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

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Date:   Fri, 9 Dec 2016 06:11:17 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        John Stultz <john.stultz@...aro.org>,
        Ingo Molnar <mingo@...nel.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>
Subject: Re: [patch 5/6] [RFD] timekeeping: Provide optional 128bit math

On Thu, Dec 08, 2016 at 08:49:39PM -0000, Thomas Gleixner wrote:

> +/*
> + * Enabled when timekeeping is supposed to deal with virtualization keeping
> + * VMs long enough scheduled out that the 64 * 32 bit multiplication in
> + * timekeeping_delta_to_ns() overflows 64bit.
> + */
> +#ifdef CONFIG_TIMEKEEPING_USE_128BIT_MATH
> +
> +#if defined(CONFIG_ARCH_SUPPORTS_INT128) && defined(__SIZEOF_INT128__)
> +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> +{
> +	unsigned __int128 nsec;
> +
> +	nsec = ((unsigned __int128)delta * tkr->mult) + tkr->xtime_nsec;
> +	return (u64) (nsec >> tkr->shift);
> +}
> +#else
> +static inline u64 timekeeping_delta_to_ns(struct tk_read_base *tkr, u64 delta)
> +{
> +	u32 dh, dl;
> +	u64 nsec;
> +
> +	dl = delta;
> +	dh = delta >> 32;
> +
> +	nsec = ((u64)dl * tkr->mult) + tkr->xtime_nsec;
> +	nsec >>= tkr->shift;
> +	if (unlikely(dh))
> +		nsec += ((u64)dh * tkr->mult) << (32 - tkr->shift);
> +	return nsec;
> +}
> +#endif
> +
> +#else /* CONFIG_TIMEKEEPING_USE_128BIT_MATH */

xtime_nsec confuses me, contrary to its name, its not actually in nsec,
its in shifted nsec units for some reason (and that might well be a good
reason, but I don't know).

In any case, it needing to be inside the shift is somewhat unfortunate
in that it doesn't allow you to use the existing mul_u64_u32_shr()