runtime: improve mstart comments

Some of the comments were unclear or outdated.

Change-Id: I02e01bf60def0074c1fa760e94aa992e9e4969b9
Reviewed-on: https://go-review.googlesource.com/c/go/+/172987
Run-TryBot: Austin Clements <austin@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Michael Knyszek <mknyszek@google.com>

diff --git a/src/runtime/proc.go b/src/runtime/proc.go
index 30ddeadff5db142ec0c93b1222c700622551bb10..57ad17d59417ce08862eb28b8c5a16861898638f 100644 (file)
--- a/src/runtime/proc.go
+++ b/src/runtime/proc.go
@@ -1125,7 +1125,7 @@ func startTheWorldWithSema(emitTraceEvent bool) int64 {
        return startTime
 }
 
-// Called to start an M.
+// mstart is the entry-point for new Ms.
 //
 // This must not split the stack because we may not even have stack
 // bounds set up yet.
@@ -1150,9 +1150,11 @@ func mstart() {
                _g_.stack.hi = uintptr(noescape(unsafe.Pointer(&size)))
                _g_.stack.lo = _g_.stack.hi - size + 1024
        }
-       // Initialize stack guards so that we can start calling
-       // both Go and C functions with stack growth prologues.
+       // Initialize stack guard so that we can start calling regular
+       // Go code.
        _g_.stackguard0 = _g_.stack.lo + _StackGuard
+       // This is the g0, so we can also call go:systemstack
+       // functions, which check stackguard1.
        _g_.stackguard1 = _g_.stackguard0
        mstart1()