Core/HW: Add Broadband Adapter (IPC).

This is a hassle-free BBA option intended for local play with multiple
Dolphin instances running *in the same system*. After selecting
**Broadband Adapter (IPC)** in the **SP1** slot in the GameCube section
in the settings, games that support LAN play will be able to discover
each other, without requiring third-party software or relatively complex
TAP setups.

The implementation is based on cpp-ipc, a high-performance inter-process
communication library that uses shared memory as transport layer.

Supported platforms are:

- [x] Linux
- [x] Windows
- [ ] macOS (cpp-ipc does not support this platform)
- [ ] FreeBSD (cpp-ipc does not support this platform)
- [ ] Android (cpp-ipc needs some adjustments; while it could work,
  launching two Dolphin instances within the same Android system may be
  both challenging and impractical)

9 files changed, 164 insertions, 1 deletions

diff --git a/Source/Core/Core/CMakeLists.txt b/Source/Core/Core/CMakeLists.txt
index 6e7ff1a0bc..cb2a3ac58a 100644
--- a/Source/Core/Core/CMakeLists.txt
+++ b/Source/Core/Core/CMakeLists.txt
@@ -797,6 +797,11 @@ if(UNIX)
   )
 endif()
 
+if(WIN32 OR LINUX)
+  target_sources(core PRIVATE HW/EXI/BBA/IPC.cpp)
+  target_link_libraries(core PRIVATE cpp-ipc::ipc)
+endif()
+
 if(MSVC)
   # Add precompiled header
   target_link_libraries(core PRIVATE use_pch)
diff --git a/Source/Core/Core/HW/EXI/BBA/IPC.cpp b/Source/Core/Core/HW/EXI/BBA/IPC.cpp
new file mode 100644
index 0000000000..e23795e7eb
--- /dev/null
+++ b/Source/Core/Core/HW/EXI/BBA/IPC.cpp
@@ -0,0 +1,101 @@
+// Copyright 2025 Dolphin Emulator Project
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+#include <cstring>
+
+#include <libipc/ipc.h>
+
+#include "Common/Logging/Log.h"
+#include "Core/HW/EXI/EXI_DeviceEthernet.h"
+
+namespace ExpansionInterface
+{
+
+bool CEXIETHERNET::IPCBBAInterface::Activate()
+{
+  if (m_active)
+    return false;
+
+  m_channel.connect("dolphin-emu-bba-ipc", ipc::sender | ipc::receiver);
+  if (!m_channel.valid())
+    return false;
+
+  m_read_enabled.Clear();
+  m_read_thread_shutdown.Clear();
+
+  m_active = true;
+
+  return RecvInit();
+}
+
+void CEXIETHERNET::IPCBBAInterface::Deactivate()
+{
+  m_read_enabled.Clear();
+  m_read_thread_shutdown.Set();
+
+  if (m_read_thread.joinable())
+  {
+    m_read_thread.join();
+  }
+
+  m_channel.disconnect();
+
+  m_active = false;
+}
+
+bool CEXIETHERNET::IPCBBAInterface::IsActivated()
+{
+  return m_active;
+}
+
+bool CEXIETHERNET::IPCBBAInterface::SendFrame(const u8* const frame, const u32 size)
+{
+  if (!m_active)
+    return false;
+
+  static constexpr u64 TIMEOUT_IN_MS{3000};
+  if (!m_channel.send(frame, size, TIMEOUT_IN_MS))
+  {
+    ERROR_LOG_FMT(SP1, "Failed to send frame");
+    return false;
+  }
+
+  m_eth_ref->SendComplete();
+
+  return true;
+}
+
+bool CEXIETHERNET::IPCBBAInterface::RecvInit()
+{
+  m_read_thread = std::thread(&CEXIETHERNET::IPCBBAInterface::ReadThreadHandler, this);
+  return true;
+}
+
+void CEXIETHERNET::IPCBBAInterface::RecvStart()
+{
+  m_read_enabled.Set();
+}
+
+void CEXIETHERNET::IPCBBAInterface::RecvStop()
+{
+  m_read_enabled.Clear();
+}
+
+void CEXIETHERNET::IPCBBAInterface::ReadThreadHandler()
+{
+  while (!m_read_thread_shutdown.IsSet())
+  {
+    const ipc::buff_t buffer{m_channel.recv(50)};
+    if (buffer.empty() || !m_read_enabled.IsSet())
+      continue;
+
+    const u8* const frame{reinterpret_cast<const u8*>(buffer.data())};
+    const u64 size{buffer.size()};
+
+    std::memcpy(m_eth_ref->mRecvBuffer.get(), frame, size);
+    m_eth_ref->mRecvBufferLength = static_cast<u32>(size);
+    m_eth_ref->RecvHandlePacket();
+  }
+}
+
+}  // namespace ExpansionInterface
diff --git a/Source/Core/Core/HW/EXI/EXI_Device.cpp b/Source/Core/Core/HW/EXI/EXI_Device.cpp
index ac92c7b723..c1300a363f 100644
--- a/Source/Core/Core/HW/EXI/EXI_Device.cpp
+++ b/Source/Core/Core/HW/EXI/EXI_Device.cpp
@@ -160,6 +160,10 @@ std::unique_ptr<IEXIDevice> EXIDevice_Create(Core::System& system, const EXIDevi
     result = std::make_unique<CEXIETHERNET>(system, BBADeviceType::BuiltIn);
     break;
 
