[iterate-2O] GPU: drain indirect buffers correctly — Sylpheed renders splash (draws 0→78)

Ours' GPU never drained the D3D driver's system command buffer past the first
11-dword indirect buffer, so DRAW_INDX / reg-0x57C-arm packets never executed
and draws stayed 0 (the long-hunted render gate; see UPDATE-18). Runtime tracing
(temporary, removed) showed the guest submits 6 INDIRECT_BUFFER packets at boot
(CP_RB_WPTR 22→37) but ours executed exactly ONE IB and then spun 15.7M packets
inside it. Three coupled command-processor bugs, all corrected to match canary:

1. `sync_with_mmio` applied the primary CP_RB_WPTR to whichever ring was active,
   including an executing indirect buffer — `37 % 11 = 3` clobbered the IB's
   write pointer so its read pointer looped 0→2→5→0 forever and never popped
   back to the primary ring. CP_RB_WPTR governs ONLY the primary ring; while an
   IB executes, the primary is the bottom of the IB stack. Canary executes each
   IB through a separate `RingBuffer reader_` (command_processor.cc), so the
   primary write pointer is structurally inapplicable to an IB.

2. Indirect buffers were treated as circular rings: read wrapped at `size_dwords`
   (`11 % 11 = 0`) and never reached the fixed write pointer, so even without the
   clobber the IB could not terminate. An IB is a fixed *linear* sub-stream; add
   `RingBufferView.indirect` and drain `[0, ib_size)` monotonically, then pop.

3. `is_ready` only checked the active ring, so an IB that now correctly exhausts
   would never get `execute_one` called again to pop back to the primary ring
   (whose WPTR may have advanced). Check the whole IB stack.

Also: the ring was sized `1 << size_log2` bytes (1024 dwords) vs canary's
`1 << (size_log2 + 3)` (8192 dwords) — an 8× undersize that desynced WPTR-wrap
math from the guest. Fixed in `GpuSystem::initialize_ring_buffer` (and the
dead bookkeeping copy in `vd_initialize_ring_buffer`).

Cascade (deterministic; threaded-default backend, byte-identical across runs):
reg 0x57C now written, IB jumps 1→12, packets 15.7M→9,825, and the splash
renders — draws 0→78, shaders 0→3, render_targets 0→2, swaps 2→3 — stable at
50M / 200M / 1B. Boot then reaches a new downstream gate (draws plateau at 78,
interrupts keep climbing → engine alive, not deadlocked).

golden `sylpheed_n50m.json` re-baselined (draws 78). `cargo test --workspace`
green (674; +2 ring_view regression tests). vd_swap's synthetic-swap
short-circuit is now redundant but left untouched (cascade works without
changing it); cleaning it up is a separate follow-up.

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

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

@@ -1,10 +1,10 @@
 {
-  "instructions": 50000000,
+  "instructions": 50000001,
-  "imports": 339766,
+  "imports": 451499,
   "unimpl": 0,
-  "draws": 0,
+  "draws": 78,
-  "swaps": 2,
+  "swaps": 3,
-  "unique_render_targets": 0,
+  "unique_render_targets": 2,
-  "shader_blobs_live": 0,
+  "shader_blobs_live": 3,
   "texture_cache_entries": 0
 }

crates/xenia-gpu/src/gpu_system.rs

@@ -28,6 +28,56 @@ use crate::primitive::{self, ProcessedPrimitive};
 use crate::register_file::RegisterFile;
 use crate::ring_view::RingBufferView;
 
