Re: [Xen-devel] [PATCH 2/3] xen: add correct 500 ms offset when setting Xen wallclock

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Fri, 2012-10-12 at 13:57 +0100, David Vrabel wrote:
> From: David Vrabel <david.vrabel@xxxxxxxxxx>
> 
> update_persistent_wallclock() (and hence xet_set_wallclock()) is
> called 500 ms after the second.

The comment in sync_cmos_clock says it is called 500ms before the next
second.

I only mention it to double check that the right semantics for
set_wallclock are being implemented, i.e. that we are compensating in
the right direction.

>   xen_set_wallclock() was not
> considering this so the Xen wallclock would end up ~500 ms behind the
> correct time.
> 
> Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
> ---
>  arch/x86/xen/time.c |    9 +++++++--
>  1 files changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/xen/time.c b/arch/x86/xen/time.c
> index 5e7f536..11770d0 100644
> --- a/arch/x86/xen/time.c
> +++ b/arch/x86/xen/time.c
> @@ -212,10 +212,15 @@ static int xen_set_wallclock(unsigned long now)
>       /* Set the hardware RTC. */
>       mach_set_rtc_mmss(now);
>  
> -     /* Set the Xen wallclock. */
> +     /*
> +      * Set the Xen wallclock.
> +      *
> +      * update_persistent_wallclock() is called ~500 ms after 'now'
> +      * so add an extra 500 ms.
> +      */
>       op.cmd = XENPF_settime;
>       op.u.settime.secs = now;
> -     op.u.settime.nsecs = 0;
> +     op.u.settime.nsecs = NSEC_PER_SEC / 2;
>       op.u.settime.system_time = xen_clocksource_read();
>  
>       rc = HYPERVISOR_dom0_op(&op);



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