Merge dispatcher-probe-audit/p0-ctor-probe-and-struct-dump (KRNBUG-AUDIT-004)

audit-findings.md

@@ -4457,3 +4457,168 @@ function arg (no constant-load), but the simple
 3. Treat 0x42450b5c independently. AUDIT-002's hook missed it because
    the parking site (PC=0x824cd4f4) isn't routed through `do_wait_single`.
    Open KRNBUG-AUDIT-004 for that wait path.
+
+---
+
+### KRNBUG-AUDIT-004 — `--ctor-probe` PC hook + `--dump-addr` struct dump; producer-indirection layer identified; "8-instance pool" hypothesis falsified
+
+**Status**: landed on master (no-ff merge of feature branch
+`dispatcher-probe-audit/p0-ctor-probe-and-struct-dump`). Diagnostic-
+only, read-only, lockstep-preserved (`instructions=100000002` at
+`-n 100M --stable-digest`).
+
+**Tests**: 586 → **588**.
+
+**What landed (`crates/xenia-kernel/src/state.rs`):**
+- `pub ctor_probe_pcs: HashSet<u32>` field on `KernelState` (default
+  empty).
+- `pub fire_ctor_probe_if_match(hw_id, mem)` — fast-rejects when set
+  is empty; on match prints a one-shot record `CTOR-PROBE pc=...
+  tid=... hw=... cycle=... sp=... r3=... lr=...` plus an 8-frame
+  back-chain with saved-r31/r30 per frame. Pure read.
+- `pub dump_addrs: Vec<u32>` field for end-of-run struct dumps.
+- 2 unit tests: empty-set no-op, set-membership invariant.
+
+**What landed (`crates/xenia-app/src/main.rs`):**
+- `--ctor-probe=0x8217C850,0x82181750,...` CLI flag (and
+  `XENIA_CTOR_PROBE`). Parsed into `kernel.ctor_probe_pcs` at
+  `cmd_exec_inner` startup.
+- `--dump-addr=0x828F3D08,0x828F4070,0x828F3EC0,...` CLI flag (and
+  `XENIA_DUMP_ADDR`). Each address gets a 128-byte hex+be32+ASCII
+  dump at end-of-run, after the per-handle FOCUS report.
+- `worker_prologue` calls `fire_ctor_probe_if_match` after reading
+  `pc` and before any thunk-dispatch / step-block branch.
+  `dump_thread_diagnostic` consumes `kernel.dump_addrs`.
+
+**Decisive findings (corrects KRNBUG-AUDIT-002/003):**
+
+1. **The "8-instance pool" hypothesis for handle 0x1004 is FALSE.**
+   Probe ran at `-n 50M --halt-on-deadlock` with PCs
+   `[0x821783D8, 0x82181750, 0x821701C8]` (the per-instance ctors
+   for handles 0x1004 / 0x100c / 0x15e0 respectively). Each fired
+   **EXACTLY ONCE**:
+   ```
+   CTOR-PROBE pc=0x821783d8 tid=1 hw=0 cycle=1401430 r3=0x828f3ec0  ← handle 0x1004
+   CTOR-PROBE pc=0x82181750 tid=1 hw=0 cycle=5363599 r3=0x828f3d08  ← handle 0x100c
+   CTOR-PROBE pc=0x821701c8 tid=1 hw=0 cycle=9203618 r3=0x828f4070  ← handle 0x15e0
+   ```
+   Handle 0x1004 has a SINGLE dispatcher at **0x828F3EC0**, not 8
+   pool members. The earlier "called 8 times" claim came from
+   counting raw entries to the OUTER getter `sub_8217C850`, but
+   `sub_8217C850` is a Meyers-style singleton-getter — its inner
+   `bl 0x821783D8` (the per-instance ctor) is gated on a one-shot
+   init flag at `[0x828F48D8] bit 0`. Subsequent `sub_8217C850`
+   calls just return the existing slot pointer.
