On 2/8/19 9:57 AM, Thomas Gleixner wrote: > On Thu, 7 Feb 2019, Rasmus Villemoes wrote: > > Cc: + Vincenzo, Will > >> On 06/02/2019 01.10, 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. >>> >>> 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. >>> >>> Unfortunately, to allow changing VDSO VMA on a running process, >>> the entry points to VDSO should have the same offsets (addresses). >>> That's needed as i.e. application that calls setns() may have already >>> resolved VDSO symbols in GOT/PLT. >> >> These (14-19, if I'm reading them right) seems to add quite a lot of >> complexity and fragility to the build, and other architectures would >> probably have to add something similar to their vdso builds. > > Yes and we really want to avoid that. The VDSO implementations are > pointlessly different accross the architectures and there is effort on the > way to consolidate them: > > https://lkml.kernel.org/r/20190115135539.24762-1-vincenzo.frascino@xxxxxxx > > I talked to Vincenzo earlier this week and he's working on a new version of > that. The timens stuff wants to go on top of the consolidation otherwise we > end up with another set of pointlessly different and differently broken > VDSO variants. That looks awesome! I've missed the tread about it, will catch the details. Thanks much, Dmitry
