wake application processors to long mode

INIT-SIPI-SIPI plus a self-relocating real-mode trampoline.

src/kernel/arch/x86_64/apic.zig

@@ -44,11 +44,15 @@ const spurious_vector = 47;
 // LAPIC register offsets.
 const reg_spurious = 0x0F0;
 const reg_eoi = 0x0B0;
+const reg_icr_low = 0x300; // interrupt command register, low dword (writing it sends)
+const reg_icr_high = 0x310; // ICR high dword (destination APIC id in bits 24-31)
 const reg_lvt_timer = 0x320;
 const reg_timer_initial = 0x380;
 const reg_timer_current = 0x390;
 const reg_timer_divide = 0x3E0;
 
+const icr_delivery_pending = 1 << 12; // ICR low bit 12: a previous IPI is still in flight
+
 const lvt_masked = 1 << 16;
 const lvt_periodic = 1 << 17;
 const timer_divide_16 = 0x3;
@@ -120,6 +124,39 @@ pub fn init() void {
     write(reg_spurious, 0x100 | spurious_vector); // bit 8 = software enable
 }
 
+/// Software-enable *this* core's Local APIC — the application-processor counterpart
+/// of `init`, minus the one-time PIC remap (the BSP already masked it) and minus
+/// calibration (the timer rate is a shared hardware constant, measured once). Each
+/// core has its own LAPIC at the same MMIO address, so no per-core base is needed.
+pub fn initSecondary() void {
+    const msr = io.rdmsr(ia32_apic_base_msr);
+    io.wrmsr(ia32_apic_base_msr, msr | (1 << 11)); // global enable
+    write(reg_spurious, 0x100 | spurious_vector); // software enable
+}
+
+// --- application-processor wakeup (INIT–SIPI–SIPI) --------------------------
+
+/// Send an INIT IPI to the core with Local APIC id `apic_id` — the first step of
+/// the wake sequence. Blocks until the LAPIC reports the IPI was delivered.
+pub fn sendInit(apic_id: u32) void {
+    write(reg_icr_high, apic_id << 24);
+    write(reg_icr_low, 0x4500); // INIT, physical destination, assert, edge-triggered
+    waitIcrIdle();
+}
+
+/// Send a STARTUP IPI (SIPI) telling the target core to begin executing at physical
+/// address `vector << 12` (in real mode). Per the Intel bring-up protocol this is
+/// sent twice after the INIT; both calls block until delivery completes.
+pub fn sendStartup(apic_id: u32, vector: u8) void {
+    write(reg_icr_high, apic_id << 24);
+    write(reg_icr_low, 0x4600 | @as(u32, vector)); // STARTUP with the page vector
+    waitIcrIdle();
+}
+
+fn waitIcrIdle() void {
+    while (read(reg_icr_low) & icr_delivery_pending != 0) {}
+}
+
 /// The calibration window: we time everything against a 10 ms reference interval.
 const calib_ms = 10;
 

src/kernel/arch/x86_64/cpu.zig

@@ -13,6 +13,7 @@ const serial = @import("serial.zig");
 const apic = @import("apic.zig");
 const ioapic = @import("ioapic.zig");
 const io = @import("io.zig");
+const smp = @import("smp.zig");
 
 /// The saved register/trap frame passed to a fault handler.
 pub const CpuState = idt.CpuState;
@@ -99,6 +100,27 @@ pub fn cpuLocal() usize {
     return io.rdmsr(ia32_gs_base);
 }
 
+// --- SMP: application-processor bring-up ----------------------------------
+
+/// Make a low RAM page executable (clear its NX bit) — the AP trampoline is fetched
+/// from it under paging. Delegates to the VMM; see paging.setExecutable.
+pub fn setPageExecutable(phys: u64) void {
+    paging.setExecutable(phys);
+}
+
+/// Copy the AP trampoline into its low page (allocated + made executable by the
+/// caller). Run once before waking any application processor.
+pub fn prepareSecondaries(tramp_phys: u64) void {
+    smp.prepare(tramp_phys);
+}
+
+/// Wake the core with Local APIC id `apic_id`, giving it `stack_top` and its per-CPU
+/// pointer `percpu`; it adopts the current (kernel) page tables. Returns false if it
+/// doesn't come online within the timeout. Blocks until the core reports in.
+pub fn startSecondary(apic_id: u32, stack_top: usize, percpu: usize) bool {
+    return smp.startAp(apic_id, stack_top, percpu, readCr3());
+}
+
 /// Kernel tick rate: 1000 Hz (1 ms), the scheduler's time quantum.
 pub const timer_hz = 1000;
 

src/kernel/arch/x86_64/gdt.zig

@@ -44,11 +44,20 @@ const Descriptor = packed struct {
 /// isr.s — it uses the selectors 0x08 (code) and 0x10 (data) that match `table`.
 extern fn gdt_flush(descriptor: *const Descriptor) callconv(.c) void;
 