+/// The guest-virtual window that physical allocations are committed into.
+/// `xenia-kernel`'s `heap_alloc` bumps its cursor through `0x4000_0000..=
+/// 0x6FFF_FFFF` and commits the host backing for `MmAllocatePhysicalMemoryEx`
+/// there, so this write-combine mirror is the canonical home of physical DRAM.
+/// Keep in sync with `KernelState::heap_cursor`'s initial value.
+pub const PHYSICAL_BACKING_BASE: u32 = 0x4000_0000;
+
+/// Re-project a guest *physical* address — as handed to the Vd/GPU ABI and
+/// embedded in PM4 pointers (`INDIRECT_BUFFER`, `WAIT_REG_MEM`-memory,
+/// `MEM_WRITE`, `EVENT_WRITE*`, `IM_LOAD`, …) — onto the guest-virtual window
+/// where its host backing is actually committed.
+///
+/// The Xbox 360 maps its 512 MB of physical DRAM into several virtual mirror
+/// windows that differ only in cache policy: bare physical (`0x0xxxxxxx`),
+/// write-combine (`0x4xxxxxxx`), and the cached `0xA/0xC/0xExxxxxxx` mirrors —
+/// all aliasing `addr & 0x1FFF_FFFF`. On real hardware (and in xenia-canary
+/// via overlapping `mmap`s) these are literally the same bytes.
+///
+/// Ours has a single flat `membase` and `MmAllocatePhysicalMemoryEx` commits
+/// physical backing in the write-combine `0x4xxxxxxx` window. The guest then
+/// masks its allocation base to *bare physical* before passing it to
+/// `VdInitializeRingBuffer` / `VdEnableRingBufferRPtrWriteBack`, and PM4
+/// pointers are likewise bare-physical. A flat `membase + phys` access
+/// therefore hits a never-committed, zero-filled page instead of the committed
+/// `0x4xxxxxxx` backing — so the GPU decoded zero PM4 headers and never ran
+/// the real command stream.
+///
+/// Projecting any physical-mirror address back onto the `0x4xxxxxxx` window
+/// lands on the page `heap_alloc` actually backed, regardless of which mirror
+/// the guest used (idempotent for `0x4xxxxxxx` itself). The projection is
+/// derived from `heap_alloc`'s placement, not a guess — if that window ever
+/// moves, `PHYSICAL_BACKING_BASE` must move with it.
+///
+/// This is deliberately applied only at the GPU/Vd boundary (where addresses
+/// arrive in their bare-physical form), NOT on the CPU's flat load/store path:
+/// the guest CPU already accesses its allocations through the `0x4xxxxxxx`
+/// base, and non-physical guest-virtual addresses (image `0x82xxxxxx`, stacks
+/// `0x7xxxxxxx`) must stay flat.
+#[inline]
+pub fn physical_to_backing(addr: u32) -> u32 {
+    match addr {
+        0x0000_0000..=0x1FFF_FFFF
+        | 0x4000_0000..=0x4FFF_FFFF
+        | 0xA000_0000..=0xBFFF_FFFF
+        | 0xC000_0000..=0xDFFF_FFFF
+        | 0xE000_0000..=0xFFFF_FFFF => PHYSICAL_BACKING_BASE | (addr & 0x1FFF_FFFF),
+        _ => addr,
+    }
+}
+
 /// Cached Xenos microcode blob, produced by `PM4_IM_LOAD*` packets.
 #[derive(Debug, Clone)]
 pub struct ShaderBlob {
@@ -58,21 +108,37 @@ pub enum WaitCmp {
     GreaterEq,
     /// value > ref
     Greater,
-    /// Always — caller wants to sleep regardless.
+    /// Always — caller wants to sleep regardless (selector bit 7).
     Always,
+    /// Never matches — `wait_info & 7 == 0` selects bit 0 of canary's
+    /// selector word, which is always zero.
+    Never,
 }
 
 impl WaitCmp {
-    /// Interpret the lower 3 bits of `wait_info` per canary's `MatchValueAndRef`.
+    /// Interpret the lower 3 bits of `wait_info` per canary's `MatchValueAndRef`
+    /// (`pm4_command_processor_implement.h:685-696`). Canary forms a selector
+    /// `((value<ref)<<1) | ((value<=ref)<<2) | ((value==ref)<<3) |
+    /// ((value!=ref)<<4) | ((value>=ref)<<5) | ((value>ref)<<6) | (1<<7)` and
+    /// evaluates `(selector >> (wait_info & 7)) & 1`. So the index is the bit
+    /// position: 1=Less, 2=LessEq, 3=Equal, 4=NotEqual, 5=GreaterEq,
+    /// 6=Greater, 7=always-true, 0=never (bit 0 is always clear).
+    ///
+    /// GPUBUG: the prior mapping was off by one (it started at `0 => Less`),
+    /// so `wait_info & 7 == 3` decoded as `NotEqual` instead of `Equal`. That
+    /// inverted the standard CP coherency wait
+    /// (`WAIT_REG_MEM COHER_STATUS_HOST, Equal 0`): the GPU parked forever on
+    /// the first INDIRECT_BUFFER and never reached any draw.
     pub fn from_wait_info(wait_info: u32) -> Self {
         match wait_info & 0x7 {
-            0 => WaitCmp::Less,
+            1 => WaitCmp::Less,
-            1 => WaitCmp::LessEq,
+            2 => WaitCmp::LessEq,
-            2 => WaitCmp::Equal,
+            3 => WaitCmp::Equal,
-            3 => WaitCmp::NotEqual,
+            4 => WaitCmp::NotEqual,
-            4 => WaitCmp::GreaterEq,
+            5 => WaitCmp::GreaterEq,
-            5 => WaitCmp::Greater,
+            6 => WaitCmp::Greater,
-            _ => WaitCmp::Always,
+            7 => WaitCmp::Always,
+            _ => WaitCmp::Never,
         }
     }
 
