Re: [Xen-devel] Question about apic ipi interface

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Wed, 2013-05-08 at 17:26 +0100, Stefan Bader wrote:
> On 23.04.2013 12:07, Stefan Bader wrote:
> > I was looking at some older patch and there is one thing I do not 
> > understand.
> > 
> > commit f447d56d36af18c5104ff29dcb1327c0c0ac3634
> >     xen: implement apic ipi interface
> > 
> > Specifically there the implementation of xen_send_IPI_mask_allbutself().
> > 
> > void xen_send_IPI_mask_allbutself(const struct cpumask *mask,
> >                                 int vector)
> > {
> >         unsigned cpu;
> >         unsigned int this_cpu = smp_processor_id();
> > 
> >         if (!(num_online_cpus() > 1))
> >                 return;
> > 
> >         for_each_cpu_and(cpu, mask, cpu_online_mask) {
> >                 if (this_cpu == cpu)
> >                         continue;
> > 
> >                 xen_smp_send_call_function_single_ipi(cpu);
> >         }
> > }
> > 
> > Why is this using xen_smp_send_call_function_single_ipi()? This dumps the
> > supplied vector and always uses XEN_CALL_FUNCTION_SINGLE_VECTOR. In 
> > contrast the
> > xen_send_IPI_all() and xen_send_IPI_self() keep the (mapped) vector.
> > 
> > Mildly wondering about whether call function would need special casing (just
> > because xen_smp_send_call_function_ipi() is special). But I don't have the 
> > big
> > picture there.
> > 
> 
> This never got really answered, so lets try this: Does the following patch 
> seem
> to make sense?

It certainly seems too. Although I traced the call chain back to
default_send_IPI_allbutself() which nothing actually seems to call. Or
my grep is faulty...

There is a call path via send_IPI_allbutself which looks to only
potentially be called as:
        apic->send_IPI_allbutself(APIC_DM_NMI);
        apic->send_IPI_allbutself(NMI_VECTOR);
(the other calls are in native_smp_foo which I assume doesn't apply).

xen_map_vector( doesn't seem to handle those two, I'm not sure if that
will constitute a regression since it doesn't seem likely that either of
those would have worked properly with the current code either.

Ian.


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