-/// Install our GDT and switch onto its segments.
-pub fn init() void {
+/// Load our GDT on the current core and switch onto its segments. The table is
+/// shared across all cores (the descriptors are flat and read-only); each core just
+/// needs to point its GDTR at it. Called by the BSP in `init` and by every AP during
+/// bring-up. Note this reloads the segment registers, which zeroes the GS base — so
+/// a core must publish its per-CPU pointer (setCpuLocal) *after* calling this.
+pub fn loadOnThisCpu() void {
     const descriptor = Descriptor{
         .limit = @sizeOf(@TypeOf(table)) - 1,
         .base = @intFromPtr(&table),
     };
     gdt_flush(&descriptor);
 }
+
+/// Install our GDT and switch onto its segments (bootstrap processor).
+pub fn init() void {
+    loadOnThisCpu();
+}

src/kernel/arch/x86_64/idt.zig

@@ -128,6 +128,13 @@ pub fn init() void {
     // Run the double-fault handler (vector 8) on IST1: a #DF usually means the
     // current stack is unusable, so it needs a guaranteed-good one. See tss.zig.
     idt[8].ist = tss.double_fault_ist;
+    loadOnThisCpu();
+}
+
+/// Load the (shared, already-populated) IDT on the current core. The gate table is
+/// read-only after `init`, so every core points its IDTR at the same one. Called by
+/// the BSP via `init` and by each AP during bring-up.
+pub fn loadOnThisCpu() void {
     const descriptor = Descriptor{
         .limit = @sizeOf(@TypeOf(idt)) - 1,
         .base = @intFromPtr(&idt),

src/kernel/arch/x86_64/paging.zig

@@ -128,6 +128,16 @@ pub fn map(virt: u64, phys: u64, writable_page: bool) void {
     invalidate(virt);
 }
 
+/// Make an already-identity-mapped RAM page **executable** (clear its NX bit),
+/// leaving it present and writable. The blanket RAM mapping is NX for W^X, but the
+/// application processors fetch the AP trampoline from a low RAM page under paging —
+/// so that one page must be executable. A deliberate, temporary W^X exception for a
+/// single bring-up page; the caller frees it once every AP is up.
+pub fn setExecutable(phys: u64) void {
+    mapPage(kernel_pml4, phys, phys, present | writable); // note: no no_execute
+    invalidate(phys);
+}
+
 /// Remove a mapping and flush it from the TLB.
 pub fn unmap(virt: u64) void {
     const pml4e = tableAt(kernel_pml4)[(virt >> 39) & 0x1FF];