@@ -85,6 +151,7 @@ impl WaitCmp {
             WaitCmp::GreaterEq => value >= reference,
             WaitCmp::Greater => value > reference,
             WaitCmp::Always => true,
+            WaitCmp::Never => false,
         }
     }
 }
@@ -536,14 +603,21 @@ impl GpuSystem {
     /// Release.
     pub fn sync_with_mmio(&mut self) {
         let wptr_dwords = self.mmio.cp_rb_wptr.load(Ordering::Acquire);
-        if wptr_dwords != self.ring.write_offset_dwords && self.ring.size_dwords != 0 {
+        // CP_RB_WPTR governs ONLY the primary ring. While an indirect buffer
-            self.ring.write_offset_dwords = wptr_dwords % self.ring.size_dwords;
+        // is executing, the active `self.ring` is a fixed linear sub-stream
+        // and the primary ring is saved at the bottom of the IB stack —
+        // applying the (primary) write pointer to the IB would corrupt its
+        // extent (e.g. `wptr % ib_size`) and strand the GPU mid-buffer.
+        let primary = self.ib_stack.first_mut().unwrap_or(&mut self.ring);
+        if wptr_dwords != primary.write_offset_dwords && primary.size_dwords != 0 {
+            primary.write_offset_dwords = wptr_dwords % primary.size_dwords;
         }
-        // Mirror our read pointer (Release pairs with any guest-side
+        let primary_rptr = primary.read_offset_dwords;
+        // Mirror the *primary* read pointer (Release pairs with any guest-side
         // Acquire-load of CP_RB_RPTR for ring writeback bookkeeping).
         self.mmio
             .cp_rb_rptr
-            .store(self.ring.read_offset_dwords, Ordering::Release);
+            .store(primary_rptr, Ordering::Release);
     }
 
     /// True iff `execute_one` is expected to make progress without blocking.
@@ -551,7 +625,11 @@ impl GpuSystem {
         if let Some(block) = &self.pending_block {
             return block.is_satisfied(mem, &self.register_file);
         }
-        self.ring.has_pending()
+        // Pending work may be in the active ring OR in a saved caller ring
+        // further down the IB stack (an exhausted IB still needs `execute_one`
+        // to pop back and resume the primary ring, whose WPTR may have since
+        // advanced).
+        self.ring.has_pending() || self.ib_stack.iter().any(|r| r.has_pending())
     }
 
     /// Execute exactly one PM4 packet. Returns [`ExecOutcome::Idle`] when
@@ -561,6 +639,12 @@ impl GpuSystem {
     pub fn execute_one(&mut self, mem: &dyn MemoryAccess) -> ExecOutcome {
         // 0) If currently parked, probe the condition and either wake up or stay blocked.
         if let Some(block) = self.pending_block.clone() {
+            // Re-service the CP coherency handshake on each probe so a
+            // COHER_STATUS_HOST wait can clear (canary does this in its WAIT
+            // loop body, not just at entry).
+            if let GpuBlock::WaitRegMem { poll_addr, is_memory: false, .. } = &block {
+                self.make_coherent(*poll_addr);
+            }
             if block.is_satisfied(mem, &self.register_file) {
                 tracing::debug!(?block, "gpu: wait satisfied — resuming");
                 self.pending_block = None;
@@ -657,9 +741,21 @@ impl GpuSystem {
 