+
+2. **The producer indirection layer IS the singleton-getter
+   itself.** Static byte-scans of `.rdata` and `.data` show 0 hits
+   for the dispatcher addresses 0x828F3D08 / 0x828F4070 — so no
+   registry table holds them. But the `xrefs` table for the OUTER
+   getters reveals:
+   ```
+   sub_821800D8 (outer for 0x828F3D08, handle 0x100c): 6 callers
+     0x821802d8 (sub_82180158+0x180)   ← non-create-chain
+     0x821806e0 (sub_821805C8+0x118)   ← non-create-chain
+     0x82180b28 (sub_82180A10+0x118)   ← non-create-chain
+     0x82180ea0 (sub_82180D90+0x110)   ← non-create-chain
+     0x82181254 (sub_821810E0+0x174)   ← non-create-chain
+     0x82181c54 (sub_82181C28+0x2C)    ← create-chain ONLY
+
+   sub_8216F618 (outer for 0x828F4070, handle 0x15e0): 5 callers
+     0x8216f9d4 (sub_8216F818+0x1BC)   ← non-create-chain
+     0x8216fc08 (sub_8216F9F0+0x218)   ← non-create-chain
+     0x821700b8 (sub_8216FF70+0x148)   ← non-create-chain
+     0x821700f4 (sub_821700E0+0x14)    ← non-create-chain
+     0x821707f4 (sub_821707C0+0x34)    ← create-chain ONLY
+   ```
+   The non-create-chain consumers all share the **canonical
+   producer pattern**:
+   ```
+   bl outer_singleton_getter   ; r3 = dispatcher ptr
+   lwz r3, OFFSET(r3)          ; r3 = an event handle / queue field
+   bl 0x824AA1D8               ; signal/wake function
+   ```
+   For 0x100c the offset is 80 (= 0x50); for 0x15e0 the offset is
+   36 (= 0x24).
+
+   So **interpretation (2) of the audit charter is confirmed**:
+   producers reference the dispatchers via a function-call layer of
+   indirection, not through direct address materialization. The
+   xref-table audit in AUDIT-003 (which only catches direct
+   constant-loads of the dispatcher base) was **necessary but not
+   sufficient** — it correctly saw "no direct producer references"
+   but missed the singleton-getter indirection.
+
+3. **Dispatcher struct layouts** (128-byte dumps at `-n 50M
+   --halt-on-deadlock`):
+   ```
+   0x828F3D08 (handle 0x100c, per-instance ctor sub_82181750):
+     +0x00 = 0xFFFFFFFF                  ; queue head/tail sentinel
+     +0x28 = 0x00000007                  ; capacity = 7
+     +0x2C = 0x01000000                  ; init flag
+     +0x3C = 0xFFFFFFFF                  ; secondary sentinel
+     +0x48 = 0x00001010                  ; thread_handle (worker thread)
+     +0x4C = 0x0000100C                  ; event_handle (= self handle 0x100c)
+     +0x50 = 0x00000000                  ; producer reads this — currently 0
+     +0x70 = 0x00000001                  ; refcount?
