Re: RESEND [PATCH v6 11/12] lib: vdso: Add support for CLOCK_BOOTTIME

[Mark Salyzyn]

On 10/01/2018 11:15 AM, Thomas Gleixner wrote:
> On Mon, 1 Oct 2018, Mark Salyzyn wrote:
> >   
> > +static notrace int do_boottime(const struct vdso_data *vd, struct timespec *ts)
> > +{
> > +	u32 seq, mult, shift;
> > +	u64 nsec, cycle_last;
> > +	vdso_wtm_clock_nsec_t wtm_nsec;
> > +
> > +	/* open coding timespec_add_ns to save a ts->tv_nsec = 0 */
> > +	ts->tv_sec = sec + __iter_div_u64_rem(nsec, NSEC_PER_SEC, &nsec);
> > +	ts->tv_nsec = nsec;
> > +
> > +	return 0;
> > +}
> > +
> Instead of adding yet another copy of the same code you might want to look
> at the rework I did for the x86 vdso in order to support CLOCK_TAI w/o
> running into the issue of the clock switch case being compiled into a jump
> table and then the compile asking for retpoline.
>
>    http://lkml.kernel.org/r/20180917124533.329334911@xxxxxxxxxxxxx
>
> Thanks,
>
> 	tglx

Great idea. Thanks!

The point of the first 12 patches is to _align_ the arm code to match 
the assembler for arm64 exactly 1:1. Then switch arm64 assembler to use 
the _same_ code in 'C' as a library. No performance degradation.

Next extend the vdso framework on arm64 to _also_ use that library for 
vDSO32 (arm32 compat on 64-bit). At this point we achieve a 0.4% power 
reduction on Android.

At that point, we would be ready for a rework that fixes all three (ARM 
vDSO, ARM64 vDSO and ARM64 vDSO32) to get the compiler to handle the 
switch statement better. One step at a time.

-- Mark