Merge pull request #13689 from JosJuice/lock-core-any-thread

Core: Let any thread call previously host-thread-only functions

11 files changed, 103 insertions, 182 deletions

diff --git a/Source/Android/jni/Host.h b/Source/Android/jni/Host.h
index 3a7a304825..0183038d9a 100644
--- a/Source/Android/jni/Host.h
+++ b/Source/Android/jni/Host.h
@@ -5,37 +5,24 @@
 
 #include <mutex>
 
-#include "Core/Core.h"
-
 // The Core only supports using a single Host thread.
 // If multiple threads want to call host functions then they need to queue
 // sequentially for access.
+// TODO: The above isn't true anymore, so we should get rid of this class.
 struct HostThreadLock
 {
-  explicit HostThreadLock() : m_lock(s_host_identity_mutex) { Core::DeclareAsHostThread(); }
+  explicit HostThreadLock() : m_lock(s_host_identity_mutex) {}
 
-  ~HostThreadLock()
-  {
-    if (m_lock.owns_lock())
-      Core::UndeclareAsHostThread();
-  }
+  ~HostThreadLock() = default;
 
   HostThreadLock(const HostThreadLock& other) = delete;
   HostThreadLock(HostThreadLock&& other) = delete;
   HostThreadLock& operator=(const HostThreadLock& other) = delete;
   HostThreadLock& operator=(HostThreadLock&& other) = delete;
 
-  void Lock()
-  {
-    m_lock.lock();
-    Core::DeclareAsHostThread();
-  }
-
-  void Unlock()
-  {
-    m_lock.unlock();
-    Core::UndeclareAsHostThread();
-  }
+  void Lock() { m_lock.lock(); }
+
+  void Unlock() { m_lock.unlock(); }
 
 private:
   static std::mutex s_host_identity_mutex;
diff --git a/Source/Core/Core/AchievementManager.cpp b/Source/Core/Core/AchievementManager.cpp
index 5423bb29e0..7a41d57c84 100644
--- a/Source/Core/Core/AchievementManager.cpp
+++ b/Source/Core/Core/AchievementManager.cpp
@@ -65,7 +65,7 @@ void AchievementManager::Init(void* hwnd)
   {
     {
       std::lock_guard lg{m_lock};
-      m_client = rc_client_create(MemoryVerifier, Request);
+      m_client = rc_client_create(MemoryPeeker, Request);
     }
     std::string host_url = Config::Get(Config::RA_HOST_URL);
     if (!host_url.empty())
@@ -170,7 +170,6 @@ void AchievementManager::LoadGame(const DiscIO::Volume* volume)
     }
     else
     {
-      rc_client_set_read_memory_function(m_client, MemoryVerifier);
       rc_client_begin_load_game(m_client, "", LoadGameCallback, NULL);
     }
     return;
@@ -210,7 +209,6 @@ void AchievementManager::LoadGame(const DiscIO::Volume* volume)
   else
   {
     u32 console_id = FindConsoleID(volume->GetVolumeType());
-    rc_client_set_read_memory_function(m_client, MemoryVerifier);
     rc_client_begin_identify_and_load_game(m_client, console_id, "", NULL, 0, LoadGameCallback,
                                            NULL);
   }
@@ -320,22 +318,6 @@ void AchievementManager::DoFrame()
   if (!(IsGameLoaded() || m_dll_found) || !Core::IsCPUThread())
     return;
   {
-#ifdef RC_CLIENT_SUPPORTS_RAINTEGRATION
-    if (m_dll_found)
-    {
-      std::lock_guard lg{m_memory_lock};
-      Core::System* system = m_system.load(std::memory_order_acquire);
-      if (!system)
-        return;
-      Core::CPUThreadGuard thread_guard(*system);
-      u32 mem2_size = (system->IsWii()) ? system->GetMemory().GetExRamSizeReal() : 0;
-      if (m_cloned_memory.size() != MEM1_SIZE + mem2_size)
-        m_cloned_memory.resize(MEM1_SIZE + mem2_size);
-      system->GetMemory().CopyFromEmu(m_cloned_memory.data(), 0, MEM1_SIZE);
-      if (mem2_size > 0)
-        system->GetMemory().CopyFromEmu(m_cloned_memory.data() + MEM1_SIZE, MEM2_START, mem2_size);
-    }
-#endif  // RC_CLIENT_SUPPORTS_RAINTEGRATION
     std::lock_guard lg{m_lock};
     rc_client_do_frame(m_client);
   }
@@ -1061,7 +1043,6 @@ void AchievementManager::LoadGameCallback(int result, const char* error_message,
   if (game == nullptr)
     return;
 
