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

[Andy Lutomirski]

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.


--Andy