ARM: VDSO support

Beginning with the upcoming 4.1 release, Linux on a subset of 32-bit
ARM hardware will provide fast user-space implementations of the
following system calls:

- gettimeofday
- clock_gettime

The kernel implementation depends on the ARMv7 Generic Timers
Extension to accelerate these system calls.  So CPUs such as
Cortex-A15 and -A7 benefit, while Cortex-A9, -A8, and pre-v7 CPUs do
not.  On systems where the VDSO does not provide any speedup, the
kernel prevents the relevant symbol lookups from succeeding.

On OMAP5 (Cortex-A15) gettimeofday latency decreases from ~350ns to
~120ns.  On BeagleBone Black (Cortex-A8) it goes from ~650ns to
~660ns, which to my mind is an acceptable cost.

Verified that no new test failures are introduced on kernels with and
without the VDSO.

	* sysdeps/unix/sysv/linux/arm/Makefile: (sysdep_routines):
	Include dl-vdso.
	* sysdeps/unix/sysv/linux/arm/init-first.c: New file:
	Use VDSO routines for gettimeofday, clock_gettime if
	available.
	* sysdeps/unix/sysv/linux/arm/libc-vdso.h: New file:
	Declare VDSO symbols.
	* sysdeps/unix/sysv/linux/arm/sysdep.h:
	[HAVE_GETTIMEOFDAY_VSYSCALL]: Define.
	[HAVE_CLOCK_GETTIME_VSYSCALL]: Define.
	* sysdeps/unix/sysv/linux/arm/Versions: Add
	__vdso_clock_gettime.

1 files changed, 44 insertions, 0 deletions

diff --git a/sysdeps/unix/sysv/linux/arm/init-first.c b/sysdeps/unix/sysv/linux/arm/init-first.c
new file mode 100644
index 0000000000..2fa4fb75a7
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/arm/init-first.c
@@ -0,0 +1,44 @@
+/* Initialization code run first thing by the ELF startup code.  Linux/ARM.
+   Copyright (C) 2015 Free Software Foundation, Inc.
+
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public License as
+   published by the Free Software Foundation; either version 2.1 of the
+   License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <http://www.gnu.org/licenses/>.  */
+
+#ifdef SHARED
+# include <dl-vdso.h>
+# include <libc-vdso.h>
+
+int (*VDSO_SYMBOL(gettimeofday)) (struct timeval *, void *) attribute_hidden;
+int (*VDSO_SYMBOL(clock_gettime)) (clockid_t, struct timespec *);
+
+static inline void
+_libc_vdso_platform_setup (void)
+{
+  PREPARE_VERSION_KNOWN (linux26, LINUX_2_6);
+
+  void *p = _dl_vdso_vsym ("__vdso_gettimeofday", &linux26);
+  PTR_MANGLE (p);
+  VDSO_SYMBOL (gettimeofday) = p;
+
+  p = _dl_vdso_vsym ("__vdso_clock_gettime", &linux26);
+  PTR_MANGLE (p);
+  VDSO_SYMBOL (clock_gettime) = p;
+}
+
+# define VDSO_SETUP _libc_vdso_platform_setup
+#endif
+
+#include <csu/init-first.c>