of the rest is
+`usable` is ~121 of ~127 MiB because the loader already folded the boot-services
-`reclaimable` boot-services memory — real RAM we'll take back once we implement
+memory into it — so the frame allocator gets it all with no special step. The ~6 MiB
-reclaiming, not memory that's gone. `total` counts only writeback-cacheable RAM,
+`reserved` is the kernel image, the boot stack's region, ACPI, and runtime services.
-so the ~12 GiB PCIe address hole is excluded (it's `mmio`), and the three RAM
+`total` counts only writeback-cacheable RAM, so the ~12 GiB PCIe address hole is
-categories summing back to the firmware's total is the sanity check that nothing
+excluded (it's `mmio`), and the RAM categories summing back to the firmware's total
-was dropped.
+is the sanity check that nothing was dropped.
 
 ## How Raspberry Pi will fit
 
@@ -150,11 +156,13 @@ never knows the difference.
 ## What's next
 
 This page is plumbing plus classification only. The map's first consumer, the
-**physical frame allocator**, is built directly on the `usable` regions here —
+**physical frame allocator**, is built directly on the `usable` regions here — which
-see [frame-allocator.md](frame-allocator.md). Still to come after that:
+already include the reclaimed boot-services memory the loader folded in (see
+[frame-allocator.md](frame-allocator.md)). Still to come:
 
-- Reclaiming `reclaimable` regions, and carefully freeing `reserved` `loader_data`
+- Freeing the `reserved` `loader_data` (these boot-time buffers) once the kernel is
-  (kernel image, these buffers) once the kernel is done reading them.
+  done reading the map.
-- Paging / the kernel's own page tables, then a heap.
+- Capturing the ACPI RSDP from the UEFI configuration table before exit (the same
+  "grab it before ExitBootServices" pattern), for when ACPI parsing arrives.
 
 See the roadmap in [efi.md](efi.md) for where this sits in the boot flow.

src/efi.zig

@@ -269,6 +269,12 @@ fn exitBootServices(bs: *uefi.tables.BootServices) !danos.MemoryMap {
 /// never sees UEFI's vocabulary — the same seam the framebuffer already uses.
 fn convertMemoryMap(map: MemoryMapSlice, out: []u8) danos.MemoryMap {
     const regions: [*]danos.MemoryRegion = @ptrCast(@alignCast(out.ptr));
+    // We're about to call boot-services memory `usable`, but our own stack lives
+    // in it and the kernel starts out running on it. Keep the region holding the
+    // current stack pointer reserved so it's never handed out.
+    const rsp = asm volatile ("mov %%rsp, %[out]"
+        : [out] "=r" (-> usize),
+    );
     var count: usize = 0;
     var i: usize = 0;
     while (i < map.info.len) : (i += 1) {
@@ -277,7 +283,10 @@ fn convertMemoryMap(map: MemoryMapSlice, out: []u8) danos.MemoryMap {
         const d: *const uefi.tables.MemoryDescriptor =
             @ptrCast(@alignCast(map.ptr + i * map.info.descriptor_size));
         if (d.number_of_pages == 0) continue;
-        const kind = classify(d);
+        var kind = classify(d);
+        // The descriptor we're executing on stays reserved (see rsp above).
+        const region_end = d.physical_start + d.number_of_pages * danos.page_size;
+        if (kind == .usable and rsp >= d.physical_start and rsp < region_end) kind = .reserved;
 
         // Coalesce with the previous region if it's the same kind and contiguous.
         if (count > 0) {
@@ -304,14 +313,16 @@ fn convertMemoryMap(map: MemoryMapSlice, out: []u8) danos.MemoryMap {
 /// a reserved address-space window (e.g. PCIe config space) — so it's `mmio`
 /// regardless of type. UEFI overloads `reserved_memory_type` for both reserved RAM
 /// and such holes, and the cache attribute is what actually tells them apart.
-/// Among RAM regions, anything we don't recognise is `reserved` — the safe
+///
-/// default; our own LoaderData (kernel image, these buffers) lands there too and
+/// Boot-services memory is folded straight into `usable`: we've already called
-/// stays reserved until the kernel reclaims it.
+/// ExitBootServices, so it's free RAM now — the kernel never needs to know it was
+/// ever the firmware's (the one live piece, our stack, is reserved by the caller).
+/// Anything unrecognised is `reserved` — the safe default; our own LoaderData (the
+/// kernel image and these buffers) lands there and stays reserved.
 fn classify(d: *const uefi.tables.MemoryDescriptor) danos.MemoryKind {
     if (!d.attribute.wb) return .mmio;
     return switch (d.@"type") {
-        .conventional_memory => .usable,
+        .conventional_memory, .boot_services_code, .boot_services_data => .usable,
-        .boot_services_code, .boot_services_data => .reclaimable,
         .acpi_reclaim_memory => .acpi_tables,
         .acpi_memory_nvs => .acpi_nvs,
         .memory_mapped_io, .memory_mapped_io_port_space => .mmio,

