Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math

From: Ingo Molnar
Date: Thu Dec 08 2016 - 23:29:16 EST

* Ingo Molnar <mingo@xxxxxxxxxx> wrote:

> * Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> > If the timekeeping CPU is scheduled out long enough by a hypervisor the
> > clocksource delta multiplication can overflow and as a result time can go
> > backwards. That's insane to begin with, but people already triggered a
> > signed multiplication overflow, so a unsigned overflow is not necessarily
> > impossible.
> >
> > Implement optional 128bit math which can be selected by a config option.
> What's the rough VM interruption time that would trigger an overflow? Given that
> the clock shift tk_read_base::mult is often 1, isn't it 32-bit nsecs, i.e. 4
> seconds?
> That doesn't sound 'insanely long'.
> Or some other value?

Ok, wasn't fully awake yet: more realistic values of the scaling factor on x86
would allow cycles input values of up to ~70 billion with 64-bit math, which would
allow deltas of up to about 1 minute with 64-bit math.

I think we should at least detect (and report?) the overflow and sanitize the
effects to the max offset instead of generating random overflown values.

That would also allow the 128-bit multiplication only be done in the rare case
when we overflow. Which in turn could then be made unconditional. Am I missing