diff --git a/llvm/lib/Target/AArch64/AArch64Arm64ECCallLowering.cpp b/llvm/lib/Target/AArch64/AArch64Arm64ECCallLowering.cpp
index 91b4f18c73c93..03d641d04413e 100644
--- a/llvm/lib/Target/AArch64/AArch64Arm64ECCallLowering.cpp
+++ b/llvm/lib/Target/AArch64/AArch64Arm64ECCallLowering.cpp
@@ -117,8 +117,8 @@ void AArch64Arm64ECCallLowering::getThunkType(FunctionType *FT,
   getThunkArgTypes(FT, AttrList, TT, Out, Arm64ArgTypes, X64ArgTypes,
                    HasSretPtr);
 
-  Arm64Ty = FunctionType::get(Arm64RetTy, Arm64ArgTypes,
-                              TT == ThunkType::Entry && FT->isVarArg());
+  Arm64Ty = FunctionType::get(Arm64RetTy, Arm64ArgTypes, false);
+
   X64Ty = FunctionType::get(X64RetTy, X64ArgTypes, false);
 }
 
@@ -156,13 +156,13 @@ void AArch64Arm64ECCallLowering::getThunkArgTypes(
       X64ArgTypes.push_back(I64Ty);
     }
 
+    // x4
+    Arm64ArgTypes.push_back(PtrTy);
+    X64ArgTypes.push_back(PtrTy);
+    // x5
+    Arm64ArgTypes.push_back(I64Ty);
     if (TT != ThunkType::Entry) {
-      // x4
-      Arm64ArgTypes.push_back(PtrTy);
-      X64ArgTypes.push_back(PtrTy);
-      // x5
-      Arm64ArgTypes.push_back(I64Ty);
-      // FIXME: x5 isn't actually passed/used by the x64 side; revisit once we
+      // FIXME: x5 isn't actually used by the x64 side; revisit once we
       // have proper isel for varargs
       X64ArgTypes.push_back(I64Ty);
     }
@@ -471,10 +471,11 @@ Function *AArch64Arm64ECCallLowering::buildEntryThunk(Function *F) {
 
   bool TransformDirectToSRet = X64RetType->isVoidTy() && !RetTy->isVoidTy();
   unsigned ThunkArgOffset = TransformDirectToSRet ? 2 : 1;
+  unsigned PassthroughArgSize = F->isVarArg() ? 5 : Thunk->arg_size();
 
   // Translate arguments to call.
   SmallVector<Value *> Args;
-  for (unsigned i = ThunkArgOffset, e = Thunk->arg_size(); i != e; ++i) {
+  for (unsigned i = ThunkArgOffset, e = PassthroughArgSize; i != e; ++i) {
     Value *Arg = Thunk->getArg(i);
     Type *ArgTy = Arm64Ty->getParamType(i - ThunkArgOffset);
     if (ArgTy->isArrayTy() || ArgTy->isStructTy() ||
@@ -491,6 +492,22 @@ Function *AArch64Arm64ECCallLowering::buildEntryThunk(Function *F) {
     Args.push_back(Arg);
   }
 
+  if (F->isVarArg()) {
+    // The 5th argument to variadic entry thunks is used to model the x64 sp
+    // which is passed to the thunk in x4, this can be passed to the callee as
+    // the variadic argument start address after skipping over the 32 byte
+    // shadow store.
+
+    // The EC thunk CC will assign any argument marked as InReg to x4.
+    Thunk->addParamAttr(5, Attribute::InReg);
+    Value *Arg = Thunk->getArg(5);
+    Arg = IRB.CreatePtrAdd(Arg, IRB.getInt64(0x20));
+    Args.push_back(Arg);
+
+    // Pass in a zero variadic argument size (in x5).
+    Args.push_back(IRB.getInt64(0));
+  }
+
   // Call the function passed to the thunk.
   Value *Callee = Thunk->getArg(0);
   Callee = IRB.CreateBitCast(Callee, PtrTy);
diff --git a/llvm/lib/Target/AArch64/AArch64CallingConvention.td b/llvm/lib/Target/AArch64/AArch64CallingConvention.td
index 78ea4a5180f70..8e67f0f5c8815 100644
--- a/llvm/lib/Target/AArch64/AArch64CallingConvention.td
+++ b/llvm/lib/Target/AArch64/AArch64CallingConvention.td
@@ -213,6 +213,9 @@ def CC_AArch64_Arm64EC_VarArg : CallingConv<[
 // address is passed in X9.
 let Entry = 1 in
 def CC_AArch64_Arm64EC_Thunk : CallingConv<[
+  // ARM64EC-specific: the InReg attribute can be used to access the x64 sp passed into entry thunks in x4 from the IR.
+  CCIfInReg<CCIfType<[i64], CCAssignToReg<[X4]>>>,
+
   // Byval aggregates are passed by pointer
   CCIfByVal<CCPassIndirect<i64>>,
 
diff --git a/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll b/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll
index 0083818def151..bb9ba05f7a272 100644
--- a/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll
+++ b/llvm/test/CodeGen/AArch64/arm64ec-entry-thunks.ll
@@ -147,7 +147,7 @@ define void @has_varargs(...) nounwind {
 ; CHECK-NEXT:     add     x29, sp, #160
 ; CHECK-NEXT:     .seh_add_fp     160
 ; CHECK-NEXT:     .seh_endprologue
-; CHECK-NEXT:     mov     x4, sp
+; CHECK-NEXT:     add     x4, x4, #32
 ; CHECK-NEXT:     mov     x5, xzr
 ; CHECK-NEXT:     blr     x9
 ; CHECK-NEXT:     adrp    x8, __os_arm64x_dispatch_ret