src/main.zig

@@ -52,25 +52,22 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     // own MemoryRegion, so this is a plain slice — no firmware layout in sight.
     const regions = @as([*]const danos.MemoryRegion, @ptrFromInt(boot_info.memory_map.regions))[0..boot_info.memory_map.len];
     var usable_pages: u64 = 0;
-    var reclaim_pages: u64 = 0;
     var reserved_pages: u64 = 0; // reserved RAM only — MMIO is device space, not RAM
     for (regions) |r| {
         switch (r.kind) {
             .usable => usable_pages += r.pages,
-            .reclaimable => reclaim_pages += r.pages,
             .reserved, .acpi_tables, .acpi_nvs => reserved_pages += r.pages,
             .mmio => {},
         }
     }
-    const total_pages = usable_pages + reclaim_pages + reserved_pages;
+    const total_pages = usable_pages + reserved_pages;
     const total_bytes = total_pages * danos.page_size;
     const gib = 1 << 30;
 
     con.write("\ndanos: physical memory\n");
     con.print("  total RAM  : {d}.{d:0>2} GiB ({d} MiB) - RAM the firmware reported\n", .{ total_bytes / gib, (total_bytes % gib) * 100 / gib, mib(total_pages) });
-    con.print("  usable     : {d} MiB - free now; owned by the frame allocator\n", .{mib(usable_pages)});
+    con.print("  usable     : {d} MiB - free RAM (incl. reclaimed boot-services memory)\n", .{mib(usable_pages)});
-    con.print("  reclaimable: {d} MiB - UEFI boot-services memory, free after exit\n", .{mib(reclaim_pages)});
+    con.print("  reserved   : {d} MiB - kernel image, boot stack, ACPI, runtime services\n", .{mib(reserved_pages)});
-    con.print("  reserved   : {d} MiB - kernel image, ACPI, runtime services\n", .{mib(reserved_pages)});
     con.print("  regions    : {d} - entries in the firmware memory map\n", .{regions.len});
 
     // Bring up the physical frame allocator over that map, and prove it works:

src/pmm.zig

@@ -58,12 +58,13 @@ fn regions(map: danos.MemoryMap) []const danos.MemoryRegion {
 pub fn init(map: danos.MemoryMap) void {
     const regs = regions(map);
 
-    // 1. Size the bitmap to cover every frame up to the highest usable address.
+    // 1. Size the bitmap to cover every frame up to the highest RAM address —
-    //    Reserved/MMIO spans above that are simply outside the map and never
+    //    including reserved RAM, so those frames are trackable (e.g. to free the
-    //    allocatable.
+    //    boot buffers later). Only MMIO (device address space) is excluded.
+    //    Everything starts unallocatable; usable regions are freed below.
     var highest: u64 = 0;
     for (regs) |r| {
-        if (r.kind != .usable) continue;
+        if (r.kind == .mmio) continue;
         const end = r.base + r.pages * page_size;
         if (end > highest) highest = end;
     }

src/root.zig

@@ -41,13 +41,14 @@ pub const page_size = 4096;
 /// 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.
+    /// 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 — never hand out.
+    /// Firmware, MMIO, the kernel image, our own boot buffers, the boot stack —
+    /// never hand out.
     reserved,
-    /// Usable once the kernel is done with boot-time structures (e.g. UEFI boot
-    /// services memory, which is free after ExitBootServices).
-    reclaimable,
     /// ACPI tables: parse, then reclaim.
     acpi_tables,
     /// ACPI non-volatile storage: preserve across sleep, do not allocate.