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

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Date:   Fri, 9 Dec 2016 05:29:05 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Liav Rehana <liavr@...lanox.com>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Richard Cochran <richardcochran@...il.com>,
        Parit Bhargava <prarit@...hat.com>,
        Laurent Vivier <lvivier@...hat.com>,
        "Christopher S. Hall" <christopher.s.hall@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math


* Ingo Molnar <mingo@...nel.org> wrote:

> 
> * Thomas Gleixner <tglx@...utronix.de> 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 
something?

Thanks,

	Ingo