-  rc_client_set_read_memory_function(instance.m_client, MemoryPeeker);
   instance.FetchGameBadges();
   instance.m_system.store(&Core::System::GetInstance(), std::memory_order_release);
   instance.update_event.Trigger({.all = true});
@@ -1337,53 +1318,11 @@ void AchievementManager::Request(const rc_api_request_t* request,
       });
 }
 
-// Currently, when rc_client calls the memory peek method provided in its constructor (or in
-// rc_client_set_read_memory_function) it will do so on the thread that calls DoFrame, which is
-// currently the host thread, with one exception: an asynchronous callback in the load game process.
-// This is done to validate/invalidate each memory reference in the downloaded assets, mark assets
-// as unsupported, and notify the player upon startup that there are unsupported assets and how
-// many. As such, all that call needs to do is return the number of bytes that can be read with this
-// call. As only the CPU and host threads are allowed to read from memory, I provide a separate
-// method for this verification. In lieu of a more convenient set of steps, I provide MemoryVerifier
-// to rc_client at construction, and in the Load Game callback, after the verification has been
-// complete, I call rc_client_set_read_memory_function to switch to the usual MemoryPeeker for all
-// future synchronous calls.
-u32 AchievementManager::MemoryVerifier(u32 address, u8* buffer, u32 num_bytes, rc_client_t* client)
-{
-  auto& system = Core::System::GetInstance();
-  u32 mem2_size = system.GetMemory().GetExRamSizeReal();
-  if (address < MEM1_SIZE + mem2_size)
-    return std::min(MEM1_SIZE + mem2_size - address, num_bytes);
-  return 0;
-}
-
 u32 AchievementManager::MemoryPeeker(u32 address, u8* buffer, u32 num_bytes, rc_client_t* client)
 {
   if (buffer == nullptr)
     return 0u;
-#ifdef RC_CLIENT_SUPPORTS_RAINTEGRATION
-  auto& instance = AchievementManager::GetInstance();
-  if (instance.m_dll_found)
-  {
-    std::lock_guard lg{instance.m_memory_lock};
-    if (u64(address) + num_bytes > instance.m_cloned_memory.size())
-    {
-      ERROR_LOG_FMT(ACHIEVEMENTS,
-                    "Attempt to read past memory size: size {} address {} write length {}",
-                    instance.m_cloned_memory.size(), address, num_bytes);
-      return 0;
-    }
-    std::copy(instance.m_cloned_memory.begin() + address,
-              instance.m_cloned_memory.begin() + address + num_bytes, buffer);
-    return num_bytes;
-  }
-#endif  // RC_CLIENT_SUPPORTS_RAINTEGRATION
   auto& system = Core::System::GetInstance();
-  if (!(Core::IsHostThread() || Core::IsCPUThread()))
-  {
-    ASSERT_MSG(ACHIEVEMENTS, false, "MemoryPeeker called from wrong thread");
-    return 0;
-  }
   Core::CPUThreadGuard thread_guard(system);
   if (address > MEM1_SIZE)
     address += (MEM2_START - MEM1_SIZE);
@@ -1607,32 +1546,17 @@ void AchievementManager::MemoryPoker(u32 address, u8* buffer, u32 num_bytes, rc_
 {
   if (buffer == nullptr)
     return;
-  if (!(Core::IsHostThread() || Core::IsCPUThread()))
-  {
-    Core::QueueHostJob([address, buffer, num_bytes, client](Core::System& system) {
-      MemoryPoker(address, buffer, num_bytes, client);
-    });
-    return;
-  }
   auto& instance = AchievementManager::GetInstance();
-  if (u64(address) + num_bytes >= instance.m_cloned_memory.size())
-  {
-    ERROR_LOG_FMT(ACHIEVEMENTS,
-                  "Attempt to write past memory size: size {} address {} write length {}",
-                  instance.m_cloned_memory.size(), address, num_bytes);
-    return;
-  }
   Core::System* system = instance.m_system.load(std::memory_order_acquire);
   if (!system)
     return;
   Core::CPUThreadGuard thread_guard(*system);
-  std::lock_guard lg{instance.m_memory_lock};
   if (address < MEM1_SIZE)
     system->GetMemory().CopyToEmu(address, buffer, num_bytes);
   else
     system->GetMemory().CopyToEmu(address - MEM1_SIZE + MEM2_START, buffer, num_bytes);
-  std::copy(buffer, buffer + num_bytes, instance.m_cloned_memory.begin() + address);
 }
+
 void AchievementManager::GameTitleEstimateHandler(char* buffer, u32 buffer_size,
                                                   rc_client_t* client)
 {
diff --git a/Source/Core/Core/AchievementManager.h b/Source/Core/Core/AchievementManager.h
index 1d2245c24c..b666b3f862 100644
--- a/Source/Core/Core/AchievementManager.h
+++ b/Source/Core/Core/AchievementManager.h
@@ -251,7 +251,6 @@ private:
 
