diff --git a/llvm/cmake/modules/CrossCompile.cmake b/llvm/cmake/modules/CrossCompile.cmake
--- a/llvm/cmake/modules/CrossCompile.cmake
+++ b/llvm/cmake/modules/CrossCompile.cmake
@@ -12,10 +12,16 @@
     message(STATUS "Setting native build dir to " ${${project_name}_${target_name}_BUILD})
   endif(NOT DEFINED ${project_name}_${target_name}_BUILD)
 
-  if (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
+  if (DEFINED CMAKE_TOOLCHAIN_FILE)
+    # A toolchain file has already been defined, so use it.
+    # Useful for projects consuming LLVM that define a toolchain.
+    set(CROSS_TOOLCHAIN_FLAGS_INIT "-DCMAKE_TOOLCHAIN_FILE=${CMAKE_TOOLCHAIN_FILE}")
+  elseif (EXISTS ${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake)
+    # User wants a specific toolchain file.
     set(CROSS_TOOLCHAIN_FLAGS_INIT
       -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\")
   else()
+    # Else make a best guess. This should work fine for most cases.
     set(CROSS_TOOLCHAIN_FLAGS_INIT
       -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
       -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}