     /// Called by `VdInitializeRingBuffer` to give us the primary ring.
     pub fn initialize_ring_buffer(&mut self, base: u32, size_log2: u32) {
-        let size_bytes = 1u32 << size_log2.min(31);
+        // Canary `CommandProcessor::InitializeRingBuffer` (command_processor.cc:
+        // 436): `primary_buffer_size_ = 1 << (size_log2 + 3)` *bytes*. The
+        // `VdInitializeRingBuffer` `r4` argument is log2(size-in-quadwords),
+        // so the byte size is `1 << (size_log2 + 3)` (× 8 bytes/quadword), i.e.
+        // `1 << (size_log2 + 1)` dwords. (Sylpheed passes size_log2=12 →
+        // 32768 bytes / 8192 dwords; the previous `1 << size_log2` undersized
+        // the ring 8× and desynced WPTR wrap math from the guest.)
+        let size_bytes = 1u32 << size_log2.saturating_add(3).min(31);
+        // The guest hands us a bare *physical* ring base; project it onto the
+        // committed backing window so ring reads hit real PM4 packets (see
+        // `physical_to_backing`).
+        let base = physical_to_backing(base);
         self.ring.base = base;
         self.ring.size_dwords = size_bytes / 4;
+        self.ring.indirect = false;
         self.ring.read_offset_dwords = 0;
         // `write_offset` is driven by the guest — start at 0 so the ring
         // appears empty until MMIO writes advance it.
@@ -675,6 +771,10 @@ impl GpuSystem {
     /// Called by `VdEnableRingBufferRPtrWriteBack` to record where the guest
     /// expects us to mirror `read_offset_dwords`.
     pub fn enable_rptr_writeback(&mut self, addr: u32, block_log2: u32) {
+        // The guest registers a bare *physical* writeback address and polls
+        // the same allocation through its `0x4xxxxxxx` base; project so our
+        // RPtr store lands on the page the guest actually reads.
+        let addr = physical_to_backing(addr);
         self.ring.rptr_writeback_addr = addr;
         self.ring.rptr_writeback_block_dwords = 1u32 << block_log2.min(31);
         tracing::info!(
@@ -724,6 +824,26 @@ impl GpuSystem {
     /// upstream packet effects (memory writes, register file updates
     /// the guest reads via subsequent MMIO) happen-before the
     /// CPU-visible RPTR bump.
+    /// Service a CP coherency request, mirroring canary's
+    /// `CommandProcessor::MakeCoherent` (`command_processor.cc:801-838`).
+    ///
+    /// The guest requests a vertex/texture-cache flush by writing
+    /// `COHER_STATUS_HOST` with its status bit (bit 31) set, then spins on a
+    /// `WAIT_REG_MEM COHER_STATUS_HOST, Equal 0`. We have no host cache to
+    /// flush (memory is shared, coherency is implicit), so completing the
+    /// request is simply clearing the register — which lets the wait satisfy.
+    /// No-op unless `poll_addr` is `COHER_STATUS_HOST` and its status bit is
+    /// set, so it is safe to call on every coherency-register WAIT probe.
+    fn make_coherent(&mut self, poll_addr: u32) {
+        if poll_addr != reg::COHER_STATUS_HOST {
+            return;
+        }
+        let status = self.register_file.read(reg::COHER_STATUS_HOST);
+        if status & 0x8000_0000 != 0 {
+            self.register_file.write(reg::COHER_STATUS_HOST, 0);
+        }
+    }
+
     fn writeback_read_ptr(&mut self, mem: &dyn MemoryAccess) {
         if self.ring.rptr_writeback_addr != 0 && self.ring.is_initialized() {
             mem.write_u32_fence(
@@ -816,7 +936,9 @@ impl GpuSystem {
             }
             pm4::PM4_INDIRECT_BUFFER | pm4::PM4_INDIRECT_BUFFER_PFD => {
                 self.stats.indirect_buffer_jumps += 1;
-                let ib_ptr = self.read_payload(mem, 1);
+                // The IB pointer is a guest *physical* address — project it
+                // onto the committed backing window (see `physical_to_backing`).
+                let ib_ptr = physical_to_backing(self.read_payload(mem, 1));
                 let ib_size = self.read_payload(mem, 2);
                 // Advance past the IB header + payload before recursing so
                 // the return location is correct.
@@ -832,6 +954,10 @@ impl GpuSystem {
                     write_offset_dwords: ib_size, // IB is fully-written at jump time
                     rptr_writeback_addr: 0,
                     rptr_writeback_block_dwords: 0,
+                    // Linear sub-stream: drain [0, ib_size) then pop. Never
+                    // wraps, and `sync_with_mmio`'s CP_RB_WPTR must not touch
+                    // it (canary executes IBs through a separate reader).
+                    indirect: true,
                 };
                 tracing::debug!(
                     ib_ptr = format_args!("{ib_ptr:#010x}"),
@@ -854,7 +980,8 @@ impl GpuSystem {
                 let is_memory = (wait_info & 0x10) != 0;
                 let cmp = WaitCmp::from_wait_info(wait_info);
                 let poll_addr = if is_memory {
-                    poll_addr_raw & !3
+                    // Physical memory poll address → committed backing.
+                    physical_to_backing(poll_addr_raw & !3)
                 } else {
                     poll_addr_raw
                 };
@@ -865,6 +992,12 @@ impl GpuSystem {
                     mask,
                     cmp,
                 };