   static void Request(const rc_api_request_t* request, rc_client_server_callback_t callback,
                       void* callback_data, rc_client_t* client);
-  static u32 MemoryVerifier(u32 address, u8* buffer, u32 num_bytes, rc_client_t* client);
   static u32 MemoryPeeker(u32 address, u8* buffer, u32 num_bytes, rc_client_t* client);
   void FetchBadge(Badge* badge, u32 badge_type, const BadgeNameFunction function,
                   const UpdatedItems callback_data);
@@ -293,8 +292,6 @@ private:
 
   bool m_dll_found = false;
 #ifdef RC_CLIENT_SUPPORTS_RAINTEGRATION
-  std::vector<u8> m_cloned_memory;
-  std::recursive_mutex m_memory_lock;
   std::string m_title_estimate;
 #endif  // RC_CLIENT_SUPPORTS_RAINTEGRATION
 
diff --git a/Source/Core/Core/Core.cpp b/Source/Core/Core/Core.cpp
index 5241e40a29..58da62b4e8 100644
--- a/Source/Core/Core/Core.cpp
+++ b/Source/Core/Core/Core.cpp
@@ -105,6 +105,11 @@ static bool s_is_throttler_temp_disabled = false;
 static bool s_frame_step = false;
 static std::atomic<bool> s_stop_frame_step;
 
+// Threads other than the CPU thread must hold this when taking on the role of the CPU thread.
+// The CPU thread is not required to hold this when doing normal work, but must hold it if writing
+// to s_state.
+static std::recursive_mutex s_core_mutex;
+
 // The value Paused is never stored in this variable. The core is considered to be in
 // the Paused state if this variable is Running and the CPU reports that it's stepping.
 static std::atomic<State> s_state = State::Uninitialized;
@@ -126,7 +131,6 @@ static Common::Event s_cpu_thread_job_finished;
 
 static thread_local bool tls_is_cpu_thread = false;
 static thread_local bool tls_is_gpu_thread = false;
-static thread_local bool tls_is_host_thread = false;
 
 static void EmuThread(Core::System& system, std::unique_ptr<BootParameters> boot,
                       WindowSystemInfo wsi);
@@ -207,11 +211,6 @@ bool IsGPUThread()
   return tls_is_gpu_thread;
 }
 
-bool IsHostThread()
-{
-  return tls_is_host_thread;
-}
-
 bool WantsDeterminism()
 {
   return s_wants_determinism;
@@ -221,6 +220,8 @@ bool WantsDeterminism()
 // BootManager.cpp
 bool Init(Core::System& system, std::unique_ptr<BootParameters> boot, const WindowSystemInfo& wsi)
 {
+  std::lock_guard lock(s_core_mutex);
+
   if (s_emu_thread.joinable())
   {
     if (!IsUninitialized(system))
@@ -266,16 +267,20 @@ static void ResetRumble()
 // Called from GUI thread
 void Stop(Core::System& system)  // - Hammertime!
 {
-  const State state = s_state.load();
-  if (state == State::Stopping || state == State::Uninitialized)
-    return;
+  {
+    std::lock_guard lock(s_core_mutex);
 
