Re: [patch 10/11] x86/vdso: Move cycle_last handling into the caller

From: Vitaly Kuznetsov
Date: Fri Sep 14 2018 - 11:26:09 EST

Thomas Gleixner <tglx@xxxxxxxxxxxxx> writes:

> Dereferencing gtod->cycle_last all over the place and foing the cycles <
> last comparison in the vclock read functions generates horrible code. Doing
> it at the call site is much better and gains a few cycles both for TSC and
> pvclock.
> Caveat: This adds the comparison to the hyperv vclock as well, but I have
> no way to test that.

While the change looks obviously correct for Hyper-V vclock too, by saying
that you encouraged me to give the whole series a shot. I did and turns
out Hyper-V vclock is broken: simple clock_gettime(CLOCK_REALTIME, )
test goes from 70 cpu cycles to 1000 (meaning that we're falling back to
syscall I guess). Bisecting now, stay tuned!