Re: [patch 00/11] x86/vdso: Cleanups, simmplifications and CLOCK_TAI support

[Andy Lutomirski <luto@kernel.org>]

On Fri, Sep 14, 2018 at 5:52 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> Matt attempted to add CLOCK_TAI support to the VDSO clock_gettime()
> implementation, which extended the clockid switch case and added yet
> another slightly different copy of the same code.
>
> Especially the extended switch case is problematic as the compiler tends to
> generate a jump table which then requires to use retpolines. If jump tables
> are disabled it adds yet another conditional to the existing maze.
>
> This series takes a different approach by consolidating the almost
> identical functions into one implementation for high resolution clocks and
> one for the coarse grained clock ids by storing the base data for each
> clock id in an array which is indexed by the clock id.
>
> This completely eliminates the switch case and allows further
> simplifications of the code base, which at the end all together gain a few
> cycles performance or at least stay on par with todays code. The resulting
> performance depends heavily on the micro architecture and the compiler.

tglx, please consider this whole series Acked-by: Andy Lutomirski
<luto@kernel.org>

Please feel free to push it top tip:x86/vdso, as long as you pull in
tip/x86/urgent first.  Once it lands, I'll email out a couple of
follow-up patches.

--Andy