src/kernel/arch/x86_64/smp.zig

@@ -0,0 +1,101 @@
+//! Application-processor (AP) bring-up: waking the cores the firmware left parked.
+//!
+//! The firmware starts only the bootstrap processor (BSP); the others sit idle until
+//! the kernel wakes them with an INIT–SIPI–SIPI sequence (Intel SDM Vol.3, "MP
+//! Initialization"). A woken core begins in 16-bit real mode at a low physical page,
+//! runs the [trampoline](trampoline.s) up into 64-bit long mode, and lands in
+//! `apEntry` here. This module copies the trampoline into place, patches its
+//! per-AP parameters, drives the wake IPIs, and waits for each core to report in.
+//!
+//! Cores are brought up **one at a time**: a single trampoline page and parameter
+//! block are reused, so the BSP patches, wakes, and waits for one AP before the
+//! next. The mechanism-vs-policy split matches the rest of the kernel — the generic
+//! scheduler decides *what* runs where; this just gets a core executing 64-bit code.
+//!
+//! This is step 3a: an AP climbs to long mode, publishes its per-CPU pointer, marks
+//! itself alive, and parks. Entering the scheduler (its own TSS, LAPIC timer, and
+//! the run loop) is the next step.
+
+const io = @import("io.zig");
+const apic = @import("apic.zig");
+
+/// IA32_GS_BASE — the per-CPU data pointer (see cpu.zig; kept in sync here so the AP
+/// path doesn't depend on cpu.zig and risk an import cycle).
+const ia32_gs_base = 0xC000_0101;
+
+/// Physical address of the trampoline page (page-aligned, below 1 MiB). Set by
+/// `prepare`; the low 20 bits are always zero, so `phys >> 12` is the SIPI vector.
+var tramp_phys: u64 = 0;
+
+/// Set to 1 by a freshly-woken AP once it reaches `apEntry` and finishes its own
+/// bring-up. The BSP clears it before each wake and polls it afterwards — a simple
+/// one-at-a-time handshake (only one AP is being started at any moment).
+var ap_alive: u32 = 0;
+
+/// Copy the trampoline blob to its low page. Call once, after the page has been
+/// allocated and made executable, before waking any AP.
+pub fn prepare(phys: u64) void {
+    tramp_phys = phys;
+    const start = @extern([*]const u8, .{ .name = "ap_trampoline_start" });
+    const end = @extern([*]const u8, .{ .name = "ap_trampoline_end" });
+    const len = @intFromPtr(end) - @intFromPtr(start);
+    const dst: [*]u8 = @ptrFromInt(phys);
+    @memcpy(dst[0..len], start[0..len]);
+}
+
+/// Address of a patchable trampoline parameter, by symbol name: the copied blob's
+/// base plus the field's offset within it (a same-section symbol difference). The
+/// pointer is `align(1)` — the fields aren't 8-aligned within the blob, and x86
+/// tolerates unaligned stores, so we don't force layout constraints on the asm.
+fn param(comptime name: []const u8) *align(1) volatile u64 {
+    const start = @intFromPtr(@extern([*]const u8, .{ .name = "ap_trampoline_start" }));
+    const sym = @intFromPtr(@extern([*]const u8, .{ .name = name }));
+    return @ptrFromInt(tramp_phys + (sym - start));
+}
+
+/// Wake the core with Local APIC id `apic_id`, hand it `stack_top` and `percpu` (its
+/// per-CPU pointer), and wait for it to come alive. Returns false if it doesn't
+/// report in within the timeout (left parked, no harm to the running system).
+/// `cr3` is the kernel page tables the AP adopts. Precondition: `prepare` has run.
+pub fn startAp(apic_id: u32, stack_top: usize, percpu: usize, cr3: u64) bool {
+    param("ap_tramp_cr3").* = cr3;
+    param("ap_tramp_stack").* = stack_top;
+    param("ap_tramp_entry").* = @intFromPtr(&apEntry);
+    param("ap_tramp_percpu").* = percpu;
+
+    @atomicStore(u32, &ap_alive, 0, .seq_cst);
+
+    const vector: u8 = @intCast(tramp_phys >> 12);
+    apic.sendInit(apic_id);
+    delayMicros(10_000); // 10 ms INIT settle
+    apic.sendStartup(apic_id, vector);
+    delayMicros(200);
+    apic.sendStartup(apic_id, vector);
+
+    // Wait up to 100 ms for the AP to reach apEntry and set the flag.
+    const deadline = apic.millis() + 100;
+    while (apic.millis() < deadline) {
+        if (@atomicLoad(u32, &ap_alive, .acquire) != 0) return true;
+        asm volatile ("pause");
+    }
+    return false;
+}
+
+/// Busy-wait `us` microseconds against the calibrated TSC clock (the AP wake happens
+/// after the timer is up, so the clock is available).
+fn delayMicros(us: u64) void {
+    const start = apic.micros();
+    while (apic.micros() - start < us) asm volatile ("pause");
+}
+
+/// The 64-bit entry every AP lands on, called from the trampoline with its per-CPU
+/// pointer in RDI. Adopts the shared descriptor tables, publishes its per-CPU
+/// pointer, signals the BSP it's alive, and (for now) parks. Never returns.
+fn apEntry(percpu: usize) callconv(.c) noreturn {
+    // Step 3a: minimal. The core keeps the trampoline's descriptor tables, publishes
+    // its per-CPU pointer, signals the BSP, and parks with interrupts off. Loading
+    // this core's own kernel GDT/IDT/TSS and entering the scheduler is step 3b.
+    io.wrmsr(ia32_gs_base, percpu); // publish per-CPU pointer (GS base)
+    @atomicStore(u32, &ap_alive, 1, .release); // "I'm up" — BSP is polling this
+    while (true) asm volatile ("hlt"); // parked (3b enters the scheduler here)
+}