+     +0x74 = 0x828F3D08                  ; self-pointer
+
+   0x828F4070 (handle 0x15e0, per-instance ctor sub_821701C8):
+     +0x00 = 0x01000000                  ; init flag
+     +0x10 = 0xFFFFFFFF                  ; queue sentinel
+     +0x1C = 0x000015E4                  ; sibling-handle (NOT in our parked
+                                         ; set — possibly a thread handle)
+     +0x20 = 0x000015E0                  ; event_handle (= self handle 0x15e0)
+     +0x24 = 0x00000000                  ; producer reads this — currently 0
+     +0x40 = 0xFFFFFFFF                  ; secondary sentinel
+
+   0x828F3EC0 (handle 0x1004, per-instance ctor sub_821783D8):
+     +0x00 = 0x01000000                  ; init flag
+     +0x10 = 0xFFFFFFFF                  ; queue sentinel
+     +0x20 = 0x40541BC0                  ; heap pointer (sub-buffer #1)
+     +0x30 = 0x00000014                  ; size 20
+     +0x34 = 0x0000002F                  ; size 47
+     +0x38 = 0x414F5F60                  ; heap-range payload (or two halfwords)
+     +0x3C = 0x40211CA0                  ; heap pointer (sub-buffer #2)
+     +0x44 = 0x405418C0                  ; heap pointer (sub-buffer #3)
+     +0x50 = 0x40111840                  ; heap pointer (sub-buffer #4)
+     +0x58 = 0xFFFFFFFF                  ; sentinel
+     +0x5C = 0xFFFFFFFF                  ; sentinel
+     +0x76 = 0x000012AC                  ; possibly thread id
+     +0x78 = 0x00001004                  ; event_handle (= self handle 0x1004)
+   ```
+   The 0x1004 dispatcher is **noticeably different**: it owns 4
+   guest-heap sub-buffers in 0x4xxxxxxx range, suggesting it
+   manages a more complex resource than the other two (which are
+   pure POD job queues). The +0x78 location of the event_handle
+   differs from 0x100c's +0x4C and 0x15e0's +0x20, so each
+   subsystem has its own struct layout (no shared base class).
+
+**Reproduce:**
+
+```bash
+cargo run --release -p xenia-app -- exec 'sylpheed.iso' \
+  --halt-on-deadlock \
+  --trace-handles-focus=0x1004,0x100c,0x15e0 \
+  --ctor-probe=0x821783D8,0x82181750,0x821701C8 \
+  --dump-addr=0x828F3D08,0x828F4070,0x828F3EC0 \
+  -n 50000000
+```
+
+Trace files saved at:
+- `audit-runs/audit-004/run-50m-probe.txt`     (outer-getter probes)
+- `audit-runs/audit-004/run-50m-probe-v2.txt`  (inner-ctor probes — singleton hypothesis confirmed)
+
+**Recommendation for next session (do not implement a fix):**
+
+Hook the entry of each non-create-chain consumer site for handle
+0x100c (5 sites: 0x821802d8, 0x821806e0, 0x82180b28, 0x82180ea0,
+0x82181254) and for handle 0x15e0 (4 sites: 0x8216f9d4, 0x8216fc08,
+0x821700b8, 0x821700f4) using `--ctor-probe=...`. If any fire, then
+the producer DOES execute and the failure mode is in the wake/signal
+chain (probably `lwz r3, OFFSET(r3)` reads zero — see dispatcher dump
+[+0x50] = 0 for 0x100c, [+0x24] = 0 for 0x15e0 — and the wake
+function 0x824AA1D8 is then called with handle=0). If none fire,
+the producer chain is gated upstream (likely a feature flag, init
+phase, or RPC handler that never fires). Either way, the next
+diagnostic narrows the bug surface dramatically.

crates/xenia-app/src/main.rs

@@ -176,6 +176,26 @@ enum Commands {
         /// `XENIA_XAUDIO_TICK=1`.
         #[arg(long)]
         xaudio_tick: bool,
+        /// Diagnostic. Comma-separated list of guest PCs at which the
+        /// worker prologue prints a one-shot `CTOR-PROBE` line capturing
+        /// live r3/lr/sp + an 8-frame back-chain. Hit-once-per-hit — for
+        /// the bridge ctor `sub_8217C850` called 8× from the
+        /// static-init driver, the probe fires 8×. Use to recover
+        /// pool-element `this` addresses that MSVC ctors fail to
+        /// preserve in r31. Read-only; lockstep digest unaffected.
+        /// Examples: `--ctor-probe=0x8217C850`,
+        /// `--ctor-probe=0x82181750,0x821701C8`.
