Re: [PATCHv6 23/36] x86/vdso: Allocate timens vdso

From: Andy Lutomirski
Date: Fri Aug 16 2019 - 11:23:12 EST

On 8/15/19 9:38 AM, Dmitry Safonov wrote:
As it has been discussed on timens RFC, adding a new conditional branch
`if (inside_time_ns)` on VDSO for all processes is undesirable.
It will add a penalty for everybody as branch predictor may mispredict
the jump. Also there are instruction cache lines wasted on cmp/jmp.

Those effects of introducing time namespace are very much unwanted
having in mind how much work have been spent on micro-optimisation
vdso code.

The propose is to allocate a second vdso code with dynamically
patched out (disabled by static_branch) timens code on boot time.

Allocate another vdso and copy original code.

I'm unconvinced that any of this magic is wise. I think you should make a special timens vvar page that causes the normal fastpath to fail (using a special vclock mode, a special seq value, or a special "last" value) and then make the failure path detect that timens is in use and use the timens path.