On some architectures, when the kernel loads any userspace program it
maps an ELF DSO into that program's address space.  This DSO is called
the vDSO and it often contains useful and highly-optimized alternatives
to real syscalls.

These functions are called just like ordinary C function according to
your platform's ABI.  Call them from a sensible context.  (For example,
if you set CS on x86 to something strange, the vDSO functions are
within their rights to crash.)  In addition, if you pass a bad
pointer to a vDSO function, you might get SIGSEGV instead of -EFAULT.

To find the DSO, parse the auxiliary vector passed to the program's
entry point.  The AT_SYSINFO_EHDR entry will point to the vDSO.

The vDSO uses symbol versioning; whenever you request a symbol from the
vDSO, specify the version you are expecting.

Programs that dynamically link to glibc will use the vDSO automatically.
Otherwise, you can use the reference parser in
tools/testing/selftests/vDSO/parse_vdso.c.

Unless otherwise noted, the set of symbols with any given version and the
ABI of those symbols is considered stable.  It may vary across architectures,
though.

(As of this writing, this ABI documentation as been confirmed for x86_64.
 The maintainers of the other vDSO-using architectures should confirm
 that it is correct for their architecture.)