runtime: add s390x support (new files and lfstack_64bit.go modifications)

Change-Id: I51c0a332e3cbdab348564e5dcd27583e75e4b881
Reviewed-on: https://go-review.googlesource.com/20946
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 19 insertions, 13 deletions

diff --git a/src/runtime/lfstack_64bit.go b/src/runtime/lfstack_64bit.go
index 07c2a141f0..8180b0a248 100644
--- a/src/runtime/lfstack_64bit.go
+++ b/src/runtime/lfstack_64bit.go
@@ -2,26 +2,32 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// +build arm64 mips64 mips64le ppc64 ppc64le
+// +build arm64 mips64 mips64le ppc64 ppc64le s390x
 
 package runtime
 
 import "unsafe"
 
-// On ppc64, Linux limits the user address space to 46 bits (see
-// TASK_SIZE_USER64 in the Linux kernel).  This has grown over time,
-// so here we allow 48 bit addresses.
-//
-// On mips64, Linux limits the user address space to 40 bits (see
-// TASK_SIZE64 in the Linux kernel).  This has grown over time,
-// so here we allow 48 bit addresses.
-//
-// In addition to the 16 bits taken from the top, we can take 3 from the
-// bottom, because node must be pointer-aligned, giving a total of 19 bits
-// of count.
 const (
+	// addrBits is the number of bits needed to represent a virtual address.
+	//
+	// In Linux the user address space for each architecture is limited as
+	// follows (taken from the processor.h file for the architecture):
+	//
+	// Architecture  Name              Maximum Value (exclusive)
+	// ---------------------------------------------------------------------
+	// arm64         TASK_SIZE_64      Depends on configuration.
+	// ppc64{,le}    TASK_SIZE_USER64  0x400000000000UL (46 bit addresses)
+	// mips64{,le}   TASK_SIZE64       0x010000000000UL (40 bit addresses)
+	// s390x         TASK_SIZE         0x020000000000UL (41 bit addresses)
+	//
+	// These values may increase over time.
 	addrBits = 48
-	cntBits  = 64 - addrBits + 3
+
+	// In addition to the 16 bits taken from the top, we can take 3 from the
+	// bottom, because node must be pointer-aligned, giving a total of 19 bits
+	// of count.
+	cntBits = 64 - addrBits + 3
 )
 
 func lfstackPack(node *lfnode, cnt uintptr) uint64 {