From 0f783bde485796c8e97789d285c825024d1ca8e4 Mon Sep 17 00:00:00 2001
From: Shenghou Ma <minux@golang.org>
Date: Tue, 23 Feb 2016 01:26:50 -0500
Subject: [PATCH] runtime, syscall: switch linux/386 to use int 0x80

Like bionic, musl also doesn't provide vsyscall helper in %gs:0x10,
and as int $0x80 is as fast as calling %gs:0x10, just use int $0x80
always.

Fixes #14476.

Change-Id: I00ec8652060700e0a3c9b524bfe3c16a810263f6
---

diff --git a/src/runtime/sys_linux_386.s b/src/runtime/sys_linux_386.s
index 1a3aaf0..29ebd5e 100644
--- a/src/runtime/sys_linux_386.s
+++ b/src/runtime/sys_linux_386.s
@@ -12,16 +12,17 @@
 
 // Most linux systems use glibc's dynamic linker, which puts the
 // __kernel_vsyscall vdso helper at 0x10(GS) for easy access from position
-// independent code and setldt in this file does the same in the statically
-// linked case. Android, however, uses bionic's dynamic linker, which does not
-// save the helper anywhere, and so the only way to invoke a syscall from
-// position independent code is boring old int $0x80 (which is also what
-// bionic's syscall wrappers use).
-#ifdef GOOS_android
+// independent code and setldt in runtime does the same in the statically
+// linked case. However, systems that use alternative libc such as Android's
+// bionic and musl, do not save the helper anywhere, and so the only way to
+// invoke a syscall from position independent code is boring old int $0x80
+// (which is also what syscall wrappers in bionic/musl use).
+//
+// The benchmarks also showed that using int $0x80 is as fast as calling
+// *%gs:0x10 even on CPUs more than 6 years old. See https://golang.org/cl/16996
+// for the benchmark program and raw data.
+//#define INVOKE_SYSCALL	CALL	0x10(GS) // non-portable
 #define INVOKE_SYSCALL	INT	$0x80
-#else
-#define INVOKE_SYSCALL	CALL	0x10(GS)
-#endif
 
 TEXT runtime·exit(SB),NOSPLIT,$0
 	MOVL	$252, AX	// syscall number
diff --git a/src/syscall/asm_linux_386.s b/src/syscall/asm_linux_386.s
index c940605..d5b1022 100644
--- a/src/syscall/asm_linux_386.s
+++ b/src/syscall/asm_linux_386.s
@@ -12,18 +12,9 @@
 // func Syscall(trap uintptr, a1, a2, a3 uintptr) (r1, r2, err uintptr);
 // Trap # in AX, args in BX CX DX SI DI, return in AX
 
-// Most linux systems use glibc's dynamic linker, which puts the
-// __kernel_vsyscall vdso helper at 0x10(GS) for easy access from position
-// independent code and setldt in runtime does the same in the statically
-// linked case. Android, however, uses bionic's dynamic linker, which does not
-// save the helper anywhere, and so the only way to invoke a syscall from
-// position independent code is boring old int $0x80 (which is also what
-// bionic's syscall wrappers use).
-#ifdef GOOS_android
+// See ../runtime/sys_linux_386.s for rationales.
+//#define INVOKE_SYSCALL	CALL	0x10(GS) // non-portable
 #define INVOKE_SYSCALL	INT	$0x80
-#else
-#define INVOKE_SYSCALL	CALL	0x10(GS)
-#endif
 
 TEXT	·Syscall(SB),NOSPLIT,$0-28
 	CALL	runtime·entersyscall(SB)