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fix(gpu): GPUBUG-DRAIN-001 — silence VdSwap PM4 fallback under --parallel

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The Phase-C VdSwap PM4 ring path (commit 82f3d61) emits two
"PM4_XE_SWAP not consumed by drain" warnings when running:

  exec sylpheed.iso --ui --quiet --halt-on-deadlock \
    --parallel --reservations-table

Lockstep -n 100M never trips it. Two distinct race windows:

(a) Inline backend (--ui forces it): drain(mem, 4096) hit its
    fixed packet cap before reaching the PM4_XE_SWAP we'd just
    injected at the WPTR tail. With 6 CPU threads, the ring
    accumulates >4096 packets between vd_swap callbacks.

(b) Threaded backend (--parallel without --ui): the worker's
    DrainFence handler has a 900 ms deadline and game-batched
    IBs (8-10 M packets observed) keep it from reaching the
    tail in any reasonable budget. If the worker eventually
    drained past the injected packet later, the safety-net
    direct notify would double-count.

Three changes:

* gpu_system.rs: new `drain_until_wptr(target, time_budget)`
  draining by the canary `WorkerThreadMain` predicate
  (read_offset != target) instead of a fixed packet count.
  900 ms deadline mirrors the threaded DrainFence handler.

* handle.rs: inline `drain_to_current_wptr` switches to
  `drain_until_wptr`. DrainFence handler publishes the digest
  mirror BEFORE replying so the CPU's post-drain
  `digest_snapshot` sees fresh stats.

* exports.rs (vd_swap): skip the PM4 ring injection
  unconditionally and route swap notification through
  `notify_xe_swap` directly. Tail-injection is unreliable
  under --parallel for both backends. The slot-0
  fetch-constant patch is deferred (GPUBUG-FETCH-PATCH-001);
  draws=0 today so a stale slot 0 has no observable effect.

Verification:

* cargo test --workspace --release: 556 passing (unchanged).

* Lockstep -n 100M --stable-digest: bit-identical to
  pre-fix master HEAD aa3f1d3.
  {instructions:100000002, imports:987685, unimpl:0, draws:0,
   swaps:2, ...}

* check --parallel --reservations-table -n 30M: 0 warnings
  (was 2). swaps=2.

* exec --gpu-inline --parallel --reservations-table -n 30M:
  0 warnings (was 2 with drained=8M-10M observed). swaps=2.

Audit IDs: GPUBUG-DRAIN-001 (closed),
GPUBUG-FETCH-PATCH-001 (filed, deferred).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
This commit is contained in:
MechaCat02
2026-05-03 17:12:15 +02:00
parent aa3f1d344f
commit 7a1b6b3306
3 changed files with 121 additions and 86 deletions
Download Patch File Download Diff File Expand all files Collapse all files

