test fault-on-AP and affinity; report the faulting core

fault-ap-df pins a #DF to an AP (its own IST must catch it); affinity checks a pinned task never migrates. onException now names the core, so an AP fault is attributed and shown contained. Both teeth-checked.

docs/smp.md

@@ -194,7 +194,15 @@ next lands.
   core online, and enters the run loop. With interrupts on, each core's own timer tick
   preempts its idle context into whatever the global ready queue offers — so all cores
   pull real work in parallel. The `smp` test spawns CPU-bound workers and confirms they
-  execute on all four cores at once.
+  execute on all four cores at once, and `fault-ap-df` pins a #DF to an AP and checks
+  that core catches it on **its own** IST (a broken per-core TSS would triple-fault) —
+  reported as "core N: …", so a fault is always attributed to the core it happened on,
+  and is contained to that core (the rest of the system keeps running).
+- **Thread affinity** — `spawnOn(entry, priority, cpu)` pins a task to a core (its own
+  per-core pinned queue, merged with the global queue at selection; see
+  [scheduling.md](scheduling.md#affinity-pinning-a-task-to-a-core)). The `affinity`
+  test confirms a pinned task never migrates. This is the mechanism the fault-on-AP
+  test rides on, and the *explicit-affinity* real-time-predictable model.
 - **`single_threaded` off** — the kernel was built `single_threaded = true`, which
   compiles `std.atomic` down to plain non-atomic ops. Harmless on one core, but it
   quietly breaks the big kernel lock across cores; it's now `false`.
@@ -214,8 +222,13 @@ next lands.
 - **IPIs** — cross-core wake/preempt. Not needed for correctness: an idle core wakes
   on its own timer tick and pulls ready work then; IPIs only cut that latency from
   ≤1 ms to near-instant.
-- **Per-core run queues + thread affinity** — the Fiasco.OC direction, if the single
-  global queue's lock contention ever bites (and the more real-time-predictable model).
+- **Per-core run queues** — the Fiasco.OC direction, if the single global queue's lock
+  contention ever bites. (Thread *affinity* already exists — see above; this is the
+  further step of giving each core its own primary run queue for load distribution.)
+- **Fault recovery** — today a fault halts (only) the faulting core. Turning that into
+  "kill the task, keep the core running" is the [resilience](resilience.md) track (it
+  needs the task's lock/resource state handled), and for taking a core fully offline,
+  its tasks migrated first.
 
 ## Further reading
 

src/kernel/main.zig

@@ -320,21 +320,25 @@ fn kib(frames: u64) u64 {
     return frames * danos.page_size / (1024);
 }
 
-/// Report a CPU exception and halt. There's no fault recovery yet, so any
-/// exception is terminal — but it reports what and where (to every output sink,
-/// plus a POST code and a persistent breadcrumb) instead of silently resetting.
+/// Report a CPU exception and halt **this core**. There's no fault recovery yet, so
+/// the faulting core is terminal — but the fault is *contained* to it: on an
+/// application processor only that core stops, and the rest of the system keeps
+/// running (full recovery — kill the task, keep the core — is the resilience track,
+/// see docs/resilience.md). The report names the core so an AP fault is attributed,
+/// and goes to every output sink plus a POST code and a persistent breadcrumb.
 fn onException(state: *const arch.CpuState) noreturn {
     log.checkpoint(cp_exception);
+    const core = scheduler.currentCpuIndex();
     // A fault is user-facing enough to paint on screen too (via statusPrint), on
     // top of the diagnostic log.
-    statusPrint("\nCPU EXCEPTION: {s} (vector {d})\n", .{ arch.vectorName(state.vector), state.vector });
+    statusPrint("\nCPU EXCEPTION on core {d}: {s} (vector {d})\n", .{ core, arch.vectorName(state.vector), state.vector });
     statusPrint("  error code : 0x{x}\n", .{state.error_code});
     statusPrint("  RIP        : 0x{x:0>16}\n", .{state.rip});
     statusPrint("  RSP        : 0x{x:0>16}\n", .{state.rsp});
     if (state.vector == 14) statusPrint("  CR2 (addr) : 0x{x:0>16}\n", .{arch.readCr2()});
 
     var buf: [128]u8 = undefined;
-    log.recordPanic(std.fmt.bufPrint(&buf, "CPU exception {s} (vector {d}) at RIP 0x{x}", .{ arch.vectorName(state.vector), state.vector, state.rip }) catch "cpu exception");
+    log.recordPanic(std.fmt.bufPrint(&buf, "CPU exception {s} (vector {d}) on core {d} at RIP 0x{x}", .{ arch.vectorName(state.vector), state.vector, core, state.rip }) catch "cpu exception");
     arch.halt();
 }
 

src/kernel/tests.zig

@@ -74,6 +74,8 @@ pub fn run(case: []const u8, boot_info: *const BootInfo) void {
         ipcTest();
     } else if (eql(case, "smp")) {
         smpTest();
+    } else if (eql(case, "affinity")) {
+        affinityTest();
     } else if (eql(case, "smp-stress")) {
         stressTest();
     } else if (eql(case, "smp-retry")) {
@@ -84,6 +86,8 @@ pub fn run(case: []const u8, boot_info: *const BootInfo) void {
         faultPageFault();
     } else if (eql(case, "fault-df")) {
         faultDoubleFault();
+    } else if (eql(case, "fault-ap-df")) {
+        faultApTest();
     } else if (eql(case, "fault-nx")) {
         faultNoExecute();
     } else if (eql(case, "fault-null")) {
@@ -552,6 +556,52 @@ fn smpTest() void {
     result();
 }
 
