Re: [Xen-devel] [PATCH] x86/xen: Add "xen_timer_slop" command line option

[luca abeni <luca.abeni@xxxxxxxxxxxxxxx>]

Hi all,

On Tue, 26 Mar 2019 10:13:32 +0100
Dario Faggioli <dfaggioli@xxxxxxxx> wrote:

> On Mon, 2019-03-25 at 09:43 -0400, Boris Ostrovsky wrote:
> > On 3/25/19 8:05 AM, luca abeni wrote:  
> > > 
> > > The picture shows the latencies measured with an unpatched guest
> > > kernel
> > > and with a guest kernel having TIMER_SLOP set to 1000 (arbitrary
> > > small
> > > value :).
> > > All the experiments have been performed booting the hypervisor
> > > with a
> > > small timer_slop (the hypervisor's one) value. So, they show that
> > > decreasing the hypervisor's timer_slop is not enough to measure
> > > low latencies with cyclictest.  
> > 
> > I have a couple of questions:
> > * Does it make sense to make this a tunable for other clockevent
> > devices
> > as well?
> >  
> So, AFAIUI, the thing is as follows. In clockevents_program_event(),
> we keep the delta between now and the next timer event within
> dev->max_delta_ns and dev->min_delta_ns:
> 
>   delta = min(delta, (int64_t) dev->max_delta_ns);
>   delta = max(delta, (int64_t) dev->min_delta_ns);
> 
> For Xen (well, for the Xen clock) we have:
> 
>   .max_delta_ns = 0xffffffff,
>   .min_delta_ns = TIMER_SLOP,
> 
> which means a guest can't ask for a timer to fire earlier than 100us
[...]

I know this is not fully related with the current discussion, but in
these days I had a look at the code again, and...
The comment for TIMER_SLOP in arch/x86/xen/time.c says:
        /* Xen may fire a timer up to this many ns early */

Isn't the comment wrong? shouldn't it be "...many ns late" instead of
"early"?



                        Thanks,
                                Luca

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