Re: [Xen-devel] [PATCH 3/3] xen/arm: set the system time in Xen via the XENPF_settime hypercall

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Mon, 9 Nov 2015, Stefano Stabellini wrote:
> On Mon, 9 Nov 2015, Arnd Bergmann wrote:
> > On Monday 09 November 2015 14:10:22 Stefano Stabellini wrote:
> > > On Thu, 5 Nov 2015, Arnd Bergmann wrote:
> > > > On Thursday 05 November 2015 17:09:45 Stefano Stabellini wrote:
> > > > > +     now = __current_kernel_time();
> > > > 
> > > > We don't have __current_kernel_time64() yet, but it is trivial
> > > > to add, just follow the example of
> > > > current_kernel_time()/current_kernel_time64() and convert the
> > > > existing __current_kernel_time() function into a static
> > > > inline wrapper for the new __current_kernel_time64().
> > > 
> > > All right. I guess something like:
> > > 
> > > struct timespec64 __current_kernel_time64(void)
> > > {
> > >   struct timekeeper *tk = &tk_core.timekeeper;
> > > 
> > >   return tk_xtime(tk);
> > > }
> > 
> > Yes, exactly.
> > 
> > Just to make sure that this is actually the correct interface
> > that you want to call:
> > 
> > __current_kernel_time{,64}() is the fastest interface we have
> > to get an approximation of the current time, while ignoring
> > all of the locking.
> > 
> > Is is possible that you instead want ktime_get_real_ts64(),
> > which gives you the time as precise as the kernel knows it,
> > but uses locking?
> 
> I am not 100% sure. Can I call ktime_get_real_ts64 from a
> pvclock_gtod notifier_call function?

It doesn't look like it: we would be getting the tk_core seqcount twice.

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