+        #[arg(long)]
+        ctor_probe: Option<String>,
+        /// Diagnostic. Comma-separated list of guest addresses to dump
+        /// (64 bytes each, hex + u32 lanes) at end-of-run, after the
+        /// per-handle FOCUS report. Used to inspect the static
+        /// dispatcher / job-queue / pool struct layouts identified by
+        /// AUDIT-003 (e.g. `--dump-addr=0x828F3D08,0x828F4070`). The
+        /// dump captures a snapshot at the moment `dump_thread_diagnostic`
+        /// runs — typically when `--halt-on-deadlock` triggers.
+        #[arg(long)]
+        dump_addr: Option<String>,
     },
     /// Browse XISO disc image contents
     Browse {
@@ -331,6 +351,8 @@ fn main() -> Result<()> {
             reservations_table,
             parallel,
             xaudio_tick,
+            ctor_probe,
+            dump_addr,
         } => cmd_exec(
             &path,
             max_instructions,
@@ -349,6 +371,8 @@ fn main() -> Result<()> {
             reservations_table,
             parallel,
             xaudio_tick,
+            ctor_probe.as_deref(),
+            dump_addr.as_deref(),
         ),
         Commands::Browse { path } => cmd_browse(&path),
         Commands::Info { path } => cmd_info(&path),
@@ -550,6 +574,8 @@ fn cmd_exec(
     reservations_table: bool,
     parallel: bool,
     xaudio_tick: bool,
+    ctor_probe: Option<&str>,
+    dump_addr: Option<&str>,
 ) -> Result<()> {
     cmd_exec_inner(
         path,
@@ -569,6 +595,8 @@ fn cmd_exec(
         reservations_table,
         parallel,
         xaudio_tick,
+        ctor_probe,
+        dump_addr,
         None,
         None,
         false,
@@ -607,6 +635,8 @@ fn cmd_check(
         reservations_table,
         parallel,
         xaudio_tick,
+        None, // ctor_probe — diagnostic, never wanted on goldens
+        None, // dump_addr — same
         out,
         expect,
         stable_digest,
@@ -631,6 +661,8 @@ fn cmd_exec_inner(
     reservations_table: bool,
     parallel: bool,
     xaudio_tick: bool,
+    ctor_probe: Option<&str>,
+    dump_addr: Option<&str>,
     digest_out: Option<&str>,
     digest_expect: Option<&str>,
     stable_digest: bool,
@@ -879,6 +911,89 @@ fn cmd_exec_inner(
         }
     }
 
+    // Diagnostic. Parse `--ctor-probe=0x8217C850,0x...` (or
+    // `XENIA_CTOR_PROBE=...`) into `kernel.ctor_probe_pcs`. The
+    // worker prologue checks this set on every step; on a hit it
+    // prints a single back-chain capture line. Empty set = no
+    // probes = no-op fast path.
+    let ctor_probe_combined: Option<String> = match (ctor_probe, std::env::var("XENIA_CTOR_PROBE").ok()) {
+        (Some(s), _) => Some(s.to_string()),
+        (None, Some(s)) if !s.is_empty() => Some(s),
+        _ => None,
+    };
+    if let Some(list) = ctor_probe_combined {
+        for token in list.split(',').map(str::trim).filter(|s| !s.is_empty()) {
+            let parsed = if let Some(hex) = token.strip_prefix("0x").or_else(|| token.strip_prefix("0X")) {
+                u32::from_str_radix(hex, 16)
+            } else {
+                token.parse::<u32>()
+            };
+            match parsed {
+                Ok(pc) => {
+                    kernel.ctor_probe_pcs.insert(pc);
+                }
+                Err(_) => {
+                    return Err(anyhow::anyhow!(
+                        "invalid PC in --ctor-probe: {token:?}"
+                    ));
+                }
+            }
+        }
+        if !quiet && !kernel.ctor_probe_pcs.is_empty() {
+            let pcs: Vec<String> = kernel
+                .ctor_probe_pcs
+                .iter()
+                .map(|p| format!("{p:#010x}"))
+                .collect();
+            tracing::info!(
+                "ctor probes armed: {} ({})",
+                kernel.ctor_probe_pcs.len(),
+                pcs.join(", ")
+            );
+        }
+    }
+
+    // Diagnostic. Parse `--dump-addr=0x828F3D08,...` (or
+    // `XENIA_DUMP_ADDR=...`) into `kernel.dump_addrs`. The contents
+    // are dumped at end-of-run by `dump_thread_diagnostic`. Pure
+    // read; never mutates guest state.
