[PATCH v7 7/9] ARM: add vdso user-space code

[Russell King - ARM Linux]

On Sun, Jun 22, 2014 at 10:11:56PM -0500, Nathan Lynch wrote:
> Place vdso-related user-space code in arch/arm/kernel/vdso/.
> 
> It is almost completely written in C with some assembly helpers to
> load the data page address, sample the counter, and fall back to
> system calls when necessary.
> 
> If CONFIG_ARM_ARCH_TIMER is not enabled, the vdso cannot service
> high-resolution clocks and falls back to syscalls.  Low-resolution
> clocks e.g. CLOCK_REALTIME_COARSE can be serviced regardless.

Okay, how is this used by userspace?

It seems that on ARM, we generate a dso with these symbols:

DYNAMIC SYMBOL TABLE:
000001e4 l    d  .eh_frame_hdr	00000000              .eh_frame_hdr
00000000 g    DO *ABS*	00000000  LINUX_3.17  LINUX_3.17
000003c4 g    DF .text	00000080  LINUX_3.17  __kernel_clock_getres
00000444 g    DF .text	0000002c  LINUX_3.17  __kernel_gettimeofday
00000298 g    DF .text	0000012c  LINUX_3.17  __kernel_clock_gettime

whereas, x86-64 generates this:

DYNAMIC SYMBOL TABLE:
ffffffffff700330 l    d  .eh_frame_hdr	0000000000000000              .eh_frame_hdr
ffffffffff700600  w   DF .text	00000000000002b9  LINUX_2.6   clock_gettime
0000000000000000 g    DO *ABS*	0000000000000000  LINUX_2.6   LINUX_2.6
ffffffffff7008c0 g    DF .text	000000000000015a  LINUX_2.6   __vdso_gettimeofday
ffffffffff700a40 g    DF .text	000000000000003d  LINUX_2.6   __vdso_getcpu
ffffffffff7008c0  w   DF .text	000000000000015a  LINUX_2.6   gettimeofday
ffffffffff700a20  w   DF .text	0000000000000016  LINUX_2.6   time
ffffffffff700a40  w   DF .text	000000000000003d  LINUX_2.6   getcpu
ffffffffff700600 g    DF .text	00000000000002b9  LINUX_2.6   __vdso_clock_gettime
ffffffffff700a20 g    DF .text	0000000000000016  LINUX_2.6   __vdso_time

hence references to gettimeofday automatically get resolved to the
vdso version without needing C library updates.

Or... do we need a glibc update to make use of this?

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