-  AchievementManager::GetInstance().CloseGame();
+    const State state = s_state.load();
+    if (state == State::Stopping || state == State::Uninitialized)
+      return;
 
-  s_state.store(State::Stopping);
+    s_state.store(State::Stopping);
+  }
 
   NotifyStateChanged(State::Stopping);
 
+  AchievementManager::GetInstance().CloseGame();
+
   // Dump left over jobs
   HostDispatchJobs(system);
 
@@ -308,21 +313,11 @@ void UndeclareAsGPUThread()
   tls_is_gpu_thread = false;
 }
 
-void DeclareAsHostThread()
-{
-  tls_is_host_thread = true;
-}
-
-void UndeclareAsHostThread()
-{
-  tls_is_host_thread = false;
-}
-
 // For the CPU Thread only.
 static void CPUSetInitialExecutionState(bool force_paused = false)
 {
   // The CPU starts in stepping state, and will wait until a new state is set before executing.
-  // SetState must be called on the host thread, so we defer it for later.
+  // SetState isn't safe to call from the CPU thread, so we ask the host thread to call it.
   QueueHostJob([force_paused](Core::System& system) {
     bool paused = SConfig::GetInstance().bBootToPause || force_paused;
     SetState(system, paused ? State::Paused : State::Running, true, true);
@@ -362,10 +357,14 @@ static void CpuThread(Core::System& system, const std::optional<std::string>& sa
       File::Delete(*savestate_path);
   }
 
-  // If s_state is Starting, change it to Running. But if it's already been set to Stopping
-  // by the host thread, don't change it.
-  State expected = State::Starting;
-  s_state.compare_exchange_strong(expected, State::Running);
+  {
+    std::unique_lock core_lock(s_core_mutex);
+
+    // If s_state is Starting, change it to Running. But if it's already been set to Stopping
+    // because another thread called Stop, don't change it.
+    State expected = State::Starting;
+    s_state.compare_exchange_strong(expected, State::Running);
+  }
 
   {
 #ifndef _WIN32
@@ -423,12 +422,17 @@ static void FifoPlayerThread(Core::System& system, const std::optional<std::stri
   {
     system.GetPowerPC().InjectExternalCPUCore(cpu_core.get());
 
-    // If s_state is Starting, change it to Running. But if it's already been set to Stopping
-    // by the host thread, don't change it.
-    State expected = State::Starting;
-    s_state.compare_exchange_strong(expected, State::Running);
+    {
+      std::lock_guard core_lock(s_core_mutex);
+
+      // If s_state is Starting, change it to Running. But if it's already been set to Stopping
+      // because another thread called Stop, don't change it.
+      State expected = State::Starting;
+      s_state.compare_exchange_strong(expected, State::Running);
+    }
 
     CPUSetInitialExecutionState();
+
     system.GetCPU().Run();
 
     system.GetPowerPC().InjectExternalCPUCore(nullptr);
@@ -517,7 +521,10 @@ static void EmuThread(Core::System& system, std::unique_ptr<BootParameters> boot
 {
   NotifyStateChanged(State::Starting);
   Common::ScopeGuard flag_guard{[] {
-    s_state.store(State::Uninitialized);
+    {
+      std::lock_guard lock(s_core_mutex);
+      s_state.store(State::Uninitialized);
+    }
 
     NotifyStateChanged(State::Uninitialized);
 
