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