Re: Request for feedback on Generic Timeofday Subsystem (B20)

[john stultz <johnstul@us.ibm.com>]

On Tue, 2006-03-07 at 14:40 -0800, Luck, Tony wrote:
> > While if its a deal breaker, I'm ok with adding those raw access info
> > back into the structure, I'd first ask why ia64 must use this very
> > constrained fsyscall method instead of something more flexible where it
> > doesn't have to be written in asm like vsyscall/VDSO which x86-64 and
> > powerpc use? 
>
> I can't quite see how gettimeofday() can be correctly implemented
> purely in userspace on a system where there is jitter in the clock source,
> but I'm clueless about how vsyscall/VDSO works.

You are right there. The jitter handling (if I recall, basically a
cmpxchg w/ the last read cycle value to be sure the clocksource doesn't
go backward) wouldn't be doable in userspace, but it seems that would
already be a pretty bad hit on performance. Is it not? And how many
systems actually use unsycned/jittery ITCs instead of alternative mmioed
clocksources?

Regardless, if its really a blocking issue, I'm not opposed to putting
the direct access methods back into the structure, or going with an
alternative solution to make these bits doable. Ingo might have a better
idea for this as well.

Do you have any other issues or questions?

thanks
-john