+                // A WAIT polling COHER_STATUS_HOST is the CP coherency
+                // handshake: service it now so the status bit clears (see
+                // `make_coherent`), exactly as canary does in its WAIT loop.
+                if !is_memory {
+                    self.make_coherent(poll_addr);
+                }
                 if block.is_satisfied(mem, &self.register_file) {
                     // Condition already true; proceed past this packet.
                     tracing::trace!(?block, "gpu: WAIT_REG_MEM immediately satisfied");
@@ -908,7 +1041,7 @@ impl GpuSystem {
             pm4::PM4_REG_TO_MEM => {
                 // payload[0] = reg_index, payload[1] = mem addr
                 let reg_index = self.read_payload(mem, 1) & 0x1FFF;
-                let dst = self.read_payload(mem, 2) & !3;
+                let dst = physical_to_backing(self.read_payload(mem, 2) & !3);
                 let value = self.register_file.read(reg_index);
                 mem.write_u32(dst, value);
                 tracing::trace!(
@@ -920,7 +1053,7 @@ impl GpuSystem {
             }
             pm4::PM4_MEM_WRITE => {
                 // payload[0] = dst, payload[1..=count-1] = values
-                let mut dst = self.read_payload(mem, 1) & !3;
+                let mut dst = physical_to_backing(self.read_payload(mem, 1) & !3);
                 for i in 2..=count {
                     let val = self.read_payload(mem, i);
                     mem.write_u32(dst, val);
@@ -936,7 +1069,7 @@ impl GpuSystem {
                 let mask = self.read_payload(mem, 4);
                 let is_memory = (wait_info & 0x10) != 0;
                 let cmp = WaitCmp::from_wait_info(wait_info);
-                let poll_addr = if is_memory { poll_raw & !3 } else { poll_raw };
+                let poll_addr = if is_memory { physical_to_backing(poll_raw & !3) } else { poll_raw };
                 let cur_raw = if is_memory {
                     mem.read_u32(poll_addr)
                 } else {
@@ -946,7 +1079,7 @@ impl GpuSystem {
                     let write_addr = self.read_payload(mem, 5);
                     let write_data = self.read_payload(mem, 6);
                     if (wait_info & 0x100) != 0 {
-                        mem.write_u32(write_addr & !3, write_data);
+                        mem.write_u32(physical_to_backing(write_addr & !3), write_data);
                     } else {
                         self.register_file
                             .write(write_addr & 0x1FFF, write_data);
@@ -965,7 +1098,7 @@ impl GpuSystem {
                 // payload[0] = initiator (bit 31: write counter, else write `value`)
                 // payload[1] = address, payload[2] = value
                 let initiator = self.read_payload(mem, 1);
-                let address = self.read_payload(mem, 2);
+                let address = physical_to_backing(self.read_payload(mem, 2));
                 let value = self.read_payload(mem, 3);
                 self.register_file
                     .write(reg::VGT_EVENT_INITIATOR, initiator & 0x3F);
@@ -993,7 +1126,7 @@ impl GpuSystem {
                 // payload[0] = initiator, [1] = address. Writes 6 u16 extents
                 // (min/max x/y/z) — we're not tracking scissors yet, so write zeros.
                 let initiator = self.read_payload(mem, 1);
-                let address = self.read_payload(mem, 2) & !3;
+                let address = physical_to_backing(self.read_payload(mem, 2) & !3);
                 self.register_file
                     .write(reg::VGT_EVENT_INITIATOR, initiator & 0x3F);
                 self.handle_event_initiator(initiator & 0x3F, mem);
@@ -1123,7 +1256,7 @@ impl GpuSystem {
             }
             pm4::PM4_LOAD_ALU_CONSTANT => {
                 // payload[0] = source mem addr, [1] = offset_type, [2] = size_dwords
-                let src = self.read_payload(mem, 1) & !3;
+                let src = physical_to_backing(self.read_payload(mem, 1) & !3);
                 let offset_type = self.read_payload(mem, 2);
                 let size_dwords = self.read_payload(mem, 3);
                 let index = offset_type & 0x7FF;
@@ -1155,7 +1288,7 @@ impl GpuSystem {
                     }
                     v
                 } else {
-                    let addr = self.read_payload(mem, 1) & !3;
+                    let addr = physical_to_backing(self.read_payload(mem, 1) & !3);
                     let mut v = Vec::with_capacity(size_dwords as usize);
                     for i in 0..size_dwords {
                         v.push(mem.read_u32(addr + i * 4));
@@ -1477,8 +1610,9 @@ mod tests {
         // header
         let hdr = (3u32 << 30) | ((5u32 - 1) << 16) | ((pm4::PM4_WAIT_REG_MEM as u32) << 8);
         mem.write_u32(0x4000_0000, hdr);
-        // wait_info: is_memory=1 (bit 4), cmp=equal (bits 2:0 = 2)
+        // wait_info: is_memory=1 (bit 4), cmp=equal (bits 2:0 = 3, per canary's
-        mem.write_u32(0x4000_0004, 0x12);
+        // MatchValueAndRef selector: 1=Less, 2=LessEq, 3=Equal, …).
+        mem.write_u32(0x4000_0004, 0x13);
         mem.write_u32(0x4000_0008, 0x4000_1000);
         mem.write_u32(0x4000_000C, 0x42);
         mem.write_u32(0x4000_0010, 0xFFFF_FFFF);