+    let dump_addr_combined: Option<String> = match (dump_addr, std::env::var("XENIA_DUMP_ADDR").ok()) {
+        (Some(s), _) => Some(s.to_string()),
+        (None, Some(s)) if !s.is_empty() => Some(s),
+        _ => None,
+    };
+    if let Some(list) = dump_addr_combined {
+        for token in list.split(',').map(str::trim).filter(|s| !s.is_empty()) {
+            let parsed = if let Some(hex) = token.strip_prefix("0x").or_else(|| token.strip_prefix("0X")) {
+                u32::from_str_radix(hex, 16)
+            } else {
+                token.parse::<u32>()
+            };
+            match parsed {
+                Ok(addr) => {
+                    kernel.dump_addrs.push(addr);
+                }
+                Err(_) => {
+                    return Err(anyhow::anyhow!(
+                        "invalid address in --dump-addr: {token:?}"
+                    ));
+                }
+            }
+        }
+        if !quiet && !kernel.dump_addrs.is_empty() {
+            let addrs: Vec<String> = kernel
+                .dump_addrs
+                .iter()
+                .map(|a| format!("{a:#010x}"))
+                .collect();
+            tracing::info!(
+                "dump addresses armed: {} ({})",
+                kernel.dump_addrs.len(),
+                addrs.join(", ")
+            );
+        }
+    }
+
     // Install the GPU register aperture MMIO region on the guest memory so
     // any `0x7FC8xxxx` access routes to our atomic mailbox. Matches canary's
     // `graphics_system.cc:141-144`. The callbacks capture Arc clones of the
@@ -1812,6 +1927,13 @@ fn worker_prologue(
 
     let pc = kernel.scheduler.ctx(hw_id).pc;
 
+    // 0) Diagnostic ctor-probe: if `pc` is in
+    // `kernel.ctor_probe_pcs`, capture live r3/lr/sp + back-chain
+    // and println one record. Read-only; lockstep digest unaffected.
+    // Empty set is the common case → single `is_empty()` test inside
+    // the helper, no overhead on the hot path.
+    kernel.fire_ctor_probe_if_match(hw_id, mem);
+
     // 1) Halt-sentinel check (per HW thread).
     if pc == LR_HALT {
         let injected_here = kernel.interrupts.saved.is_some()
@@ -3405,6 +3527,56 @@ fn dump_thread_diagnostic(
             }
         }
     }
+
+    // Diagnostic. `--dump-addr` content dump at end-of-run. Each
+    // address gets a 64-byte dump in three forms (raw bytes, u32
+    // big-endian lanes, ASCII guess). The lanes form is gold for
+    // pool-element / dispatcher struct reverse-engineering: at
+    // [base+0]=-1 means "empty queue" sentinel from AUDIT-003, while
+    // any image-range pointer `0x82xxxxxx` at a fixed offset hints
+    // at an embedded silph::Event* / vtable / next-link.
