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[PATCH v2] ARM: VDSO support


Beginning with 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.


2015-05-29  Nathan Lynch  <nathan_lynch@codesourcery.com>

	* 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 and __vdso_gettimeofday.
---

This version has been reworked on top of Adhemerval's recent VDSO
refactoring.

 sysdeps/unix/sysv/linux/arm/Makefile     |  1 +
 sysdeps/unix/sysv/linux/arm/Versions     |  2 ++
 sysdeps/unix/sysv/linux/arm/init-first.c | 43 ++++++++++++++++++++++++++++++++
 sysdeps/unix/sysv/linux/arm/libc-vdso.h  | 32 ++++++++++++++++++++++++
 sysdeps/unix/sysv/linux/arm/sysdep.h     |  4 +++
 5 files changed, 82 insertions(+)
 create mode 100644 sysdeps/unix/sysv/linux/arm/init-first.c
 create mode 100644 sysdeps/unix/sysv/linux/arm/libc-vdso.h

diff --git a/sysdeps/unix/sysv/linux/arm/Makefile b/sysdeps/unix/sysv/linux/arm/Makefile
index 50410b67b5fc..270cb73f70e5 100644
--- a/sysdeps/unix/sysv/linux/arm/Makefile
+++ b/sysdeps/unix/sysv/linux/arm/Makefile
@@ -1,4 +1,5 @@
 ifeq ($(subdir),elf)
+sysdep_routines += dl-vdso
 sysdep-rtld-routines += aeabi_read_tp libc-do-syscall
 endif
 
diff --git a/sysdeps/unix/sysv/linux/arm/Versions b/sysdeps/unix/sysv/linux/arm/Versions
index a251b0fd7d87..7bc5936372bb 100644
--- a/sysdeps/unix/sysv/linux/arm/Versions
+++ b/sysdeps/unix/sysv/linux/arm/Versions
@@ -10,5 +10,7 @@ libc {
   GLIBC_PRIVATE {
     # A copy of sigaction lives in libpthread, and needs these.
     __default_sa_restorer; __default_rt_sa_restorer;
+    __vdso_clock_gettime;
+    __vdso_gettimeofday;
   }
 }
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 000000000000..f4868b834cf1
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/arm/init-first.c
@@ -0,0 +1,43 @@
+/* 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>
diff --git a/sysdeps/unix/sysv/linux/arm/libc-vdso.h b/sysdeps/unix/sysv/linux/arm/libc-vdso.h
new file mode 100644
index 000000000000..2ebcc62cd284
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/arm/libc-vdso.h
@@ -0,0 +1,32 @@
+/* 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/>.  */
+
+#ifndef _LIBC_VDSO_H
+#define _LIBC_VDSO_H
+
+#ifdef SHARED
+
+# include <sysdep-vdso.h>
+
+extern int (*VDSO_SYMBOL(gettimeofday)) (struct timeval *, void *)
+   attribute_hidden;
+extern int (*VDSO_SYMBOL(clock_gettime)) (clockid_t, struct timespec *);
+
+#endif
+
+#endif /* _LIBC_VDSO_H */
diff --git a/sysdeps/unix/sysv/linux/arm/sysdep.h b/sysdeps/unix/sysv/linux/arm/sysdep.h
index 37eac192b1e2..200f77a62ef1 100644
--- a/sysdeps/unix/sysv/linux/arm/sysdep.h
+++ b/sysdeps/unix/sysv/linux/arm/sysdep.h
@@ -387,6 +387,10 @@ __local_syscall_error:						\
 #undef INTERNAL_SYSCALL_ERRNO
 #define INTERNAL_SYSCALL_ERRNO(val, err)	(-(val))
 
+/* List of system calls which are supported as vsyscalls.  */
+#define HAVE_CLOCK_GETTIME_VSYSCALL	1
+#define HAVE_GETTIMEOFDAY_VSYSCALL	1
+
 #define LOAD_ARGS_0()
 #define ASM_ARGS_0
 #define LOAD_ARGS_1(a1)				\
-- 
2.1.0


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