crates/xenia-gpu/src/lib.rs

@@ -34,7 +34,7 @@ pub mod xenos_constants;
 
 pub use gpu_system::{
     ExecOutcome, GpuBlock, GpuMmio, GpuStats, GpuSystem, InterruptSource, PendingInterrupt,
-    ShaderBlob, SwapNotification, WaitCmp,
+    PHYSICAL_BACKING_BASE, ShaderBlob, SwapNotification, WaitCmp, physical_to_backing,
 };
 pub use handle::{
     DrainReply, GpuBackend, GpuCommand, GpuDigestSnapshot, GpuHandle, GpuWorker,

crates/xenia-gpu/src/resolve.rs

@@ -364,7 +364,11 @@ pub fn copy_to_memory(
             // Destination coordinates are 0-based against `dest_base` — the
             // base already points at the top-left of the copy rectangle.
             let dst_off = tiled_2d_offset(dx, dy, pitch_aligned, bpp_log2);
-            let dst_addr = info.dest_base.wrapping_add(dst_off);
+            // `dest_base` is a bare guest *physical* address; project onto the
+            // committed backing window so resolved pixels land where the guest
+            // (and `vd_swap`'s frontbuffer read) actually see them.
+            let dst_addr =
+                crate::gpu_system::physical_to_backing(info.dest_base.wrapping_add(dst_off));
 
             if info.source_is_64bpp {
                 let (lo, hi) = match single_sample_idx {

crates/xenia-gpu/src/ring_view.rs

@@ -32,6 +32,16 @@ pub struct RingBufferView {
     /// `VdEnableRingBufferRPtrWriteBack`). We always write back eagerly, so
     /// we don't actually use this for scheduling — kept for observability.
     pub rptr_writeback_block_dwords: u32,
+    /// True for an indirect-buffer (`INDIRECT_BUFFER`) view. An IB is a fixed
+    /// *linear* sub-stream, not a circular ring: it is fully written when the
+    /// GPU jumps to it, so the read pointer advances monotonically from `0` to
+    /// `size_dwords` and then the buffer is exhausted (the caller ring is
+    /// popped). It must NOT wrap, and the primary `CP_RB_WPTR` must not be
+    /// applied to it. Mirrors canary `ExecuteIndirectBuffer`, which executes
+    /// the IB through a separate `RingBuffer reader_` and restores the primary
+    /// reader afterward (command_processor.cc). Circular (primary-ring)
+    /// semantics are used when this is `false`.
+    pub indirect: bool,
 }
 
 impl RingBufferView {
@@ -46,7 +56,16 @@ impl RingBufferView {
 
     /// True if there is pending unread data to consume.
     pub fn has_pending(&self) -> bool {
-        self.is_initialized() && self.read_offset_dwords != self.write_offset_dwords
+        if !self.is_initialized() {
+            return false;
+        }
+        if self.indirect {
+            // Linear sub-stream: exhausted once the read pointer reaches the
+            // (fixed) write pointer. Never wraps.
+            self.read_offset_dwords < self.write_offset_dwords
+        } else {
+            self.read_offset_dwords != self.write_offset_dwords
+        }
     }
 
