From 4a78428c6b51bf4420a2997a1efe0f064bea0fee Mon Sep 17 00:00:00 2001
From: "Herman S." <429230+has207@users.noreply.github.com>
Date: Tue, 10 Mar 2026 21:51:58 +0900
Subject: [PATCH] [CPU] Skip QueryProtect in SIGSEGV handler on Linux

std::ifstream read of /proc/self/maps is not async-signal-safe and runs
on every access violation fault. Skip the race-condition check and go
straight to the callback, which handles cleared watches correctly.

Fixes severe performance issues seen in MFSMW, MCLA, etc.
---
 src/xenia/cpu/mmio_handler.cc | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/src/xenia/cpu/mmio_handler.cc b/src/xenia/cpu/mmio_handler.cc
index 4662f50ba..c7cc7815e 100644
--- a/src/xenia/cpu/mmio_handler.cc
+++ b/src/xenia/cpu/mmio_handler.cc
@@ -440,6 +440,22 @@ bool MMIOHandler::ExceptionCallback(Exception* ex) {
     // clears the watch we just hit).
     // Do this under the lock so we don't introduce another race condition.
     auto lock = global_critical_region_.Acquire();
+#if XE_PLATFORM_LINUX
+    // On Linux, exception handling runs inside a signal handler (SIGSEGV).
+    // QueryProtect uses std::ifstream to read /proc/self/maps, which is NOT
+    // async-signal-safe and can corrupt heap state or deadlock if the signal
+    // interrupted malloc or another non-reentrant function.
+    // Instead, skip the race-condition check and go straight to the callback.
+    // The callback will handle it correctly — if watches were already cleared,
+    // TriggerCallbacks will find no watches and the page will be unprotected
+    // by the time the instruction retries.
+    if (access_violation_callback_) {
+      return access_violation_callback_(std::move(lock),
+                                        access_violation_callback_context_,
+                                        fault_host_address, is_write);
+    }
+    return false;
+#else
     memory::PageAccess cur_access;
     size_t page_length = memory::page_size();
     memory::QueryProtect(fault_host_address, page_length, cur_access);
@@ -457,6 +473,7 @@ bool MMIOHandler::ExceptionCallback(Exception* ex) {
                                         fault_host_address, is_write);
     }
     return false;
+#endif
   }
 
   auto rip = ex->pc();