@@ -682,35 +689,39 @@ static void EmuThread(Core::System& system, std::unique_ptr<BootParameters> boot
 void SetState(Core::System& system, State state, bool report_state_change,
               bool override_achievement_restrictions)
 {
-  // State cannot be controlled until the CPU Thread is operational
-  if (s_state.load() != State::Running)
-    return;
-
-  switch (state)
   {
-  case State::Paused:
-#ifdef USE_RETRO_ACHIEVEMENTS
-    if (!override_achievement_restrictions && !AchievementManager::GetInstance().CanPause())
+    std::lock_guard lock(s_core_mutex);
+
+    // State cannot be controlled until the CPU Thread is operational
+    if (s_state.load() != State::Running)
       return;
+
+    switch (state)
+    {
+    case State::Paused:
+#ifdef USE_RETRO_ACHIEVEMENTS
+      if (!override_achievement_restrictions && !AchievementManager::GetInstance().CanPause())
+        return;
 #endif  // USE_RETRO_ACHIEVEMENTS
-    // NOTE: GetState() will return State::Paused immediately, even before anything has
-    //   stopped (including the CPU).
-    system.GetCPU().SetStepping(true);  // Break
-    Wiimote::Pause();
-    ResetRumble();
+      // NOTE: GetState() will return State::Paused immediately, even before anything has
+      //   stopped (including the CPU).
+      system.GetCPU().SetStepping(true);  // Break
+      Wiimote::Pause();
+      ResetRumble();
 #ifdef USE_RETRO_ACHIEVEMENTS
-    AchievementManager::GetInstance().DoIdle();
+      AchievementManager::GetInstance().DoIdle();
 #endif  // USE_RETRO_ACHIEVEMENTS
-    break;
-  case State::Running:
-  {
-    system.GetCPU().SetStepping(false);
-    Wiimote::Resume();
-    break;
-  }
-  default:
-    PanicAlertFmt("Invalid state");
-    break;
+      break;
+    case State::Running:
+    {
+      system.GetCPU().SetStepping(false);
+      Wiimote::Resume();
+      break;
+    }
+    default:
+      PanicAlertFmt("Invalid state");
+      break;
+    }
   }
 
   // Certain callers only change the state momentarily. Sending a callback for them causes
@@ -781,7 +792,7 @@ void SaveScreenShot(std::string_view name)
 
 static bool PauseAndLock(Core::System& system)
 {
-  // WARNING: PauseAndLock is not fully threadsafe so is only valid on the Host Thread
+  s_core_mutex.lock();
 
   if (!IsRunning(system))
     return true;
@@ -804,7 +815,7 @@ static bool PauseAndLock(Core::System& system)
 
 static void RestoreStateAndUnlock(Core::System& system, const bool unpause_on_unlock)
 {
-  // WARNING: RestoreStateAndUnlock is not fully threadsafe so is only valid on the Host Thread
+  Common::ScopeGuard scope_guard([] { s_core_mutex.unlock(); });
 
   if (!IsRunning(system))
     return;
@@ -824,8 +835,7 @@ static void RestoreStateAndUnlock(Core::System& system, const bool unpause_on_un
 void RunOnCPUThread(Core::System& system, Common::MoveOnlyFunction<void()> function,
                     bool wait_for_completion)
 {
-  // If the CPU thread is not running, assume there is no active CPU thread we can race against.
-  if (!IsRunning(system) || IsCPUThread())
+  if (IsCPUThread())
   {
     function();
     return;
@@ -834,10 +844,15 @@ void RunOnCPUThread(Core::System& system, Common::MoveOnlyFunction<void()> funct
   // Pause the CPU (set it to stepping mode).
   const bool was_running = PauseAndLock(system);
 
-  // Queue the job function.
-  if (wait_for_completion)
+  if (!IsRunning(system))
   {
-    // Trigger the event after executing the function.
+    // If the core hasn't been started, there is no active CPU thread we can race against.
+    function();
+    wait_for_completion = false;
+  }
+  else if (wait_for_completion)
+  {
+    // Queue the job function followed by triggering the event.
     s_cpu_thread_job_finished.Reset();
     system.GetCPU().AddCPUThreadJob([&function] {
       function();
@@ -846,6 +861,7 @@ void RunOnCPUThread(Core::System& system, Common::MoveOnlyFunction<void()> funct
   }
   else
   {
+    // Queue the job function.
     system.GetCPU().AddCPUThreadJob(std::move(function));
   }
 
@@ -943,6 +959,8 @@ void NotifyStateChanged(Core::State state)
 
 void UpdateWantDeterminism(Core::System& system, bool initial)
 {
+  const Core::CPUThreadGuard guard(system);
+
   // For now, this value is not itself configurable.  Instead, individual
   // settings that depend on it, such as GPU determinism mode. should have
   // override options for testing,
@@ -951,7 +969,6 @@ void UpdateWantDeterminism(Core::System& system, bool initial)
   {
     NOTICE_LOG_FMT(COMMON, "Want determinism <- {}", new_want_determinism ? "true" : "false");
 