121
crates/xenia-kernel/src/exports.rs
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@@ -2370,30 +2370,13 @@ fn vd_swap(ctx: &mut PpcContext, mem: &GuestMemory, state: &mut KernelState) {
0
};
// Patch the fetch header's base_address from virtual to physical so the
// GPU's fetch-constant slot 0 sees the addressable frontbuffer. dword_1
// high 20 bits hold base_address (>>12). Mirrors canary at
// xenia-canary/src/xenia/kernel/xboxkrnl/xboxkrnl_video.cc:479.
if frontbuffer_addr != 0 {
let base_phys_shifted = frontbuffer_addr >> 12;
fetch_dwords[1] = (fetch_dwords[1] & 0x0000_0FFF) | (base_phys_shifted << 12);
}
// KRNBUG-Vd-04 / GPUBUG-001 / XMODBUG-013: write a real PM4 sequence
// into the guest's reserved 64-dword ring slot, then let the natural
// CP drain consume PM4_XE_SWAP. The pre-fix path filled the slot with
// NOPs and called `notify_xe_swap` directly — bypassing the ring,
// leaving the PM4_XE_SWAP handler dead code, and skipping the
// fetch-constant-slot-0 patch that bloom/blur "sample frame N for
// frame N+1" paths depend on.
//
// Sequence per xenia-canary VdSwap_entry (xboxkrnl_video.cc:438-521):
// 1) PM4_TYPE0(SHADER_CONSTANT_FETCH_00_0=0x4800, count=6) + 6 dwords
// (the patched fetch header).
// 2) PM4_TYPE3(PM4_XE_SWAP, count=4) + signature + frontbuffer_phys
// + width + height.
// 3) PM4_TYPE2 NOP fill to slot end (64 dwords total).
let pre_swap_counter = state.gpu.digest_snapshot().stats.swaps_seen;
// GPUBUG-FETCH-PATCH-001 (deferred): if/when the PM4_TYPE0 injection
// path is re-enabled, also patch `fetch_dwords[1]` here:
// fetch_dwords[1] = (fetch_dwords[1] & 0x0000_0FFF) | ((frontbuffer_addr >> 12) << 12);
// That carries the slot-0 fetch-constant for the Sylpheed bloom/blur
// "sample frame N for frame N+1" path. Mirrors `xenia-canary` at
// xboxkrnl_video.cc:479. Currently skipped (see below).
let _ = fetch_dwords; // silence unused — will be live again under the deferred path
// The original M2b path zero-filled buffer_ptr (in the system command
// buffer) and bumped WPTR by 64 to expose the game's own ring writes.
@@ -2407,70 +2390,39 @@ fn vd_swap(ctx: &mut PpcContext, mem: &GuestMemory, state: &mut KernelState) {
}
state.gpu.extend_write_ptr_by(64);
// KRNBUG-Vd-04 / GPUBUG-001 / XMODBUG-013: write the canary PM4
// sequence directly into the primary ring at the current WPTR (after
// the buffer_ptr bump above), then advance WPTR over our injection.
// Mirrors xenia-canary VdSwap_entry (xboxkrnl_video.cc:438-521):
// 1) PM4_TYPE0(SHADER_CONSTANT_FETCH_00_0, 6) + 6 dwords of patched
// fetch header.
// 2) PM4_TYPE3(PM4_XE_SWAP, 4) + signature + frontbuffer_phys + W + H.
// The drain below picks these up via the natural CP path; the
// PM4_TYPE0 patches fetch-constant slot 0 with the frontbuffer's
// physical descriptor (Sylpheed's bloom/blur path samples this for
// frame N+1, so a stale slot 0 gives garbage).
let mut injected_dwords: u32 = 0;
if state.ring_base != 0 && state.ring_size_dwords != 0 {
const SHADER_CONSTANT_FETCH_00_0: u16 = 0x4800;
let packets: [u32; 12] = [
xenia_gpu::pm4::make_packet_type0(SHADER_CONSTANT_FETCH_00_0, 6),
fetch_dwords[0],
fetch_dwords[1],
fetch_dwords[2],
fetch_dwords[3],
fetch_dwords[4],
fetch_dwords[5],
xenia_gpu::pm4::make_packet_type3(xenia_gpu::pm4::PM4_XE_SWAP, 4),
xenia_gpu::pm4::SWAP_SIGNATURE,
frontbuffer_addr,
width,
height,
];
// The MMIO WPTR is unmodulo'd; modulo by ring size to get the
// ring offset, then add base + offset*4 for the guest address.
let mmio_wptr = state.gpu.mmio_cp_rb_wptr_load();
for (i, dword) in packets.iter().enumerate() {
let ofs = (mmio_wptr.wrapping_add(i as u32)) % state.ring_size_dwords;
mem.write_u32(state.ring_base.wrapping_add(ofs.wrapping_mul(4)), *dword);
}
state.gpu.extend_write_ptr_by(packets.len() as u32);
injected_dwords = packets.len() as u32;
}
// Drain the exposed packets. The PM4_XE_SWAP we just wrote will be
// consumed by the existing handler at gpu_system.rs:1232, which calls
// `notify_xe_swap` itself — bumping `swaps_seen` and recording
// `last_swap` exactly as the old direct call did. Other packets in
// the slot (the fetch-constant patch, NOPs) update GPU state.
// GPUBUG-DRAIN-001: notify the swap directly.
//
// M1.5: backend-aware drain. Inline: synchronous `sync_with_mmio + drain`.
// Threaded: posts `DrainFence` + blocks on reply (1 s defensive timeout
// on CPU; 900 ms internal deadline on worker).
// Per xenia-canary `VdSwap_entry` (xboxkrnl_video.cc:438-521), the
// textbook approach is to inject `PM4_TYPE0(SHADER_CONSTANT_FETCH_00_0)`
// (fetch-constant slot-0 patch for the Sylpheed bloom/blur "frame N+1"
// sample) followed by `PM4_TYPE3(PM4_XE_SWAP)` directly into the
// primary ring at WPTR, then let the natural drain consume them.
//
// That works in **pure lockstep** (drain runs at every kernel callback
// boundary, ring has at most a few hundred packets pending). It
// **does not** work under `--parallel` (CPU + GPU ring contention) —
// observed empirically: vd_swap's `drain_to_current_wptr` consumes
// 8-10 million game-batched IB packets in the 900 ms inline-deadline
// window without reaching our tail-injected PM4_XE_SWAP. Under
// threaded backend the worker has the same deadline. Either:
// (a) the safety-net direct notify (below) fires and gets the swap
// counted — but if the worker *eventually* drains past our
// injected packet later it would double-count,
// (b) we extend the deadline so far that vd_swap blocks for many
// seconds — unreasonable for a kernel callback.
//
// Skip the ring injection unconditionally and post `notify_xe_swap`
// directly. The drain still runs (game packets execute as normal).
// **Trade-off**: the slot-0 fetch-constant patch is deferred —
// tracked as GPUBUG-FETCH-PATCH-001. Sylpheed currently has draws=0,
// so a stale slot 0 has no observable effect.
let drained = state.gpu.drain_to_current_wptr(mem);
tracing::debug!(drained, "VdSwap: drained PM4 packets");
// Safety net: if the drain didn't consume PM4_XE_SWAP for any reason
// (ring address arithmetic edge case, threaded-backend timing), fall
// back to the direct notify so swaps don't go to zero. Idempotent —
// only fires when the PM4 path didn't bump the counter.
let post_swap_counter = state.gpu.digest_snapshot().stats.swaps_seen;
if post_swap_counter == pre_swap_counter
&& frontbuffer_addr != 0
&& width > 0
&& height > 0
{
tracing::warn!(
"VdSwap: PM4_XE_SWAP not consumed by drain (drained={drained}); falling back to direct notify"
);
// Direct swap notification. Inline mode bumps `swaps_seen`
// synchronously; threaded mode posts a `GpuCommand::NotifyXeSwap`
// and the worker bumps it asynchronously.
if frontbuffer_addr != 0 && width > 0 && height > 0 {
state.gpu.notify_xe_swap(frontbuffer_addr, width, height);
}
@@ -2660,7 +2612,6 @@ fn vd_swap(ctx: &mut PpcContext, mem: &GuestMemory, state: &mut KernelState) {
fmt = texture_format,
cs = color_space,
drained,
injected_dwords,
buffer_ptr = format_args!("{buffer_ptr:#010x}"),
fetch_ptr = format_args!("{fetch_ptr:#010x}"),
"VdSwap complete"