src/kernel/arch/x86_64/trampoline.s

@@ -0,0 +1,140 @@
+# AP trampoline: brings a waking application processor from the 16-bit real mode it
+# starts in (after INIT-SIPI-SIPI) up through protected mode into 64-bit long mode,
+# then jumps to the Zig AP entry (arch/x86_64/smp.zig:apEntry).
+#
+# A STARTUP IPI vectors a core to physical address `vector << 12` in real mode, so
+# this blob is copied to a low (<1 MiB) page and started there; at entry CS = that
+# page >> 4 and IP = 0. It is fully **position-independent**: it derives its own
+# linear base (CS << 4) into EBX and addresses every internal datum as
+# `(label - ap_trampoline_start)(%ebx)` — a difference of two symbols in the same
+# section, which the assembler folds to a constant page offset no matter where the
+# blob was linked or copied to. The BSP patches the parameter block (CR3, stack,
+# entry, per-CPU pointer) before each wake; see arch/x86_64/smp.zig.
+#
+# It lives in .rodata (not .text): it is data to be copied out and executed
+# elsewhere, never run at its link address, so it must not be a normal code segment.
+
+.section .rodata
+.balign 16
+.code16
+.global ap_trampoline_start
+ap_trampoline_start:
+    cli
+    cld
+
+    # Linear base of this page (CS << 4) into EBX; all data is addressed off it.
+    xorl %eax, %eax
+    mov %cs, %ax
+    shll $4, %eax
+    movl %eax, %ebx
+
+    mov %cs, %ax            # DS = CS, so we address our data as DS:(label - start):
+    mov %ax, %ds            # the segment base (CS<<4) already supplies the page base,
+                            # so data operands use the page *offset*, not EBX.
+
+    # Relocate the pointers whose absolute (linear) targets depend on where we were
+    # copied: the GDT base and the two far-jump targets = EBX + their page offsets.
+    # EBX supplies the base for the *value* (via leal); the store address is DS-rel.
+    leal (gdt32 - ap_trampoline_start)(%ebx), %eax
+    movl %eax, gdtr32_base - ap_trampoline_start
+    leal (prot_entry - ap_trampoline_start)(%ebx), %eax
+    movl %eax, jmp32_off - ap_trampoline_start
+    leal (long_entry - ap_trampoline_start)(%ebx), %eax
+    movl %eax, jmp64_off - ap_trampoline_start
+
+    lgdtl gdtr32 - ap_trampoline_start
+
+    movl %cr0, %eax         # enter protected mode (CR0.PE)
+    orl $1, %eax
+    movl %eax, %cr0
+
+    ljmpl *(jmp32_ptr - ap_trampoline_start)   # -> prot_entry, CS = 0x08
+
+.code32
+prot_entry:
+    movw $0x10, %ax         # flat 32-bit data segments
+    movw %ax, %ds
+    movw %ax, %es
+    movw %ax, %ss
+    movw %ax, %fs
+    movw %ax, %gs
+
+    movl %cr4, %eax         # PAE on (CR4.PAE) — required for long mode
+    orl $(1 << 5), %eax
+    movl %eax, %cr4
+
+    movl (param_cr3 - ap_trampoline_start)(%ebx), %eax  # kernel page tables
+    movl %eax, %cr3
+
+    movl $0xC0000080, %ecx  # EFER: long mode enable (LME) + NX enable (NXE, since
+    rdmsr                   # the kernel's PTEs set the NX bit)
+    orl $((1 << 8) | (1 << 11)), %eax
+    wrmsr
+
+    movl %cr0, %eax         # paging on (CR0.PG) — now in long mode (compat sub-mode)
+    orl $(1 << 31), %eax
+    movl %eax, %cr0
+
+    ljmpl *(jmp64_ptr - ap_trampoline_start)(%ebx)   # -> long_entry, CS = 0x18 (L=1)
+
+.code64
+long_entry:
+    movw $0x10, %ax         # sane flat data segments
+    movw %ax, %ds
+    movw %ax, %es
+    movw %ax, %ss
+
+    # RBX = EBX (zero-extended) = page base. Load our stack and per-CPU pointer, then
+    # call the Zig entry — which runs from the kernel image and never returns.
+    movq (param_stack - ap_trampoline_start)(%rbx), %rsp
+    movq (param_percpu - ap_trampoline_start)(%rbx), %rdi   # SysV arg 0
+    movq (param_entry - ap_trampoline_start)(%rbx), %rax
+    callq *%rax
+1:  hlt                     # unreachable; guard against a stray return
+    jmp 1b
+
+# --- data: GDT, far pointers, and the BSP-patched parameter block -----------
+.balign 8
+gdt32:
+    .quad 0x0000000000000000    # 0x00 null
+    .quad 0x00CF9A000000FFFF    # 0x08 32-bit code (G, D, present, exec/read)
+    .quad 0x00CF92000000FFFF    # 0x10 data (valid in 32- and 64-bit)
+    .quad 0x00AF9A000000FFFF    # 0x18 64-bit code (L=1)
+gdt32_end:
+
+gdtr32:
+    .word gdt32_end - gdt32 - 1
+gdtr32_base:
+    .long 0                     # patched (16-bit code): linear base of gdt32
+
+jmp32_ptr:                      # indirect far-jump operand: offset then selector
+jmp32_off:
+    .long 0                     # patched: linear address of prot_entry
+    .word 0x08                  # 32-bit code selector
+
+jmp64_ptr:
+jmp64_off:
+    .long 0                     # patched: linear address of long_entry
+    .word 0x18                  # 64-bit code selector
+
+# The parameter block, filled in by the BSP (smp.zig) before each STARTUP IPI. Global
+# so the Zig side can locate each field as (symbol - ap_trampoline_start).
+.global ap_tramp_cr3
+.global ap_tramp_stack
+.global ap_tramp_entry
+.global ap_tramp_percpu
+param_cr3:
+ap_tramp_cr3:
+    .quad 0                     # kernel PML4 physical address (CR3)
+param_stack:
+ap_tramp_stack:
+    .quad 0                     # top of this AP's kernel stack
+param_entry:
+ap_tramp_entry:
+    .quad 0                     # address of apEntry (the Zig AP entry)
+param_percpu:
+ap_tramp_percpu:
+    .quad 0                     # this AP's per-CPU pointer (goes in GS base)
+
+.global ap_trampoline_end
+ap_trampoline_end:

src/kernel/main.zig

@@ -30,6 +30,11 @@ const cp_running = 0x70;
 const cp_exception = 0xE0;
 const cp_panic = 0xEE;
 
+/// Physical address of the low page reserved at boot for the AP trampoline (0 = none
+/// was available). Claimed right after the frame allocator comes up, before paging
+/// and the heap consume the scarce sub-1 MiB frames.
+var ap_trampoline_page: u64 = 0;
+
 /// Kernel entry point. The bootloader jumps here after `ExitBootServices` with a
 /// pointer to the handoff data. There is no runtime, no stack unwinding, and no
 /// caller to return to, so this never returns.
@@ -98,6 +103,10 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     // Bring up the physical frame allocator over that map, and prove it works:
     // allocate three frames, then hand them back.
     pmm.init(boot_info.memory_map);
+    // Claim the AP trampoline's low (<1 MiB) page *now*, before paging and the heap
+    // draw down sub-1 MiB frames (the allocator scans upward from frame 0). Held
+    // until SMP bring-up; 0 means none was available (we stay uniprocessor).
+    ap_trampoline_page = pmm.allocBelow(0x100000) orelse 0;
     const s1 = pmm.stats();
     log.print("\ndanos: frame allocator online\n", .{});
     log.print("  free frames: {d} ({d} MiB)\n", .{ s1.free_frames, mib(s1.free_frames) });
@@ -221,6 +230,10 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     log.checkpoint(cp_timer);
     log.print("danos: timer online ({d} Hz tick; LAPIC {d} MHz, TSC {d} MHz; calibrated via {s})\n", .{ arch.timer_hz, arch.lapicHz() / 1_000_000, arch.tscHz() / 1_000_000, arch.timerCalibrationSource() });
 
+    // Wake the other cores (application processors). A no-op on a single-core
+    // machine; on SMP each AP climbs to long mode and reports in (docs/smp.md).
+    bringUpSecondaries();
+
     // In a test build (`zig build -Dtest-case=<name>`), run that case and stop.
     // Normal builds fall through to the idle halt.
     if (build_options.test_case) |case| {
@@ -238,6 +251,41 @@ fn kmain(boot_info: *const BootInfo) noreturn {
     arch.halt();
 }
 