+  case EXIDeviceType::EthernetIPC:
+    result = std::make_unique<CEXIETHERNET>(system, BBADeviceType::IPC);
+    break;
+
   case EXIDeviceType::ModemTapServer:
     result = std::make_unique<CEXIModem>(system, ModemDeviceType::TAPSERVER);
     break;
diff --git a/Source/Core/Core/HW/EXI/EXI_Device.h b/Source/Core/Core/HW/EXI/EXI_Device.h
index 25263af425..129f7a2308 100644
--- a/Source/Core/Core/HW/EXI/EXI_Device.h
+++ b/Source/Core/Core/HW/EXI/EXI_Device.h
@@ -40,6 +40,7 @@ enum class EXIDeviceType : int
   EthernetTapServer,
   EthernetBuiltIn,
   ModemTapServer,
+  EthernetIPC,
   None = 0xFF
 };
 
@@ -86,7 +87,7 @@ std::unique_ptr<IEXIDevice> EXIDevice_Create(Core::System& system, EXIDeviceType
 
 template <>
 struct fmt::formatter<ExpansionInterface::EXIDeviceType>
-    : EnumFormatter<ExpansionInterface::EXIDeviceType::ModemTapServer>
+    : EnumFormatter<ExpansionInterface::EXIDeviceType::EthernetIPC>
 {
   static constexpr array_type names = {
       _trans("Dummy"),
@@ -104,6 +105,7 @@ struct fmt::formatter<ExpansionInterface::EXIDeviceType>
       _trans("Broadband Adapter (tapserver)"),
       _trans("Broadband Adapter (HLE)"),
       _trans("Modem Adapter (tapserver)"),
+      _trans("Broadband Adapter (IPC)"),
   };
 
   constexpr formatter() : EnumFormatter(names) {}
diff --git a/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.cpp b/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.cpp
index 202dc557f8..311f1e2226 100644
--- a/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.cpp
+++ b/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.cpp
@@ -60,6 +60,11 @@ CEXIETHERNET::CEXIETHERNET(Core::System& system, BBADeviceType type) : IEXIDevic
         this, Config::Get(Config::MAIN_BBA_BUILTIN_DNS), Config::Get(Config::MAIN_BBA_BUILTIN_IP));
     INFO_LOG_FMT(SP1, "Created Built in network interface.");
     break;
+  case BBADeviceType::IPC:
+    mac_addr = Common::GenerateMacAddress(Common::MACConsumer::BBA);  // Always randomize
+    m_network_interface = std::make_unique<IPCBBAInterface>(this);
+    INFO_LOG_FMT(SP1, "Created IPC-based network interface.");
+    break;
   case BBADeviceType::XLINK:
     // TODO start BBA with network link down, bring it up after "connected" response from XLink
 
diff --git a/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.h b/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.h
index 21852f341c..67a0317831 100644
--- a/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.h
+++ b/Source/Core/Core/HW/EXI/EXI_DeviceEthernet.h
@@ -15,6 +15,9 @@
 #endif
 
 #include <SFML/Network.hpp>
+#if defined(WIN32) || (defined(__linux__) && !defined(__ANDROID__))
+#include <libipc/ipc.h>
+#endif
 
