runtime: parse auxv for page size on dragonfly

Decode AT_PAGESZ to determine physPageSize on dragonfly.

Change-Id: I7236d7cbe43433f16dffddad19c1655bc0c7f31d
Reviewed-on: https://go-review.googlesource.com/103257
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/runtime/auxv_none.go

@@ -4,6 +4,7 @@
 
 // +build !linux
 // +build !darwin
+// +build !dragonfly
 // +build !freebsd
 // +build !netbsd
 

src/runtime/os_dragonfly.go

@@ -4,7 +4,10 @@
 
 package runtime
 
-import "unsafe"
+import (
+	"runtime/internal/sys"
+	"unsafe"
+)
 
 const (
 	_NSIG        = 33
@@ -149,7 +152,9 @@ func newosproc(mp *m, stk unsafe.Pointer) {
 
 func osinit() {
 	ncpu = getncpu()
-	physPageSize = getPageSize()
+	if physPageSize == 0 {
+		physPageSize = getPageSize()
+	}
 }
 
 var urandom_dev = []byte("/dev/urandom\x00")
@@ -242,3 +247,33 @@ func sigdelset(mask *sigset, i int) {
 
 func (c *sigctxt) fixsigcode(sig uint32) {
 }
+
+func sysargs(argc int32, argv **byte) {
+	n := argc + 1
+
+	// skip over argv, envp to get to auxv
+	for argv_index(argv, n) != nil {
+		n++
+	}
+
+	// skip NULL separator
+	n++
+
+	auxv := (*[1 << 28]uintptr)(add(unsafe.Pointer(argv), uintptr(n)*sys.PtrSize))
+	sysauxv(auxv[:])
+}
+
+const (
+	_AT_NULL   = 0
+	_AT_PAGESZ = 6
+)
+
+func sysauxv(auxv []uintptr) {
+	for i := 0; auxv[i] != _AT_NULL; i += 2 {
+		tag, val := auxv[i], auxv[i+1]
+		switch tag {
+		case _AT_PAGESZ:
+			physPageSize = val
+		}
+	}
+}