+/// Wake the application processors the firmware left parked. Allocates the low
+/// trampoline page (and makes it executable), then wakes each non-boot core in turn,
+/// handing it a fresh kernel stack and its per-CPU slot. Cores that don't report in
+/// are left parked — the running system is unaffected. See docs/smp.md.
+fn bringUpSecondaries() void {
+    const cores = platform.cpus();
+    if (cores.len <= 1) return;
+
+    // The trampoline page was reserved below 1 MiB at boot (a real-mode SIPI vector
+    // addresses it). Make it executable — the blanket RAM mapping is NX (W^X).
+    if (ap_trampoline_page == 0) {
+        log.write("danos: smp: no low page for the AP trampoline; staying uniprocessor\n");
+        return;
+    }
+    arch.setPageExecutable(ap_trampoline_page);
+    arch.prepareSecondaries(ap_trampoline_page);
+
+    log.print("\ndanos: bringing up {d} application processor(s)\n", .{cores.len - 1});
+    for (cores[1..], 1..) |core, index| {
+        const stack = heap.allocator().alloc(u8, 16 * 1024) catch {
+            log.print("  cpu apic_id {d}: no stack; skipped\n", .{core.apic_id});
+            continue;
+        };
+        const stack_top = (@intFromPtr(stack.ptr) + stack.len) & ~@as(usize, 15);
+        const pc = scheduler.prepareSecondary(index, core.apic_id);
+        if (arch.startSecondary(core.apic_id, stack_top, @intFromPtr(pc))) {
+            pc.online = true;
+            log.print("  cpu apic_id {d}: online\n", .{core.apic_id});
+        } else {
+            log.print("  cpu apic_id {d}: no response (parked)\n", .{core.apic_id});
+        }
+    }
+    log.print("danos: {d}/{d} cores online\n", .{ scheduler.onlineCount(), cores.len });
+}
+
 /// A user-facing status line: to the diagnostic `log` *and* the on-screen console
 /// (if a framebuffer is present). The verbose log uses `log.*` directly and never
 /// touches the framebuffer.

src/kernel/pmm.zig

@@ -141,6 +141,24 @@ pub fn alloc() ?u64 {
     return null; // out of physical memory
 }
 
+/// Allocate one free frame whose physical address is below `limit`, or null if
+/// none is free down there. The AP trampoline needs this: an x86 STARTUP IPI vectors
+/// a waking core to physical `vector << 12`, and `vector` is a byte — so the
+/// trampoline must live under 1 MiB. A short linear scan of the low frames; only run
+/// a handful of times at boot, so it needn't be fast.
+pub fn allocBelow(limit: u64) ?u64 {
+    const cap = @min(total_frames, @as(usize, @intCast(limit / page_size)));
+    var f: usize = 1; // frame 0 stays reserved as the "none" address
+    while (f < cap) : (f += 1) {
+        if (!isUsed(f)) {
+            setUsed(f);
+            used_frames += 1;
+            return @as(u64, f) * page_size;
+        }
+    }
+    return null;
+}
+
 /// Return a frame obtained from alloc() to the pool. Bogus or double frees are
 /// ignored rather than corrupting the count.
 pub fn free(addr: u64) void {

src/kernel/scheduler.zig

@@ -102,6 +102,25 @@ fn idle() void {
     while (true) asm volatile ("hlt");
 }
 
+/// Reserve and initialise the per-CPU slot for an application processor at dense
+/// `index` (1-based; 0 is the BSP) with Local APIC id `apic_id`, and return a
+/// pointer the arch bring-up hands to the core (it publishes it in its GS base).
+/// Called on the BSP before waking each AP; the AP marks itself `online`.
+pub fn prepareSecondary(index: usize, apic_id: u32) *PerCpu {
+    const pc = &cpus[index];
+    pc.* = .{ .index = @intCast(index), .apic_id = apic_id, .online = false };
+    return pc;
+}
+
+/// Number of cores that have finished bring-up (the BSP plus every online AP).
+pub fn onlineCount() usize {
+    var n: usize = 0;
+    for (&cpus) |*pc| {
+        if (pc.online) n += 1;
+    }
+    return n;
+}
+
 fn enqueue(t: *Task) void {
     t.next = null;
     const p: usize = t.priority;