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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.
34 lines
1.1 KiB
C
34 lines
1.1 KiB
C
/* VDSO function pointer declarations. 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
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License as published by the Free Software Foundation; either
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version 2.1 of the 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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#ifndef _LIBC_VDSO_H
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#define _LIBC_VDSO_H
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#ifdef SHARED
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# include <sysdep-vdso.h>
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extern int (*VDSO_SYMBOL(gettimeofday)) (struct timeval *, void *)
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attribute_hidden;
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extern int (*VDSO_SYMBOL(clock_gettime)) (clockid_t, struct timespec *);
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#endif
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#endif /* _LIBC_VDSO_H */
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