Re: [PATCH v2 7/7] xen/arm: set the system time in Xen via the XENPF_settime64 hypercall

[Stefano Stabellini <stefano.stabellini@eu.citrix.com>]

On Tue, 10 Nov 2015, Arnd Bergmann wrote:
> On Tuesday 10 November 2015 11:57:55 Stefano Stabellini wrote:
> > +       op.u.settime64.nsecs = now.tv_nsec;
> > +       op.u.settime64.system_time = arch_timer_read_counter() * (u64)NSEC_PER_SEC;
> > +       do_div(op.u.settime64.system_time, arch_timer_get_rate());
> 
> NSEC_PER_SEC is a large number, doesn't that multiplication overflow the
> 64-bit system_time variable?

It could be a concern, you are right. I think I can solve this issue and
remove multiple do_divs by calculating nsec_per_tick at initialization
time and use it here and in xen_read_wallclock.


> > +       printk("GTOD: Setting to %llu.%u at %llu\n",
> 
> 				   %llu.%09u
> 
> so you get the leading zeroes.
> 
> > +              op.u.settime64.secs,
> > +              op.u.settime64.nsecs,
> > +              op.u.settime64.system_time);
> > +       (void)HYPERVISOR_platform_op(&op);
> 
> 	Arnd
>