 #include "Common/Flag.h"
 #include "Common/Network.h"
@@ -210,6 +213,7 @@ enum class BBADeviceType
   XLINK,
   TAPSERVER,
   BuiltIn,
+  IPC,
 };
 
 class CEXIETHERNET : public IEXIDevice
@@ -474,6 +478,43 @@ private:
     const Common::MACAddress& ResolveAddress(u32 inet_ip);
   };
 
+  class IPCBBAInterface : public NetworkInterface
+  {
+  public:
+    explicit IPCBBAInterface(CEXIETHERNET* const eth_ref) : NetworkInterface(eth_ref) {}
+
+#if defined(WIN32) || (defined(__linux__) && !defined(__ANDROID__))
+
+    bool Activate() override;
+    void Deactivate() override;
+    bool IsActivated() override;
+    bool SendFrame(const u8* frame, u32 size) override;
+    bool RecvInit() override;
+    void RecvStart() override;
+    void RecvStop() override;
+
+  private:
+    void ReadThreadHandler();
+
+    bool m_active{};
+    ipc::channel m_channel;
+    std::thread m_read_thread;
+    Common::Flag m_read_enabled;
+    Common::Flag m_read_thread_shutdown;
+
+#else
+
+    bool Activate() override { return false; }
+    void Deactivate() override {}
+    bool IsActivated() override { return false; }
+    bool SendFrame(const u8* const frame, const u32 size) override { return false; }
+    bool RecvInit() override { return false; }
+    void RecvStart() override {}
+    void RecvStop() override {}
+
+#endif
+  };
+
   std::unique_ptr<NetworkInterface> m_network_interface;
 
   std::unique_ptr<u8[]> mRecvBuffer;
diff --git a/Source/Core/DolphinLib.props b/Source/Core/DolphinLib.props
index 062d7f979a..35d170a0d5 100644
--- a/Source/Core/DolphinLib.props
+++ b/Source/Core/DolphinLib.props
@@ -962,6 +962,7 @@
     <ClCompile Include="Core\HW\DVD\DVDThread.cpp" />
     <ClCompile Include="Core\HW\DVD\FileMonitor.cpp" />
     <ClCompile Include="Core\HW\EXI\BBA\BuiltIn.cpp" />
+    <ClCompile Include="Core\HW\EXI\BBA\IPC.cpp" />
     <ClCompile Include="Core\HW\EXI\BBA\TAP_Win32.cpp" />
     <ClCompile Include="Core\HW\EXI\BBA\TAPServerConnection.cpp" />
     <ClCompile Include="Core\HW\EXI\BBA\TAPServerBBA.cpp" />
diff --git a/Source/Core/DolphinLib.vcxproj b/Source/Core/DolphinLib.vcxproj
index 0d4925c47e..104b2d86ed 100644
--- a/Source/Core/DolphinLib.vcxproj
+++ b/Source/Core/DolphinLib.vcxproj
@@ -30,6 +30,7 @@
   </ItemGroup>
   <Import Project="$(ExternalsDir)Bochs_disasm\exports.props" />
   <Import Project="$(ExternalsDir)bzip2\exports.props" />
+  <Import Project="$(ExternalsDir)cpp-ipc\exports.props" />
   <Import Project="$(ExternalsDir)cpp-optparse\exports.props" />
   <Import Project="$(ExternalsDir)cubeb\exports.props" />
   <Import Project="$(ExternalsDir)curl\exports.props" />
diff --git a/Source/Core/DolphinQt/Settings/GameCubePane.cpp b/Source/Core/DolphinQt/Settings/GameCubePane.cpp
index 1cd5dc5a2f..b1fee0471b 100644
--- a/Source/Core/DolphinQt/Settings/GameCubePane.cpp
+++ b/Source/Core/DolphinQt/Settings/GameCubePane.cpp
@@ -145,6 +145,9 @@ void GameCubePane::CreateWidgets()
            EXIDeviceType::EthernetXLink,
            EXIDeviceType::EthernetTapServer,
            EXIDeviceType::EthernetBuiltIn,
+#if defined(WIN32) || (defined(__linux__) && !defined(__ANDROID__))
+           EXIDeviceType::EthernetIPC,
+#endif
            EXIDeviceType::ModemTapServer,
        })
   {