[iterate-2S] GPU: implement CP SCRATCH_REG memory writeback — arms Sylpheed's swap-callback slot

Sylpheed renders the splash (draws=78, iterate-2O) then plateaus: the
title's per-frame manager (sub_821741C8) only re-fires when "clock B"
([gfx+15160], swap count) changes, which only the CP swap-complete
callback sub_824CE2B8 increments. The graphics ISR sub_824BE9A0
indirect-calls that callback via [[gfx+10772]+16] on CP (source=1)
interrupts, but the slot stayed NULL so the callback never ran.

Root (runtime-verified, ours-side GPU): the guest arms the slot through
the Xenos CP scratch-register writeback path, which ours never
implemented. The arming IB (drained by ours at 0x4adf5180) contains a
Type-0 register write of the callback PC 0x824ce2b8 into SCRATCH_REG4
(0x057C). On hardware/canary, writing a SCRATCH_REG{n} mirrors the value
to SCRATCH_ADDR + n*4 in memory when the matching SCRATCH_UMSK bit is
set. Runtime values: SCRATCH_ADDR=0x0b1d5000 (the [gfx+10772]
descriptor), SCRATCH_UMSK=0x20033 (bit 4 set), so SCRATCH_REG4 ->
0x0b1d5010 = descriptor+16 = the callback slot (0x4b1d5010). Ours
decoded the Type-0 write into the register file but performed no
writeback (case a: drained-but-mishandled), so the slot stayed NULL.

Fix mirrors canary's CommandProcessor::HandleSpecialRegisterWrite
(command_processor.cc:545-552): a scratch_register_writeback() helper
called from handle_type0/handle_type1 after every register write; for
SCRATCH_REG0..7 with the UMSK bit set, it writes the value (big-endian,
as mem.write_u32 already stores) to SCRATCH_ADDR + n*4 (projected via
physical_to_backing). Deterministic given identical register state;
proven by unit test.

Cascade (verified by runtime probe): slot 0x4b1d5010 now armed with
0x824ce2b8; on the 2-3 CP interrupts that fire, the ISR reads the slot
and bcctrl's into sub_824CE2B8 (runs 2x; 0x cascade on master);
sub_824CE2B8 increments clock B ([gfx+15160]). The cascade does NOT yet
reach draws>78: there are only ~3 CP interrupts (from the initial 9825-
packet batch), and the title render loop stalls upstream (the iterate-2Q
title-respawn gate) before it submits more PM4_INTERRUPT work, so the
callback can't bootstrap a self-sustaining loop. This is the remaining
update-17/18 arming gap closed; the upstream stall is the next gate.

The default threaded GPU backend drains the ring on a separate host
thread, so with the callback now doing work the exact CP-interrupt
delivery instruction varies run to run (pre-existing GPU-thread race).
Pin the n50m oracle test to --gpu-inline (instruction-count
deterministic) and re-baseline its golden; bit-exact across repeated
runs. New unit test scratch_reg_write_mirrors_to_memory_when_umsk_enabled.

Tests: 675 pass (was 674). Golden re-baselined + determinism verified.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

crates/xenia-app/tests/golden/sylpheed_n50m.json

@@ -1,6 +1,6 @@
 {
-  "instructions": 50000001,
-  "imports": 451499,
+  "instructions": 50000013,
+  "imports": 451497,
   "unimpl": 0,
   "draws": 78,
   "swaps": 3,

crates/xenia-app/tests/sylpheed_oracles.rs

@@ -57,6 +57,16 @@ fn run_oracle(label: &str, max_instr: u64, golden_rel: &str) {
             &iso,
             "-n",
             &max_instr_str,
+            // Pin the inline (single-threaded) GPU backend. The default
+            // threaded backend drains the ring on a separate host thread,
+            // so the exact instruction at which a CP interrupt is queued —
+            // and therefore when the guest's swap-complete ISR callback runs
+            // (iterate-2S armed it via SCRATCH_REG writeback) — varies run to
+            // run. Inline draining is instruction-count-deterministic, which
+            // is what a regression golden needs. (The threaded path is the
+            // documented "GPU thread race" the stable-digest already warns
+            // about.)
+            "--gpu-inline",
             "--stable-digest",
             "--expect",
             &golden_str,

