From: Arnd Bergmann <arnd@arndb.de>
To: linux-arm-kernel@lists.infradead.org
Cc: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        xen-devel@lists.xensource.com, linux-kernel@vger.kernel.org,
        Ian.Campbell@citrix.com
Subject: Re: [PATCH v3 5/6] xen/arm: introduce xen_read_wallclock
Date: Wed, 11 Nov 2015 21:33:57 +0100
Message-ID: <6299736.g7Jp9b7byl@wuerfel>
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On Wednesday 11 November 2015 16:51:35 Stefano Stabellini wrote:
> +static void xen_read_wallclock(struct timespec64 *ts)
> +{
> +       u32 version;
> +       u64 delta;
> +       struct timespec64 now;
> +       struct shared_info *s = HYPERVISOR_shared_info;
> +       struct pvclock_wall_clock *wall_clock = &(s->wc);
> +
> +       /* get wallclock at system boot */
> +       do {
> +               version = wall_clock->version;
> +               rmb();          /* fetch version before time */
> +               now.tv_sec  = ((uint64_t)wall_clock->sec_hi << 32) | wall_clock->sec;
> +               now.tv_nsec = wall_clock->nsec;
> +               rmb();          /* fetch time before checking version */
> +       } while ((wall_clock->version & 1) || (version != wall_clock->version));
> +
> +       /* time since system boot */
> +       delta = ktime_get_ns();
> +       delta += now.tv_sec * (u64)NSEC_PER_SEC + now.tv_nsec;
> +
> +       *ts = ns_to_timespec64(delta);
> +}

Looks correct to me and better than the previous versions, but you are still
converting from timespec64 to nanoseconds and back. While I previously
recommended going all the way to nanoseconds here, I guess this you
can even avoid the ns_to_timespec64() if you stay within timespec64
domain and replace the last lines with

	ktime_get_ts64(&ts_monotonic);
	*ts = timespec64_add(now, ts_monotonic);

which avoids both the multiplication and division.

	Arnd
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