WOW64: Support unwinding past FEX from within syscall handlers

This is required by recent wine changes to use longjmp for user
callbacks. Switch to saving the context at every simulate call and
setting the unwind SP/PC to that context with a small SEH trampoline
for the syscall handler.

Source/Windows/WOW64/Module.cpp

@@ -368,13 +368,27 @@ void Init() {
 }
 } // namespace Logging
 
+// Calls a 2-argument function `Func` setting the parent unwind frame information to the given SP and PC
+__attribute__((naked)) extern "C" uint64_t SEHFrameTrampoline2Args(void* Arg0, void* Arg1, void* Func, uint64_t Sp, uint64_t Pc) {
+  asm(".seh_proc SEHFrameTrampoline2Args;"
+      "stp x3, x4, [sp, #-0x10]!;"
+      ".seh_pushframe;"
+      "stp x29, x30, [sp, #-0x10]!;"
+      ".seh_save_fplr_x 16;"
+      ".seh_endprologue;"
+      "blr x2;"
+      "ldp x29, x30, [sp], 0x20;"
+      "ret;"
+      ".seh_endproc;");
+}
+
 class WowSyscallHandler : public FEXCore::HLE::SyscallHandler, public FEXCore::Allocator::FEXAllocOperators {
 public:
   WowSyscallHandler() {
     OSABI = FEXCore::HLE::SyscallOSABI::OS_WIN32;
   }
 
-  uint64_t HandleSyscall(FEXCore::Core::CpuStateFrame* Frame, FEXCore::HLE::SyscallArguments* Args) override {
+  static uint64_t HandleSyscallImpl(FEXCore::Core::CpuStateFrame* Frame, FEXCore::HLE::SyscallArguments* Args) {
     const uint64_t ReturnRIP = *(uint32_t*)(Frame->State.gregs[FEXCore::X86State::REG_RSP]); // Return address from the stack
     uint64_t ReturnRSP = Frame->State.gregs[FEXCore::X86State::REG_RSP] + 4;                 // Stack pointer after popping return address
     uint64_t ReturnRAX = 0;
@@ -410,6 +424,16 @@ public:
     return 0;
   }
 
+  uint64_t HandleSyscall(FEXCore::Core::CpuStateFrame* Frame, FEXCore::HLE::SyscallArguments* Args) override {
+    // Stash the the context pointer on the stack, as Simulate can be called from this syscall handler which would overwrite it
+    CONTEXT* EntryContext = GetTLS().EntryContext();
+    // Call the syscall handler with unwind information pointing to Simulate as its caller
+    uint64_t Ret = SEHFrameTrampoline2Args(reinterpret_cast<void*>(Frame), reinterpret_cast<void*>(Args),
+                                           reinterpret_cast<void*>(&HandleSyscallImpl), EntryContext->Sp, EntryContext->Pc);
+    GetTLS().EntryContext() = EntryContext;
+    return Ret;
+  }
+
   FEXCore::HLE::SyscallABI GetSyscallABI(uint64_t Syscall) override {
     return {.NumArgs = 0, .HasReturn = false, .HostSyscallNumber = -1};
   }
@@ -548,12 +572,7 @@ NTSTATUS BTCpuSetContext(HANDLE Thread, HANDLE Process, void* Unknown, WOW64_CON
 void BTCpuSimulate() {
   CONTEXT entry_context;
   RtlCaptureContext(&entry_context);
-
-  // APC handling calls BTCpuSimulate from syscalls and then use NtContinue to return to the previous context,
-  // to avoid the saved context being clobbered in this case only save the entry context highest in the stack
-  if (!GetTLS().EntryContext() || GetTLS().EntryContext()->Sp <= entry_context.Sp) {
-    GetTLS().EntryContext() = &entry_context;
-  }
+  GetTLS().EntryContext() = &entry_context;
 
   Context::LockJITContext();
   CTX->ExecuteThread(GetTLS().ThreadState());