Add a comment on WASM_STACK_GUARD_SIZE (#3332)

core/config.h

@@ -445,7 +445,29 @@
 #endif
 
 /* Reserved bytes to the native thread stack boundary, throw native
-   stack overflow exception if the guard boudary is reached */
+ * stack overflow exception if the guard boudary is reached
+ *
+ * WASM_STACK_GUARD_SIZE needs to be large enough for:
+ *
+ * - native functions
+ *   w/o hw bound check, the overhead (aot_call_function etc) + the native
+ *   function itself. as of writing this, the former is about 1000 bytes
+ *   on macOS amd64.
+ *   with hw bound check, theoretically, only needs to cover the logic to
+ *   set up the jmp_buf stack.
+ *
+ * - aot runtime functions
+ *   eg. aot_enlarge_memory.
+ *
+ * - w/o hw bound check, the intepreter loop
+ *
+ * - stack check wrapper functions generated by the aot compiler
+ *   (--stack-bounds-checks=1)
+ *
+ * Note: on platforms with lazy function binding, don't forget to consider
+ * the symbol resolution overhead on the first call. For example,
+ * on Ubuntu amd64 20.04, it seems to consume about 1500 bytes.
+ */
 #ifndef WASM_STACK_GUARD_SIZE
 #if WASM_ENABLE_UVWASI != 0
 /* UVWASI requires larger native stack */