Re: [Xen-devel] [PATCH for-4.5 7/8] xen/irq: Handle multiple action per IRQ

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Tue, 11 Mar 2014, Jan Beulich wrote:
> >>> On 11.03.14 at 16:16, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
> > Hello Jan,
> > 
> > On 02/24/2014 02:48 PM, Julien Grall wrote:
> >> On 02/24/2014 02:32 PM, Jan Beulich wrote:
> >>>>>> On 24.02.14 at 15:08, Julien Grall <julien.grall@xxxxxxxxxx> wrote:
> >>>> (Adding Jan for x86 part).
> >>>>
> >>>> On 02/20/2014 09:29 PM, Julien Grall wrote:
> >>>>> Hi Ian,
> >>>>>
> >>>>> On 02/19/2014 11:55 AM, Ian Campbell wrote:
> >>>>>> On Fri, 2014-01-24 at 16:43 +0000, Julien Grall wrote:
> >>>>>>> On ARM, it may happen (eg ARM SMMU) to setup multiple handler for the 
> >>>>>>> same
> >>>>>>> interrupt.
> >>>>>>
> >>>>>> Mention here that you are therefore creating a linked list of actions
> >>>>>> for each interrupt.
> >>>>>>
> >>>>>> If you use xen/list.h for this then you get a load of helpers and
> >>>>>> iterators which would save you open coding them.
> >>>>>
> >>>>> After thinking, using xen/list.h won't really remove open code, except
> >>>>> removing "action_ptr" in release_dt_irq.
> >>>>>
> >>>>> Calling release_dt_irq to an IRQ with multiple action shouldn't be
> >>>>> called often. Therefore, having both prev and next is a waste of space.
> >>>>
> >>>> Jan, as it's common code, do you have any thoughts?
> >>>
> >>> In fact I'm not convinced this action chaining is correct in the first
> >>> place, as mentioned by Ian too (considering the potential sharing
> >>> between hypervisor and guest). Furthermore, if this is really just
> >>> about IOMMU handlers, why can't the SMMU code register a single
> >>> action and disambiguate by the dev_id argument passed to the
> >>> handler?
> >> 
> >> The patch #3 of this serie protects the IRQ to be shared with the domain.
> >> 
> >> I should have remove "eg ARM SMMU" in the description. ARM SMMU is not
> >> the only the case, we don't know in advance if the IRQ will be shared
> >> (except browsing the DT and checking if this IRQ was used by another
> >> devices...). We may have the same thing with other devices.
>
> >> The logic is painful to handle internally in ARM SMMU driver while we
> >> can handle it generically. No need to duplicate the code when a new
> >> driver will have the same problem.
> > 
> > I haven't heard any answer from you. Shall I take as a "go"?
> 
> I'm sorry, this got lost between other stuff. Honestly I'm still not
> convinced generic multi-action IRQ support is indeed useful. 

I agree.
In general if an IRQ is shared among multiple devices, it is likely to
go to Dom0 and have a single action from Xen point of view.
An IRQ shared between Xen and a guest is a very bad idea.

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