crates/xenia-gpu/src/gpu_system.rs

@@ -844,6 +844,38 @@ impl GpuSystem {
         }
     }
 
+    /// CP scratch-register memory writeback, mirroring canary's
+    /// `CommandProcessor::HandleSpecialRegisterWrite`
+    /// (`command_processor.cc:545-552`). Every register write runs through
+    /// here; when the target is one of the eight `SCRATCH_REG{n}`
+    /// (`0x0578..=0x057F`) **and** the matching bit in `SCRATCH_UMSK` is set,
+    /// the value is also written (big-endian, as `mem.write_u32` already
+    /// stores) to `SCRATCH_ADDR + n*4` in guest physical memory.
+    ///
+    /// Sylpheed arms its CP swap-complete interrupt callback through this
+    /// path: it programs `SCRATCH_ADDR` to the GPU command-block descriptor
+    /// (`[gfx+10772]`, runtime `0x0b1d5000`), `SCRATCH_UMSK` bit 4, then a
+    /// Type-0 write of the callback PC `0x824ce2b8` into `SCRATCH_REG4`
+    /// (`0x057C`). The writeback lands it at descriptor+16 (`0x4b1d5010`),
+    /// which the graphics ISR (`sub_824BE9A0`) reads via `[[gfx+10772]+16]`
+    /// and `bcctrl`s to fire the swap-complete callback. Without this
+    /// writeback the slot stayed NULL, the ISR skipped the callback, the
+    /// swap counter never advanced, and the title's per-frame manager
+    /// re-fired once then plateaued.
+    fn scratch_register_writeback(&self, mem: &dyn MemoryAccess, index: u32, value: u32) {
+        if !(reg::SCRATCH_REG0..=reg::SCRATCH_REG7).contains(&index) {
+            return;
+        }
+        let scratch_reg = index - reg::SCRATCH_REG0;
+        let umsk = self.register_file.read(reg::SCRATCH_UMSK);
+        if (1u32 << scratch_reg) & umsk == 0 {
+            return;
+        }
+        let scratch_addr = self.register_file.read(reg::SCRATCH_ADDR);
+        let mem_addr = physical_to_backing(scratch_addr.wrapping_add(scratch_reg * 4));
+        mem.write_u32(mem_addr, value);
+    }
+
     fn writeback_read_ptr(&mut self, mem: &dyn MemoryAccess) {
         if self.ring.rptr_writeback_addr != 0 && self.ring.is_initialized() {
             mem.write_u32_fence(
@@ -868,6 +900,7 @@ impl GpuSystem {
             let value = mem.read_u32(dword_addr);
             let target = if write_one { base_index } else { base_index + i };
             self.register_file.write(target, value);
+            self.scratch_register_writeback(mem, target, value);
         }
         tracing::trace!(
             base = format_args!("{base_index:#x}"),
@@ -890,6 +923,8 @@ impl GpuSystem {
         let b = mem.read_u32(b_addr);
         self.register_file.write(reg_index_1, a);
         self.register_file.write(reg_index_2, b);
+        self.scratch_register_writeback(mem, reg_index_1, a);
+        self.scratch_register_writeback(mem, reg_index_2, b);
         tracing::trace!(
             r1 = format_args!("{reg_index_1:#x}"),
             r2 = format_args!("{reg_index_2:#x}"),
@@ -1511,6 +1546,17 @@ pub mod reg {
     /// `XE_GPU_REG_COHER_STATUS_HOST` — coherency bits
     /// (Canary `register_table.inc:530`).
     pub const COHER_STATUS_HOST: u32 = 0x0A31;
+    /// `XE_GPU_REG_SCRATCH_UMSK` — bitmask of which `SCRATCH_REG{n}` writes are
+    /// mirrored to memory (Canary `register_table.inc:139`).
+    pub const SCRATCH_UMSK: u32 = 0x01DC;
+    /// `XE_GPU_REG_SCRATCH_ADDR` — base physical address of the scratch
+    /// writeback block (Canary `register_table.inc:141`).
+    pub const SCRATCH_ADDR: u32 = 0x01DD;
+    /// `XE_GPU_REG_SCRATCH_REG0` — first of 8 CP scratch registers
+    /// (`0x0578..=0x057F`, Canary `register_table.inc:331-338`).
+    pub const SCRATCH_REG0: u32 = 0x0578;
+    /// `XE_GPU_REG_SCRATCH_REG7` — last CP scratch register.
+    pub const SCRATCH_REG7: u32 = 0x057F;
 }
 
 /// 32-bit FNV-1a over a u32 seed + a slice of u32s. Used to derive a
@@ -1601,6 +1647,38 @@ mod tests {
         assert_eq!(gpu.register_file.read(0x101), 0xCAFE_BABE);
     }
 
+    #[test]
+    fn scratch_reg_write_mirrors_to_memory_when_umsk_enabled() {
+        // Mirrors Sylpheed's CP swap-callback arming: SCRATCH_ADDR points at a
+        // descriptor, SCRATCH_UMSK enables bit 4, and a Type-0 write of the
+        // callback PC into SCRATCH_REG4 (0x57C) must land at SCRATCH_ADDR + 16.
+        let mut gpu = GpuSystem::new();
+        let mut mem = build_mem();
+        gpu.initialize_ring_buffer(0x4000_0000, 10);
+        // Program SCRATCH_ADDR = 0x4000_1000 (physical-mirror identity), and
+        // SCRATCH_UMSK = bit 4 only (so SCRATCH_REG4 mirrors, REG3 does not).
+        gpu.register_file.write(reg::SCRATCH_ADDR, 0x4000_1000);
+        gpu.register_file.write(reg::SCRATCH_UMSK, 1 << 4);
+        // Type0 write run: base = SCRATCH_REG3 (0x57B), count = 2 → writes
+        // 0x11111111 → SCRATCH_REG3 (UMSK bit 3 clear), 0x824CE2B8 →
+        // SCRATCH_REG4 (UMSK bit 4 set → mirrored to ADDR + 4*4 = +16).
+        const SCRATCH_REG3: u32 = 0x057B;
+        let hdr = (1u32 << 16) | SCRATCH_REG3;
+        mem.write_u32(0x4000_0000, hdr);
+        mem.write_u32(0x4000_0004, 0x1111_1111);
+        mem.write_u32(0x4000_0008, 0x824C_E2B8);
+        gpu.extend_write_ptr(3);
+        assert!(matches!(gpu.execute_one(&mut mem), ExecOutcome::Stepped { .. }));
+        // SCRATCH_REG3 (bit 3 clear) must NOT mirror; SCRATCH_REG4 (bit 4 set)
+        // must mirror to SCRATCH_ADDR + 16.
+        assert_eq!(mem.read_u32(0x4000_1000 + 12), 0, "reg3 must not mirror");
+        assert_eq!(
+            mem.read_u32(0x4000_1000 + 16),
+            0x824C_E2B8,
+            "reg4 must mirror to SCRATCH_ADDR+16"
+        );
+    }
+
     #[test]
     fn wait_reg_mem_blocks_then_unblocks_when_mem_changes() {
         let mut gpu = GpuSystem::new();