enumerate usable CPU cores from the MADT

Keep each Local APIC's id and expose platform.cpus().

src/device/acpi.zig

@@ -91,6 +91,35 @@ pub const PlatformInfo = struct {
 /// Filled in by `discover`; the arch layer reads it during bring-up.
 pub var platform_info: PlatformInfo = .{};
 
+/// One usable logical processor, from a MADT type-0 (Local APIC) record. The
+/// `apic_id` is the Local APIC ID that SMP bring-up targets to wake this core
+/// (INIT–SIPI–SIPI); `processor_id` is the ACPI namespace handle. Only processors
+/// the firmware marks *enabled* are recorded — a disabled one can't be started.
+pub const Cpu = struct {
+    processor_id: u8,
+    apic_id: u8,
+    /// MADT flags bit 1: usable but firmware-started offline (hot-plug / deferred
+    /// bring-up), as opposed to already available. Informational for now.
+    online_capable: bool,
+};
+
+/// The set of usable logical processors the MADT listed — the hardware's degree of
+/// parallelism. Includes the bootstrap processor danos already runs on; the rest
+/// are the application processors SMP bring-up would start (see docs/smp.md).
+pub const CpuInfo = struct {
+    /// A static pool sized well above any danos target (a desktop, two 4-core Pis).
+    /// If the MADT ever lists more, the surplus is dropped and counted in `dropped`
+    /// so the truncation is never silent.
+    cpus: [max_cpus]Cpu = undefined,
+    count: usize = 0,
+    dropped: usize = 0,
+};
+
+const max_cpus = 64;
+
+/// Filled in by `discover` (from the MADT); SMP bring-up reads it to wake the APs.
+pub var cpu_info: CpuInfo = .{};
+
 /// Integrity/diagnostics for the AML parse. `consumed == total` means the parser
 /// walked every byte of the DSDT/SSDTs without desyncing.
 pub const AmlStats = struct {
@@ -438,6 +467,18 @@ fn parseMadt(dt: *DeviceTree, header: *const SystemDescriptorTableHeader) !void
                     var nb: [24]u8 = undefined;
                     const nm = std.fmt.bufPrint(&nb, "cpu{d}", .{la.processor_id}) catch "cpu";
                     _ = try dt.addChild(dt.root, .processor, nm);
+                    // Also record it as a schedulable core (with the APIC ID an AP
+                    // wake needs, which the device node name doesn't preserve).
+                    if (cpu_info.count < cpu_info.cpus.len) {
+                        cpu_info.cpus[cpu_info.count] = .{
+                            .processor_id = la.processor_id,
+                            .apic_id = la.apic_id,
+                            .online_capable = la.flags & 2 != 0,
+                        };
+                        cpu_info.count += 1;
+                    } else {
+                        cpu_info.dropped += 1;
+                    }
                 }
             },
             1 => {