+    if !kernel.dump_addrs.is_empty() {
+        println!("\n=== Dump-addr ===");
+        const DUMP_BYTES: usize = 128;
+        for &addr in &kernel.dump_addrs {
+            println!("  addr={:#010x}", addr);
+            let mut bytes = [0u8; DUMP_BYTES];
+            for i in 0..DUMP_BYTES {
+                bytes[i] = mem.read_u8(addr.wrapping_add(i as u32));
+            }
+            for row in 0..(DUMP_BYTES / 16) {
+                let off = row * 16;
+                let hex: String = (0..16)
+                    .map(|i| format!("{:02x}", bytes[off + i]))
+                    .collect::<Vec<_>>()
+                    .join(" ");
+                let words: String = (0..4)
+                    .map(|i| {
+                        let b = &bytes[off + i * 4..off + i * 4 + 4];
+                        format!(
+                            "{:08x}",
+                            u32::from_be_bytes([b[0], b[1], b[2], b[3]])
+                        )
+                    })
+                    .collect::<Vec<_>>()
+                    .join(" ");
+                let ascii: String = (0..16)
+                    .map(|i| {
+                        let b = bytes[off + i];
+                        if (0x20..=0x7E).contains(&b) {
+                            b as char
+                        } else {
+                            '.'
+                        }
+                    })
+                    .collect();
+                println!(
+                    "    +{:#04x}: {} | be32={} | {}",
+                    off, hex, words, ascii,
+                );
+            }
+        }
+    }
 }
 
 #[allow(clippy::too_many_arguments)]

crates/xenia-kernel/src/state.rs

@@ -179,6 +179,30 @@ pub struct KernelState {
     /// backend (which lives on the worker thread under `--gpu-thread`).
     pub ring_base: u32,
     pub ring_size_dwords: u32,
+    /// Diagnostic. PCs at which the worker prologue fires a one-shot
+    /// stack/back-chain dump capturing live `r3` (= `this` in MSVC
+    /// PPC ctors), `lr` (= return site), and the cycle/tid that hit
+    /// the PC. Populated from `--ctor-probe=0x8217C850,0x...` /
+    /// `XENIA_CTOR_PROBE`. Empty by default → check is a single
+    /// `is_empty()` test, no extra cost on the unprobed hot path.
+    /// Read-only diagnostic — no guest state is mutated, so the
+    /// `sylpheed_n*m.json` lockstep digest is preserved.
+    ///
+    /// **Why a per-PC probe instead of per-handle?** The MSVC ctors
+    /// at `sub_8217C850` (and friends) don't preserve `this` in r31
+    /// across the inner `bl` to `silph::Event::Construct`, so the
+    /// AUDIT-002 multi-frame back-chain at `NtCreateEvent` only
+    /// recovers stack-relative pointers — never the pool-element
+    /// `this`. Hooking the ctor's PRE-prologue PC captures r3 = this
+    /// before any save/restore can clobber it.
+    pub ctor_probe_pcs: std::collections::HashSet<u32>,
+    /// Diagnostic. Guest addresses to dump (64 bytes each, hex + u32
+    /// lanes) at end-of-run. Populated from `--dump-addr=0x828F3D08,
+    /// 0x828F4070`. Used to inspect static dispatcher / job-queue /
+    /// pool struct layouts identified by AUDIT-003. Read-only — the
+    /// dump is performed by `dump_thread_diagnostic`, never during
+    /// the hot interpreter loop, so lockstep determinism is unaffected.
+    pub dump_addrs: Vec<u32>,
 }
 
 impl KernelState {
@@ -230,6 +254,8 @@ impl KernelState {
             ring_base: 0,
             ring_size_dwords: 0,
             parallel_active: false,
+            ctor_probe_pcs: std::collections::HashSet::new(),
+            dump_addrs: Vec::new(),
         };
         crate::exports::register_exports(&mut state);
         crate::xam::register_exports(&mut state);
@@ -522,6 +548,48 @@ impl KernelState {
         self.audit.record_wake(handle, entry);
     }
 
+    /// Diagnostic. If the live PC for HW slot `hw_id` is in
+    /// `self.ctor_probe_pcs`, emit a single `CTOR-PROBE` line with
+    /// the current cycle, tid, hw_id, sp, r3, lr, plus an 8-frame
+    /// back-chain walk. Read-only — no guest state is mutated, so a
+    /// run with the probe set is byte-identical to one without (the
+    /// probe only adds println noise).
+    ///
+    /// Intended call site: top of `worker_prologue`, after `pc` has
+    /// been read but before any thunk-dispatch / step-block branch.
