vDSO gettimeofday

There is a Virtual Dynamic Shared Object (VDSO) implemented in the glibc runtime library. The VDSO maps some of the kernel code, which is necessary to read gettimeofday in the user-space. Standard Red Hat Enterprise Linux 5.5 allows the gettimeofday function to be performed entirely in user-space, removing the system call overhead.

2.6. gettimeofday speedup

x86-64 functions
The table below lists the symbols exported by the vDSO. All of these symbols are also available without the "__vdso_" prefix, but you should ignore those and stick to the names below.

symbol version
__vdso_clock_gettime LINUX_2.6
__vdso_getcpu LINUX_2.6
__vdso_gettimeofday LINUX_2.6
__vdso_time LINUX_2.6

vdso(7) - Linux manual page

Xen pvclock; does not support vDSO

x86: vdso: pvclock gettime support
Improve performance of time system calls when using Linux pvclock,
by reading time info from fixmap visible copy of pvclock data.