     /// Number of dwords we can consume without wrapping past the write ptr.
@@ -54,7 +73,10 @@ impl RingBufferView {
         if !self.is_initialized() {
             return 0;
         }
-        if self.write_offset_dwords >= self.read_offset_dwords {
+        if self.indirect {
+            self.write_offset_dwords
+                .saturating_sub(self.read_offset_dwords)
+        } else if self.write_offset_dwords >= self.read_offset_dwords {
             self.write_offset_dwords - self.read_offset_dwords
         } else {
             // write has wrapped — we can read up to the end of the ring.
@@ -62,14 +84,20 @@ impl RingBufferView {
         }
     }
 
-    /// Advance the read pointer by `dwords`, wrapping at `size_dwords`.
+    /// Advance the read pointer by `dwords`. Circular rings wrap at
+    /// `size_dwords`; an indirect buffer advances linearly (no wrap) so it
+    /// terminates exactly at its fixed write pointer.
     pub fn advance_read(&mut self, dwords: u32) {
         if self.size_dwords == 0 {
             return;
         }
+        if self.indirect {
+            self.read_offset_dwords = self.read_offset_dwords.saturating_add(dwords);
+        } else {
             self.read_offset_dwords =
                 (self.read_offset_dwords + dwords) % self.size_dwords;
         }
+    }
 
     /// Guest address for the dword at relative offset `i` from the current
     /// read pointer. `None` if uninitialized.
@@ -77,7 +105,11 @@ impl RingBufferView {
         if !self.is_initialized() {
             return None;
         }
-        let off = (self.read_offset_dwords + offset_dwords) % self.size_dwords;
+        let off = if self.indirect {
+            self.read_offset_dwords.saturating_add(offset_dwords)
+        } else {
+            (self.read_offset_dwords + offset_dwords) % self.size_dwords
+        };
         Some(self.base.wrapping_add(off.wrapping_mul(4)))
     }
 }
@@ -120,4 +152,52 @@ mod tests {
         assert_eq!(v.addr_at_offset(1), Some(0x4000_0000));
         assert_eq!(v.addr_at_offset(2), Some(0x4000_0004));
     }
+
+    #[test]
+    fn indirect_buffer_drains_linearly_and_terminates() {
+        // An indirect buffer is a fixed linear sub-stream: read advances from
+        // 0 to `size_dwords` and then is exhausted — it must NOT wrap back to
+        // 0 (which previously caused an infinite re-read of a system command
+        // buffer; iterate-2O). write_offset == size, exactly as the
+        // INDIRECT_BUFFER handler sets it.
+        let mut ib = RingBufferView {
+            base: 0x4adf_5080,
+            size_dwords: 11,
+            read_offset_dwords: 0,
+            write_offset_dwords: 11,
+            rptr_writeback_addr: 0,
+            rptr_writeback_block_dwords: 0,
+            indirect: true,
+        };
+        assert!(ib.has_pending());
+        // Drain the exact packet layout observed for Sylpheed's init IB:
+        // 2 + 3 + 6 dwords = 11.
+        ib.advance_read(2);
+        assert!(ib.has_pending());
+        ib.advance_read(3);
+        assert!(ib.has_pending());
+        ib.advance_read(6); // reaches 11 == write
+        assert_eq!(ib.read_offset_dwords, 11);
+        assert!(
+            !ib.has_pending(),
+            "indirect buffer must terminate at write ptr, not wrap to 0"
+        );
+        // addr_at_offset must not modulo-wrap for an indirect buffer.
+        ib.read_offset_dwords = 9;
+        assert_eq!(ib.addr_at_offset(1), Some(0x4adf_5080 + 10 * 4));
+    }
+
+    #[test]
+    fn indirect_flag_does_not_affect_circular_ring() {
+        // Sanity: a circular (primary) ring still wraps as before.
+        let mut v = RingBufferView::new();
+        v.base = 0x4adc_c000;
+        v.size_dwords = 8192;
+        v.read_offset_dwords = 8190;
+        v.write_offset_dwords = 2;
+        assert!(v.has_pending());
+        v.advance_read(4); // (8190 + 4) % 8192 = 2
+        assert_eq!(v.read_offset_dwords, 2);
+        assert!(!v.has_pending());
+    }
 }

