FEXCore: Warn if MDWE is set

This will result in FEX not being able to allocate executable memory.
We can use shared memory in the future to work around this but for now
we don't support that as a fix.

FEXCore/Source/Interface/Core/CPUBackend.cpp

@@ -5,6 +5,10 @@
 #include "Interface/Core/Dispatcher/Dispatcher.h"
 #include <FEXCore/Core/CPUBackend.h>
 
+#ifndef _WIN32
+#include <sys/prctl.h>
+#endif
+
 namespace FEXCore {
 namespace CPU {
 
@@ -349,6 +353,31 @@ auto CPUBackend::GetEmptyCodeBuffer() -> CodeBuffer * {
 }
 
 auto CPUBackend::AllocateNewCodeBuffer(size_t Size) -> CodeBuffer {
+#ifndef _WIN32
+// MDWE (Memory-Deny-Write-Execute) is a new Linux 6.3 feature.
+// It's equivalent to systemd's `MemoryDenyWriteExecute` but implemented entirely in the kernel.
+//
+// MDWE prevents applications from creating RWX memory mappings.
+// This prevents FEX from doing anything JIT related, as FEX uses RWX for JIT memory mappings.
+//
+// A potential workaround to make FEX work with MDWE is to call mprotect every time we need to write or modify code.
+// Alternatively, FEX could use a memory mirror where one half is mapped as RW and the other is RX.
+//
+// Once MDWE is enabled with the prctl, the feature is sealed and it can /NOT/ be turned off.
+//
+// Status of MDWE is queried through prctl using `PR_GET_MDWE`:
+// -1: The kernel doesn't support MDWE
+// 0: MDWE is supported but disabled
+// >0: MDWE is enabled, hence prohibiting RWX mappings
+#ifndef PR_GET_MDWE
+#define PR_GET_MDWE 66
+#endif
+  int MDWE = ::prctl(PR_GET_MDWE, 0, 0, 0, 0);
+  if (MDWE != -1 && MDWE != 0) {
+    LogMan::Msg::EFmt("MDWE was set to 0x{:x} which means FEX can't allocate executable memory", MDWE);
+  }
+#endif
+
   CodeBuffer Buffer;
   Buffer.Size = Size;
   Buffer.Ptr = static_cast<uint8_t *>(