[COMMITTED] aarch64: Use an ifunc/VDSO to implement gettimeofday in shared glibc

[Szabolcs Nagy <szabolcs dot nagy at arm dot com>]

On 22/05/18 21:25, Steve Ellcey wrote:
> Any chance you could add the comments that you think are needed and
> check the patch in?  It seems like you and Adhemerval are both happy
> with the code, it is just the comments (and the test if you want to use
> the nanosleep test instead of adding the new ones) that need fixing.

i removed the test as it cannot really test if vdso is in use or not,
i might do something with ifunc + bindnow testing separately.
committed as:

This patch uses an ifunc to implement gettimeofday in the shared libc.
This is faster compared to the vsyscall mechanism that has to check a
global pointer, demangle it and call it indirectly when the VDSO is
present. Resolving the gettimeofday symbol directly to the VDSO code
is safe because there are no failures that the libc has to handle by
setting errno like in a generic vsyscall (the only failure when the
VDSO code falls back to a syscall is EFAULT, but passing an invalid
pointer is undefined behaviour so returning -EFAULT is fine).

If the kernel supports the VDSO interface we use it for extern calls,
otherwise the old vsyscall method is used which falls back to a syscall.
The static version of gettimeofday continues to use a syscall, libc.so
internal calls use the old vsyscall method.

2018-06-14  Steve Ellcey  <sellcey@caviumnetworks.com>
	    Szabolcs Nagy  <szabolcs.nagy@arm.com>

	* sysdeps/unix/sysv/linux/aarch64/gettimeofday.c: New file.

diff --git a/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c b/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c
new file mode 100644
index 0000000000..8ff83e4e7d
--- /dev/null
+++ b/sysdeps/unix/sysv/linux/aarch64/gettimeofday.c
@@ -0,0 +1,71 @@
+/* Copyright (C) 2018 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/>.  */
+
+/* Get the current time of day and timezone information,
+   putting it into *tv and *tz.  If tz is null, *tz is not filled.
+   Returns 0 on success, -1 on errors.  */
+
+#ifdef SHARED
+
+# define __gettimeofday __redirect___gettimeofday
+# include <sys/time.h>
+# undef __gettimeofday
+# define HAVE_VSYSCALL
+# include <dl-vdso.h>
+# include <sysdep-vdso.h>
+
+/* Used as a fallback in the ifunc resolver if VDSO is not available
+   and for libc.so internal __gettimeofday calls.  */
+
+static int
+__gettimeofday_vsyscall (struct timeval *tv, struct timezone *tz)
+{
+  return INLINE_VSYSCALL (gettimeofday, 2, tv, tz);
+}
+
+/* PREPARE_VERSION will need an __LP64__ ifdef when ILP32 support
+   goes in.  See _libc_vdso_platform_setup in
+   sysdeps/unix/sysv/linux/aarch64/init-first.c.  */
+
+# undef INIT_ARCH
+# define INIT_ARCH() \
+	   PREPARE_VERSION (linux_version, "LINUX_2.6.39", 123718537); \
+	   void *vdso_gettimeofday = \
+	     _dl_vdso_vsym ("__kernel_gettimeofday", &linux_version);
+
+libc_ifunc_hidden (__redirect___gettimeofday, __gettimeofday,
+		   vdso_gettimeofday ?: (void *) __gettimeofday_vsyscall)
+
+__hidden_ver1 (__gettimeofday_vsyscall, __GI___gettimeofday,
+	       __gettimeofday_vsyscall);
+
+#else
+
+# include <sys/time.h>
+# include <sysdep.h>
+int
+__gettimeofday (struct timeval *tv, struct timezone *tz)
+{
+  return INLINE_SYSCALL (gettimeofday, 2, tv, tz);
+}
+libc_hidden_def (__gettimeofday)
+
+#endif
+
+weak_alias (__gettimeofday, gettimeofday)
+libc_hidden_weak (gettimeofday)