-    const Core::CPUThreadGuard guard(system);
     s_wants_determinism = new_want_determinism;
     const auto ios = system.GetIOS();
     if (ios)
@@ -1013,6 +1030,9 @@ void DoFrameStep(Core::System& system)
     OSD::AddMessage("Frame stepping is disabled in RetroAchievements hardcore mode");
     return;
   }
+
+  std::lock_guard lock(s_core_mutex);
+
   if (GetState(system) == State::Paused)
   {
     // if already paused, frame advance for 1 frame
diff --git a/Source/Core/Core/Core.h b/Source/Core/Core/Core.h
index a6e40d2257..815d049656 100644
--- a/Source/Core/Core/Core.h
+++ b/Source/Core/Core/Core.h
@@ -92,10 +92,9 @@ enum class ConsoleType : u32
   ReservedTDEVSystem = 0x20000007,
 };
 
-// This is an RAII alternative to using PauseAndLock. If constructed from the host thread, the CPU
-// thread is paused, and the current thread temporarily becomes the CPU thread. If constructed from
-// the CPU thread, nothing special happens. This should only be constructed on the CPU thread or the
-// host thread.
+// This is an RAII alternative to using PauseAndLock. If constructed from any thread other than the
+// CPU thread, the CPU thread is paused, and the current thread temporarily becomes the CPU thread.
+// If constructed from the CPU thread, nothing special happens.
 //
 // Some functions use a parameter of this type to indicate that the function should only be called
 // from the CPU thread. If the parameter is a pointer, the function has a fallback for being called
@@ -119,6 +118,8 @@ private:
   bool m_was_unpaused = false;
 };
 
+// These three are normally called from the Host thread. However, they can be called from any thread
+// that isn't launched by the emulator core.
 bool Init(Core::System& system, std::unique_ptr<BootParameters> boot, const WindowSystemInfo& wsi);
 void Stop(Core::System& system);
 void Shutdown(Core::System& system);
@@ -127,8 +128,6 @@ void DeclareAsCPUThread();
 void UndeclareAsCPUThread();
 void DeclareAsGPUThread();
 void UndeclareAsGPUThread();
-void DeclareAsHostThread();
-void UndeclareAsHostThread();
 
 std::string StopMessage(bool main_thread, std::string_view message);
 
@@ -141,11 +140,11 @@ bool IsUninitialized(Core::System& system);
 
 bool IsCPUThread();  // this tells us whether we are the CPU thread.
 bool IsGPUThread();
-bool IsHostThread();
 
 bool WantsDeterminism();
 
-// [NOT THREADSAFE] For use by Host only
+// SetState can't be called by the CPU thread, but can be called by any thread that isn't launched
+// by the emulator core.
 void SetState(Core::System& system, State state, bool report_state_change = true,
               bool override_achievement_restrictions = false);
 State GetState(Core::System& system);
@@ -160,7 +159,6 @@ void FrameUpdateOnCPUThread();
 void OnFrameEnd(Core::System& system);
 
 // Run a function on the CPU thread, asynchronously.
-// This is only valid to call from the host thread, since it uses PauseAndLock() internally.
 void RunOnCPUThread(Core::System& system, Common::MoveOnlyFunction<void()> function,
                     bool wait_for_completion);
 
@@ -170,7 +168,6 @@ using StateChangedCallbackFunc = std::function<void(Core::State)>;
 [[nodiscard]] Common::EventHook AddOnStateChangedCallback(StateChangedCallbackFunc callback);
 void NotifyStateChanged(Core::State state);
 
-// Run on the Host thread when the factors change. [NOT THREADSAFE]
 void UpdateWantDeterminism(Core::System& system, bool initial = false);
 
