Re: [PATCH v4 05/17] xen/arm: ITS: implement hw_irq_controller for LPIs

[Julien Grall <julien.grall@citrix.com>]

Hi Ian,

On 15/07/2015 11:32, Ian Campbell wrote:
>>>> Why can't we store the event ID in the irq_guest? As said on v3, this is not
>>>
>>> Are you referring to irq_desc in above statement?
>>
>> Yes sorry.
>
> I'm afraid I don't follow your suggestion here, are you suggesting that
> the vid field added above should be moved to irq_desc?

Yes,

> But the vid _is_ domain specific, it is the virtual event ID which is
> per-domain (it's the thing looked up in the ITT to get a vLPI to be
> injected). I think it is a pretty direct analogue of the virq field used
> for non-LPI irq_guest structs.

No, vid is not specific to a domain but a device. The virtual event ID 
is always the same as the physical event ID (See your design document 
[1]). Furthermore, all the usage of the irq_to_vid in this series are 
for physical command (see lpi_set_config within this patch).

>
> Your proposal on v3 looks to be around moving the its_device pointer to
> the irq_desc, which appears to have been done here, along with turning
> the virq+vid into a union as requested there too.

On v3 I said: "The event ID and
the its_device assigned are known when the device is added to Xen and
hence can be set in irq_desc (with a small memory impact, but we have
plenty of memory on ARM64)."

Sorry if it was confusing.

>
>>>> It has been suggested by Ian to move col_id in the its_device in the
>>>> previous version [4]. Any reason to not doing it?
>>>
>>> In round robin fashion each plpi is attached to col_id. So storing
>>> in its_device is not possible. In linux latest col_id is stored in its_device
>>> structure for which set_affinity is called.
>
> Are you saying that in Linux all Events/LPIs associated with a given ITS
> device are routed to the same collection?
>
>> You could do round robin on its_device... It would be exactly the same
>
> Routing all LPIs associated with a given its_device to the same
> collection is not exactly the same as round robin-ing all LPIs from the
> device over the collections.

Yes, sorry I was a bit lax on the writing. I wanted to meant that there 
is not much difference to do it.

>> and save 2 byte if not more with the alignment per irq_desc.
>
> If this is a concern then I would say we would either want a separate
> array of per-pLPI information which we do not want in irq_desc because
> it is irq specific, or do add a pointer to its_desc which points to an
> array of per-event information.

That would be a good solution. Although, as I said, I don't really care 
for Xen 4.6. It's more an optimization for 4.7.

Regards,

[1] http://xenbits.xen.org/people/ianc/vits/draftG.html#event-id-event

-- 
Julien Grall