PatchEngine: Handle MSR more cleanly

Instead of fiddling with the MSR value, just reschedule and try again
after the game fixes it itself.

Source/Core/Core/Boot/Boot_BS2Emu.cpp

@@ -2,6 +2,7 @@
 // Licensed under GPLv2+
 // Refer to the license.txt file included.
 
+#include "Common/Assert.h"
 #include "Common/CommonPaths.h"
 #include "Common/CommonTypes.h"
 #include "Common/FileUtil.h"
@@ -178,7 +179,8 @@ bool CBoot::EmulatedBS2_GC(bool skipAppLoader)
   PatchEngine::LoadPatches();
 
   // If we have any patches that need to be applied very early, here's a good place
-  PatchEngine::ApplyFramePatches();
+  bool patched = PatchEngine::ApplyFramePatches();
+  _assert_(patched);
 
   return true;
 }

Source/Core/Core/GeckoCode.cpp

@@ -157,25 +157,11 @@ static Installation InstallCodeHandlerLocked()
   return Installation::Installed;
 }
 
-void RunCodeHandler(u32 msr_reg)
+void RunCodeHandler()
 {
   if (!SConfig::GetInstance().bEnableCheats)
     return;
 
-  // Dolphin's hook mechanism is less 'precise' than Gecko OS' which detects particular
-  // instruction sequences (uses configuration, not automatic) that only run once per frame
-  // which includes a BLR as one of the instructions. It then overwrites the BLR with a
-  // "B 0x800018A8" to establish the hook. Dolphin uses its own internal VI interrupt which
-  // means the PC is non-deterministic and could be anywhere.
-  UReg_MSR msr = msr_reg;
-  if (!msr.DR || !msr.IR)
-  {
-    WARN_LOG(ACTIONREPLAY, "GeckoCode: Skipping frame update. MSR.IR/DR is currently disabled. "
-                           "PC = 0x%08X, MSR = 0x%08X",
-             PC, msr_reg);
-    return;
-  }
-
   std::lock_guard<std::mutex> codes_lock(s_active_codes_lock);
   // Don't spam retry if the install failed. The corrupt / missing disk file is not likely to be
   // fixed within 1 frame of the last error.

Source/Core/Core/GeckoCode.h

@@ -52,6 +52,6 @@ constexpr u32 HLE_TRAMPOLINE_ADDRESS = INSTALLER_END_ADDRESS - 4;
 constexpr u32 MAGIC_GAMEID = 0xD01F1BAD;
 
 void SetActiveCodes(const std::vector<GeckoCode>& gcodes);
-void RunCodeHandler(u32 msr_reg);
+void RunCodeHandler();
 
 }  // namespace Gecko

Source/Core/Core/HW/SystemTimers.cpp

@@ -166,11 +166,29 @@ s64 GetLocalTimeRTCOffset()
   return s_localtime_rtc_offset;
 }
 
-static void PatchEngineCallback(u64 userdata, s64 cyclesLate)
+static void PatchEngineCallback(u64 userdata, s64 cycles_late)
 {
-  // Patch mem and run the Action Replay
-  PatchEngine::ApplyFramePatches();
-  CoreTiming::ScheduleEvent(VideoInterface::GetTicksPerField() - cyclesLate, et_PatchEngine);
+  // We have 2 periods, a 1000 cycle error period and the VI period.
+  // We have to carefully combine these together so that we stay on the VI period without drifting.
+  u32 vi_interval = VideoInterface::GetTicksPerField();
+  s64 cycles_pruned = (userdata + cycles_late) % vi_interval;
+  s64 next_schedule = 0;
+
+  // Try to patch mem and run the Action Replay
+  if (PatchEngine::ApplyFramePatches())
+  {
+    next_schedule = vi_interval - cycles_pruned;
+    cycles_pruned = 0;
+  }
+  else
+  {
+    // The patch failed, usually because the CPU is in an inappropriate state (interrupt handler).
+    // We'll try again after 1000 cycles.
+    next_schedule = 1000;
+    cycles_pruned += next_schedule;
+  }
+
+  CoreTiming::ScheduleEvent(next_schedule, et_PatchEngine, cycles_pruned);
 }
 
 static void ThrottleCallback(u64 last_time, s64 cyclesLate)

Source/Core/Core/PatchEngine.cpp

@@ -203,22 +203,29 @@ static void ApplyPatches(const std::vector<Patch>& patches)
   }
 }
 
-void ApplyFramePatches()
+bool ApplyFramePatches()
 {
-  // TODO: Messing with MSR this way is really, really, evil; we should
-  // probably be using some sort of Gecko OS-style hooking mechanism
-  // so the emulated CPU is in a predictable state when we process cheats.
-  u32 oldMSR = MSR;
-  UReg_MSR newMSR = oldMSR;
-  newMSR.IR = 1;
-  newMSR.DR = 1;
-  MSR = newMSR.Hex;
+  // Because we're using the VI Interrupt to time this instead of patching the game with a
+  // callback hook we can end up catching the game in an exception vector.
+  // We deal with this by returning false so that SystemTimers will reschedule us in a few cycles
+  // where we can try again after the CPU hopefully returns back to the normal instruction flow.
+  UReg_MSR msr = MSR;
+  if (!msr.DR || !msr.IR)
+  {
+    INFO_LOG(
+        ACTIONREPLAY,
+        "Need to retry later. CPU configuration is currently incorrect. PC = 0x%08X, MSR = 0x%08X",
+        PC, MSR);
+    return false;
+  }
+
   ApplyPatches(onFrame);
 
   // Run the Gecko code handler
-  Gecko::RunCodeHandler(oldMSR);
+  Gecko::RunCodeHandler();
   ActionReplay::RunAllActive();
-  MSR = oldMSR;
+
+  return true;
 }
 
 void Shutdown()

Source/Core/Core/PatchEngine.h

@@ -43,7 +43,7 @@ int GetSpeedhackCycles(const u32 addr);
 void LoadPatchSection(const std::string& section, std::vector<Patch>& patches, IniFile& globalIni,
                       IniFile& localIni);
 void LoadPatches();
-void ApplyFramePatches();
+bool ApplyFramePatches();
 void Shutdown();
 
 inline int GetPatchTypeCharLength(PatchType type)