From a3aa3cc7d621c50ea3b4ac9c6074b1264a1c86f1 Mon Sep 17 00:00:00 2001
From: MechaCat02 <fabian@diekaulbachs.de>
Date: Thu, 18 Jun 2026 15:28:49 +0200
Subject: [PATCH] [iterate-3Q] draw_capture: read vertex window via physical
 alias

The UI geometry-capture read the vertex-fetch base at its bare low VA
(~0x0adf_xxxx), which is unmapped in ours, so it copied all-zeros.

The fetch constant's address:30 field is a guest *physical* dword
address (canary reads it via Memory::TranslatePhysical, draw_util.cc:961).
Ours only maps the cached-physical window at 0x4000_0000
(physical_to_backing). Rebase a low physical base onto that mapped alias
when the raw VA is unmapped; window_base_dwords still carries the
original base so the shader's rebase indexes the uploaded window.

Decode itself was verified correct against canary (xe_gpu_vertex_fetch_t
+ GetVertexFetch + ucode.h vfetch const_index*3+const_index_sel): for
the splash draws const_index_sel==0, so ours' stride-6 register offset
lands on the exact same constant as canary's stride-2 offset; raw dwords
match byte-for-byte.

UI-only path (frame_captures is None in headless), so the deterministic
--gpu-inline golden is byte-identical (verified) and 679 tests stay green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
---
 crates/xenia-gpu/src/draw_capture.rs | 23 ++++++++++++++++++++---
 1 file changed, 20 insertions(+), 3 deletions(-)

diff --git a/crates/xenia-gpu/src/draw_capture.rs b/crates/xenia-gpu/src/draw_capture.rs
index 2453e01..df4df75 100644
--- a/crates/xenia-gpu/src/draw_capture.rs
+++ b/crates/xenia-gpu/src/draw_capture.rs
@@ -115,18 +115,35 @@ fn resolve_vertex_window(
     let fc = fetch_const? as u32;
     let dword0 = rf.read(CONST_BASE_FETCH + fc * 6);
     let dword1 = rf.read(CONST_BASE_FETCH + fc * 6 + 1);
-    // address:30 at bits[31:2] of dword0 (in bytes once masked).
+    // address:30 at bits[31:2] of dword0 (in bytes once masked). The fetch
+    // constant carries a guest *physical* dword address — canary reads the
+    // vertex buffer via `Memory::TranslatePhysical(fetch.address * 4)`
+    // (`draw_util.cc:961`). On the Xbox 360 the physical range is mirrored at
+    // several virtual windows; ours only maps the cached-physical window at
+    // `0x4000_0000` (`gpu_system::physical_to_backing`). Reading the bare low
+    // address (`0x0adf_xxxx`) hits an unmapped VA and returns zeros, so rebase
+    // a low physical base onto the mapped `0x4000_0000` alias when the raw VA
+    // is not itself mapped. `window_base_dwords` keeps the *original* base so
+    // the shader's rebase against the (unmodified) fetch-constant address still
+    // indexes the uploaded window correctly.
     let base_bytes = dword0 & 0xFFFF_FFFC;
     if base_bytes == 0 {
         return None;
     }
+    let read_base = if mem.translate(base_bytes).is_some() {
+        base_bytes
+    } else if base_bytes < 0x2000_0000 && mem.translate(base_bytes | 0x4000_0000).is_some() {
+        base_bytes | 0x4000_0000
+    } else {
+        base_bytes
+    };
     // size:24 at bits[25:2] of dword1, in dwords. Clamp to our window cap.
     let size_dwords = ((dword1 >> 2) & 0x00FF_FFFF).clamp(1, MAX_WINDOW_DWORDS);
     let window_base_dwords = base_bytes >> 2;
     let mut dwords = Vec::with_capacity(size_dwords as usize);
     for i in 0..size_dwords {
-        let addr = base_bytes.wrapping_add(i * 4);
-        if addr < base_bytes {
+        let addr = read_base.wrapping_add(i * 4);
+        if addr < read_base {
             break; // wrap guard
         }
         // `read_u32` composes big-endian bytes into the u32 value; the WGSL's