From b64d7b217f43933e06acf2e2f4a1d47c50978bfe Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Henrik=20Rydg=C3=A5rd?= <hrydgard@gmail.com>
Date: Tue, 6 Jan 2026 20:38:06 +0100
Subject: [PATCH] Add a --root= command line argument to override what host0:
 is mounted to.

Fixes #20191
---
 Core/Config.h        | 2 ++
 Core/HLE/sceMpeg.cpp | 2 +-
 UI/EmuScreen.cpp     | 2 +-
 UI/NativeApp.cpp     | 5 +++++
 4 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/Core/Config.h b/Core/Config.h
index 33fe8d45d6..b4c7d405f5 100644
--- a/Core/Config.h
+++ b/Core/Config.h
@@ -662,6 +662,8 @@ public:
 	Path internalDataDirectory;
 	Path appCacheDirectory;
 
+	Path mountRoot;  // Actually, mount as host0. keeping consistent with headless args.
+
 	void Load(const char *iniFileName = nullptr, const char *controllerIniFilename = nullptr);
 	bool Save(const char *saveReason);
 	void Reload();
diff --git a/Core/HLE/sceMpeg.cpp b/Core/HLE/sceMpeg.cpp
index 5d01e0f655..30a42df40f 100644
--- a/Core/HLE/sceMpeg.cpp
+++ b/Core/HLE/sceMpeg.cpp
@@ -451,7 +451,7 @@ static u32 sceMpegRingbufferConstruct(u32 ringbufferAddr, u32 numPackets, u32 da
 	if (ring->semaID != 0) {
 		// I'm starting to think this is just padding or something.
 		// It's not written by this function.
-		WARN_LOG(Log::Mpeg, "Detected semaID %d", ring->semaID);
+		WARN_LOG(Log::Mpeg, "Detected 'semaID' (might not be) %d (%08x)", ring->semaID, ring->semaID);
 	}
 	ring->mpeg = 0;
 	// This isn't in ver 0104, but it is in 0105.
diff --git a/UI/EmuScreen.cpp b/UI/EmuScreen.cpp
index ba74075b78..9b20938092 100644
--- a/UI/EmuScreen.cpp
+++ b/UI/EmuScreen.cpp
@@ -306,7 +306,7 @@ void EmuScreen::ProcessGameBoot(const Path &filename) {
 	coreParam.enableSound = g_Config.bEnableSound;
 	coreParam.fileToStart = filename;
 	coreParam.mountIso.clear();
-	coreParam.mountRoot.clear();
+	coreParam.mountRoot = g_Config.mountRoot;
 	coreParam.startBreak = !g_Config.bAutoRun;
 	coreParam.headLess = false;
 
diff --git a/UI/NativeApp.cpp b/UI/NativeApp.cpp
index 7afce4ebe7..ff8064c997 100644
--- a/UI/NativeApp.cpp
+++ b/UI/NativeApp.cpp
@@ -530,6 +530,8 @@ void NativeInit(int argc, const char *argv[], const char *savegame_dir, const ch
 		forceLogLevel = true;
 	};
 
+	// TODO: Need a much better command line argument parser.
+
 	for (int i = 1; i < argc; i++) {
 		if (argv[i][0] == '-') {
 #if defined(__APPLE__)
@@ -581,6 +583,9 @@ void NativeInit(int argc, const char *argv[], const char *savegame_dir, const ch
 					g_Config.iForceFullScreen = 1;
 					System_ToggleFullscreenState("1");
 				}
+				if (!strncmp(argv[i], "--root=", strlen("--root=")) && strlen(argv[i]) > strlen("--root=")) {
+					g_Config.mountRoot = Path(argv[i] + strlen("--root="));
+				}
 				if (!strcmp(argv[i], "--windowed")) {
 					g_Config.iForceFullScreen = 0;
 					System_ToggleFullscreenState("0");