 // Queue an arbitrary function to asynchronously run once on the Host thread later.
diff --git a/Source/Core/DolphinNoGUI/MainNoGUI.cpp b/Source/Core/DolphinNoGUI/MainNoGUI.cpp
index 2a7d7c8113..9c8f938836 100644
--- a/Source/Core/DolphinNoGUI/MainNoGUI.cpp
+++ b/Source/Core/DolphinNoGUI/MainNoGUI.cpp
@@ -189,8 +189,6 @@ static std::unique_ptr<Platform> GetPlatform(const optparse::Values& options)
 
 int main(const int argc, char* argv[])
 {
-  Core::DeclareAsHostThread();
-
   const auto parser =
       CommandLineParse::CreateParser(CommandLineParse::ParserOptions::OmitGUIOptions);
   parser->add_option("-p", "--platform")
diff --git a/Source/Core/DolphinQt/Host.cpp b/Source/Core/DolphinQt/Host.cpp
index 57586300cf..9407e5424b 100644
--- a/Source/Core/DolphinQt/Host.cpp
+++ b/Source/Core/DolphinQt/Host.cpp
@@ -8,6 +8,7 @@
 #include <QAbstractEventDispatcher>
 #include <QApplication>
 #include <QLocale>
+#include <QThread>
 
 #include <imgui.h>
 
@@ -245,7 +246,8 @@ bool Host_TASInputHasFocus()
 
 void Host_YieldToUI()
 {
-  qApp->processEvents(QEventLoop::ExcludeUserInputEvents);
+  if (qApp->thread() == QThread::currentThread())
+    qApp->processEvents(QEventLoop::ExcludeUserInputEvents);
 }
 
 void Host_UpdateDisasmDialog()
diff --git a/Source/Core/DolphinQt/Main.cpp b/Source/Core/DolphinQt/Main.cpp
index 49436475ae..ce85ee1f0d 100644
--- a/Source/Core/DolphinQt/Main.cpp
+++ b/Source/Core/DolphinQt/Main.cpp
@@ -127,8 +127,6 @@ int main(int argc, char* argv[])
   }
 #endif
 
-  Core::DeclareAsHostThread();
-
 #ifdef __APPLE__
   // On macOS, a command line option matching the format "-psn_X_XXXXXX" is passed when
   // the application is launched for the first time. This is to set the "ProcessSerialNumber",
diff --git a/Source/Core/DolphinQt/Settings.cpp b/Source/Core/DolphinQt/Settings.cpp
index 76363d8e10..f74c6589cc 100644
--- a/Source/Core/DolphinQt/Settings.cpp
+++ b/Source/Core/DolphinQt/Settings.cpp
@@ -17,6 +17,7 @@
 #include <QSize>
 #include <QStyle>
 #include <QStyleHints>
+#include <QThread>
 #include <QWidget>
 
 #include "AudioCommon/AudioCommon.h"
@@ -76,7 +77,7 @@ Settings::Settings()
   });
 
   m_hotplug_event_hook = g_controller_interface.RegisterDevicesChangedCallback([this] {
-    if (Core::IsHostThread())
+    if (qApp->thread() == QThread::currentThread())
     {
       emit DevicesChanged();
     }
diff --git a/Source/Core/DolphinTool/ToolMain.cpp b/Source/Core/DolphinTool/ToolMain.cpp
index 224ead3d7b..058bccc7b5 100644
--- a/Source/Core/DolphinTool/ToolMain.cpp
+++ b/Source/Core/DolphinTool/ToolMain.cpp
@@ -32,8 +32,6 @@ static void PrintUsage()
 
 int main(int argc, char* argv[])
 {
-  Core::DeclareAsHostThread();
-
   if (argc < 2)
   {
     PrintUsage();
diff --git a/Source/UnitTests/UnitTestsMain.cpp b/Source/UnitTests/UnitTestsMain.cpp
index 4eed0c6099..3edb1c1909 100644
--- a/Source/UnitTests/UnitTestsMain.cpp
+++ b/Source/UnitTests/UnitTestsMain.cpp
@@ -25,7 +25,6 @@ int main(int argc, char** argv)
 {
   fmt::print(stderr, "Running main() from UnitTestsMain.cpp\n");
   Common::RegisterMsgAlertHandler(TestMsgHandler);
-  Core::DeclareAsHostThread();
 
   ::testing::InitGoogleTest(&argc, argv);
   return RUN_ALL_TESTS();