Re: [PATCHv6 25/36] vdso: Introduce vdso_static_branch_unlikely()

On Thu, 15 Aug 2019, Dmitry Safonov wrote:

From: Andrei Vagin <redacted>

As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.

Addressing those problems, there are two versions of VDSO's .so:
for host tasks (without any penalty) and for processes inside of time
namespace with clk_to_ns() that subtracts offsets from host's time.

Introduce vdso_static_branch_unlikely(), which is similar to
static_branch_unlikely(); alias it with timens_static_branch_unlikely()
under CONFIG_TIME_NS.

The timens code in vdso will look like this:

   if (timens_static_branch_unlikely()) {
	   clk_to_ns(clk, ts);

Please name that clk_to_namespace(). _ns() is widely used for nanoseconds.

   }

The version of vdso which is compiled from sources will never execute
clk_to_ns(). And then we can patch the 'no-op' in the straight-line
codepath with a 'jump' instruction to the out-of-line true branch and
get the timens version of the vdso library.

Colour me confused. Why do we need that static branch at all?

Why don't we compile VDSO_NO_NAMESPACE and VDSO_NAMESPACE right away? One
has the clk_to_namespace() one does not. The you can spare the whole static
key patching and the NO_NAMESPACE variant does not have extra 5 NOPS.

The VDSO is one page IIRC, so having the extra namespace variant around
does really not matter at all.

Thanks,

	tglx