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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.
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