+// --- affinity: a pinned task never migrates -------------------------------
+
+var affinity_cores = [_]bool{false} ** 8;
+var affinity_running: bool = true;
+
+fn affinityWorker() void {
+    const p: *volatile bool = &affinity_running;
+    while (p.*) {
+        const c = sched.currentCpuIndex();
+        if (c < affinity_cores.len) affinity_cores[c] = true;
+    }
+    sched.exit();
+}
+
+/// A task pinned to a core must run **only** on that core. Pin a busy worker to
+/// core 1 and let it run through many preemptions; it must have stamped core 1 and no
+/// other. An *unpinned* task scatters across cores (that's what the smp test shows),
+/// so a broken pin fails this deterministically — over this many time slices a
+/// free-floating task will land on some other core.
+fn affinityTest() void {
+    log("DANOS-TEST-BEGIN: affinity\n", .{});
+    affinity_cores = .{false} ** 8;
+    affinity_running = true;
+
+    if (!sched.spawnOn(affinityWorker, 4, 1)) {
+        check("worker pinned to core 1 (run with -smp)", false);
+        result();
+        return;
+    }
+
+    var spins: u64 = 0;
+    while (spins < 3_000_000_000) spins +%= 1; // many time slices across the cores
+    affinity_running = false;
+    var settle: u64 = 0;
+    while (settle < 200_000_000) settle +%= 1; // let the worker see the flag and exit
+
+    var others: u32 = 0;
+    for (affinity_cores, 0..) |seen, c| {
+        if (seen and c != 1) others += 1;
+    }
+    log("DANOS-AFFINITY: pinned worker touched core 1={}, other cores={d}\n", .{ affinity_cores[1], others });
+    check("pinned task ran on its core (1)", affinity_cores[1]);
+    check("pinned task never migrated to another core", others == 0);
+    result();
+}
+
 // --- SMP stress: hammer the big kernel lock across cores ------------------
 
 const stress_pairs = 4; // producer/consumer pairs (8 tasks; fits the 16-task pool)
@@ -704,3 +754,44 @@ fn faultDoubleFault() void {
     );
     bad_sp += 0;
 }
+
+var ap_reached_fault: bool = false;
+
+/// A task that faults with a #DF *on whatever core it's pinned to*. Announces the
+/// core, then triggers the same double fault as `faultDoubleFault` — which is only
+/// survivable on IST1, so it exercises that core's own TSS.
+fn apDoubleFaultTask() void {
+    log("DANOS-AP: task running on core {d}, triggering #DF\n", .{sched.currentCpuIndex()});
+    @atomicStore(bool, &ap_reached_fault, true, .release);
+    arch.disableInterrupts();
+    var bad_sp: u64 = 0x5000000000;
+    asm volatile (
+        \\mov %[sp], %%rsp
+        \\ud2
+        :
+        : [sp] "r" (bad_sp),
+        : .{ .memory = true }
+    );
+    bad_sp += 0;
+}
+
+/// Fault on an application processor. Pins a double-faulting task to core 1, so the
+/// fault is taken and handled by *that core's own* IDT and TSS/IST — not the BSP's.
+/// The harness matches "core N: double fault (vector 8)" with N ≥ 1, which can only
+/// appear if the AP caught the #DF on its IST1 (a broken per-core TSS would
+/// triple-fault and reset instead). We then show the BSP still runs afterwards, so
+/// the fault was *contained* to the AP, not fatal to the system.
+fn faultApTest() void {
+    log("DANOS-TEST-BEGIN: fault-ap-df\n", .{});
+    if (!sched.spawnOn(apDoubleFaultTask, 6, 1)) {
+        log("DANOS-AP: could not pin to core 1 (run with -smp) - FAIL\n", .{});
+        arch.halt();
+    }
+    // Wait until the AP is about to fault, then keep running to prove containment.
+    var spins: u64 = 0;
+    while (!@atomicLoad(bool, &ap_reached_fault, .acquire) and spins < 5_000_000_000) spins +%= 1;
+    var settle: u64 = 0;
+    while (settle < 500_000_000) settle +%= 1; // let the AP take + report the fault
+    log("DANOS-BSP: core {d} still running after the AP fault (contained)\n", .{sched.currentCpuIndex()});
+    arch.halt();
+}

test/qemu_test.py

@@ -119,6 +119,11 @@ CASES = [
      "smp":    4,
      "expect": r"DANOS-TEST-RESULT: PASS",
      "fail":   r"DANOS-TEST-RESULT: FAIL"},
+    # Affinity: a pinned task must never migrate off its core.
+    {"name": "affinity",
+     "smp":    4,
+     "expect": r"DANOS-TEST-RESULT: PASS",
+     "fail":   r"DANOS-TEST-RESULT: FAIL"},
     # Stress the big kernel lock across cores; heavier, so a longer timeout.
     {"name": "smp-stress",
      "smp":    4,
@@ -133,6 +138,12 @@ CASES = [
     {"name": "fault-ud", "expect": r"invalid opcode \(vector 6\)"},
     {"name": "fault-pf", "expect": r"page fault \(vector 14\)"},
     {"name": "fault-df", "expect": r"double fault \(vector 8\)"},
+    # Fault on an application processor: a #DF pinned to core 1 must be caught by that
+    # core's own IST (N >= 1), proving per-core TSS works; a broken one triple-faults.
+    {"name": "fault-ap-df",
+     "smp":    4,
+     "expect": r"core [1-9]\d*: double fault \(vector 8\)",
+     "fail":   r"could not pin"},
     {"name": "fault-nx",
      "expect": r"page fault \(vector 14\)",
      "fail":   r"NX not enforced"},