+    /// Fires once per hit — if the same PC is reached again (e.g.
+    /// the bridge ctor sub_8217C850 called 8 times by the static-
+    /// init driver), it fires 8 times, which is exactly what we want
+    /// for pool-element identification.
+    pub fn fire_ctor_probe_if_match(&self, hw_id: u8, mem: &GuestMemory) {
+        if self.ctor_probe_pcs.is_empty() {
+            return;
+        }
+        let ctx = self.scheduler.ctx(hw_id);
+        let pc = ctx.pc;
+        if !self.ctor_probe_pcs.contains(&pc) {
+            return;
+        }
+        let tid = self.scheduler.tid(hw_id).unwrap_or(0);
+        let r3 = ctx.gpr[3] as u32;
+        let lr = ctx.lr as u32;
+        let sp = ctx.gpr[1] as u32;
+        let cycle = ctx.cycle_count;
+        let frames = walk_guest_back_chain(sp, lr, mem, 8);
+        println!(
+            "CTOR-PROBE pc={:#010x} tid={} hw={} cycle={} sp={:#010x} r3={:#010x} lr={:#010x}",
+            pc, tid, hw_id, cycle, sp, r3, lr,
+        );
+        for (i, (fp, frame_lr)) in frames.iter().enumerate() {
+            let saved_r31 = mem.read_u32(fp.wrapping_sub(12));
+            let saved_r30 = mem.read_u32(fp.wrapping_sub(16));
+            println!(
+                "  CTOR-PROBE frame={} fp={:#010x} lr={:#010x} saved-r31={:#010x} saved-r30={:#010x}",
+                i, fp, frame_lr, saved_r31, saved_r30,
+            );
+        }
+    }
+
     /// Read a TLS slot for the currently running HW thread.
     pub fn tls_get(&self, index: u32) -> u64 {
         self.scheduler.tls_get(index)
@@ -1231,6 +1299,38 @@ mod tests {
     /// A NUL-terminated ASCII string is read up to `max`; non-printable
     /// bytes mark the candidate as bogus (return empty string). The
     /// `.?A` prefix gating in `read_class_at_this` then rejects them.
+    /// `fire_ctor_probe_if_match` only emits when `pc` matches a
+    /// configured PC. We assert it's a no-op on miss and a no-panic
+    /// on hit (the println goes to stdout; we just check the helper
+    /// reads the back-chain without faulting).
+    #[test]
+    fn fire_ctor_probe_if_match_no_op_on_empty_set() {
+        let mem = GuestMemory::new().expect("memory init");
+        let state = KernelState::new();
+        // No probes set → must be a no-op even when the scheduler
+        // ctx has whatever PC.
+        state.fire_ctor_probe_if_match(0, &mem);
+        assert!(state.ctor_probe_pcs.is_empty());
+    }
+
+    #[test]
+    fn fire_ctor_probe_if_match_only_fires_on_listed_pc() {
+        // We can't easily redirect stdout under cargo-test, so this
+        // test mostly verifies the membership check + that no panic
+        // occurs when frame walking encounters zero/sentinel pages.
+        // The empty-stack walk returns just `[(sp, lr)]`, exercising
+        // the loop body once safely.
+        let mem = GuestMemory::new().expect("memory init");
+        let mut state = KernelState::new();
+        state.ctor_probe_pcs.insert(0x8217_C850);
+        // The default PpcContext on slot 0 has pc=0 (idle sentinel),
+        // so the probe set membership test misses → no fire.
+        state.fire_ctor_probe_if_match(0, &mem);
+        // Sanity: an unrelated PC isn't claimed.
+        assert!(!state.ctor_probe_pcs.contains(&0x8200_0000));
+        assert!(state.ctor_probe_pcs.contains(&0x8217_C850));
+    }
+
     #[test]
     fn read_ascii_cstring_handles_termination_and_garbage() {
         use xenia_memory::page_table::MemoryProtect;