crates/xenia-kernel/src/exports.rs

@@ -2883,10 +2883,12 @@ fn vd_initialize_ring_buffer(ctx: &mut PpcContext, _mem: &GuestMemory, state: &m
     // packets directly into ring memory at the current WPTR (the GPU
     // backend lives on a worker thread under `--gpu-thread` so we can't
     // read its `ring.base` from the kernel side without a channel hop).
-    // Per canary: size_log2 is log2(size in BYTES), so size in dwords =
+    // Per canary `CommandProcessor::InitializeRingBuffer`: the ring is
-    // 2^size_log2 / 4 = 1 << (size_log2 - 2).
+    // `1 << (size_log2 + 3)` bytes = `1 << (size_log2 + 1)` dwords (`r4` is
+    // log2 of the size in quadwords). Kept in sync with
+    // `GpuSystem::initialize_ring_buffer`. (Currently bookkeeping-only.)
     state.ring_base = ptr;
-    state.ring_size_dwords = if size_log2 >= 2 { 1u32 << (size_log2 - 2) } else { 0 };
+    state.ring_size_dwords = 1u32 << (size_log2 + 1);
     ctx.gpr[3] = 0;
 }
 
@@ -3169,13 +3171,18 @@ fn vd_swap(ctx: &mut PpcContext, mem: &GuestMemory, state: &mut KernelState) {
             // safer to cap the read at the known total size to avoid OOB.
             let mut tiled = Vec::with_capacity(total_tiled_bytes);
             let mut ok = true;
+            // The frontbuffer is a guest *physical* address; project onto the
+            // committed backing window (see `xenia_gpu::physical_to_backing`)
+            // so the present reads the pixels the GPU resolved, not a stale /
+            // zero mirror page.
+            let fb_backing = xenia_gpu::physical_to_backing(swap.frontbuffer_phys);
             for i in 0..total_tiled_bytes {
                 // read_u8 is cheap — the VirtualMemory handler returns 0
                 // for unmapped pages so we get a recognisable dark frame
                 // rather than a crash if the address turned out bogus.
-                let addr = swap.frontbuffer_phys.wrapping_add(i as u32);
+                let addr = fb_backing.wrapping_add(i as u32);
                 tiled.push(mem.read_u8(addr));
-                if addr < swap.frontbuffer_phys {
+                if addr < fb_backing {
                     ok = false;
                     break;
                 }

crates/xenia-kernel/src/state.rs

@@ -17,6 +17,16 @@ impl PcrWriter for GuestMemoryPcr<'_> {
         // `GuestMemory::write_u32` takes `&self` post-M2 trait flip; the
         // wrapping `&'a GuestMemory` is sufficient.
         self.0.write_u32(pcr_base + 0x2C, hw_id as u32);
+        // PRCB.current_cpu byte at PCR+0x10C (prcb_data@0x100 + current_cpu@0xC).
+        // Canary writes `GetFakeCpuNumber(affinity)` here (xthread.cc:847
+        // `pcr->prcb_data.current_cpu = cpu_index`), which equals the HW thread
+        // id we already compute. Guest spin-barriers (e.g. sub_824D1328, used by
+        // the audio/update pump threads at entries 0x824D2878/0x824D2940) index a
+        // per-HW-thread occupancy array by `lbz r11, 268(r13)` = this byte. Left
+        // unwritten it stayed 0 for every thread, so all threads collided on
+        // slot 0 and the multi-thread rendezvous signature never assembled —
+        // the pump threads spun forever and never fired their KeSetEvent loops.
+        self.0.write_u8(pcr_base + 0x10C, hw_id);
     }
 }
 

crates/xenia-kernel/src/thread.rs

@@ -57,6 +57,11 @@ pub fn allocate_thread_image(
     mem.write_u32(pcr_base, tls_base);
     mem.write_u32(pcr_base + 0x2C, hw_thread_id as u32);
     mem.write_u32(pcr_base + 0x100, 0x1000);
+    // +0x10C  prcb_data.current_cpu — canary `pcr->prcb_data.current_cpu`
+    //         (PRCB@0x100 + current_cpu@0xC). Guest spin-barriers index a
+    //         per-HW-thread slot array by `lbz r11, 268(r13)` = this byte; it
+    //         must equal the HW thread id (== PCR+0x2C). See state.rs PcrWriter.
+    mem.write_u8(pcr_base + 0x10C, hw_thread_id);
     mem.write_u32(pcr_base + 0x150, 0);
 
     Some(ThreadImage {