[EXTERNAL] Re: [RFC 1/1] irqchip/gic-v3-its: Add irq domain and chip for Direct LPI without ITS

[Robin Murphy]

On 2021-08-03 03:11, Sunil Muthuswamy wrote:
>   On Saturday, July 31, 2021 2:52 AM,
>   Marc Zyngier <maz at kernel.org> wrote:
>>
>> [...]
>>
>>>> I also want to understand *how* you are going to plumb this into both
>>>> ACPI and DT, given that neither understand how to link a PCI endpoint
>>>> to a set of RDs.
>>>>
>>>> 	M.
>>>
>>> One way to do this for NUMA-aware systems would be to use the NUMA
>>> related information that is available with PCI endpoints or root complex, to
>>> pick a Redistributor/CPU that is in the NUMA node, as specified by the PCI
>>> endpoint/root complex. In DT PCI devices can specify this using
>>> 'numa-node-id' and in ACPI using the '_PXM (Proximity)'. For systems that
>>> are not NUMA-aware, we can go with *any* Redistributor/CPU.
>>
>> This makes zero sense. From the point of view of a device, all the RDs
>> should be reachable, and firmware has no say in it. Dealing with
>> interrupt affinity is the responsibility of the endpoint driver, and
>> NUMA affinity is only a performance optimisation.
>>
>>> Is there any additional information we would be able to gather from ACPI
>>> or DT that's not there currently, that would be useful here?
>>
>> You will need some firmware information describing that a given set of
>> devices must use the RDs for their MSIs. Just like we currently
>> describe it in IORT for the ITS. You cannot /assume/ things. At the
>> moment, there is nothing at all, because no-one (including Microsoft)
>> thought it would be a good idea not to have an ITS, which is also why
>> ACPI doesn't describe MBIs as a potential MSI provider.
>>
> I am a little bit confused by your above comment. Maybe you can help me
> understand the ask. You indicate that from the point of the view of the
> device, all the RDs should be reachable. But, then if we define a mapping
> between PCI endpoint and RD in the firmware, we would be doing exactly
> the opposite. i.e. restricting the RDs that are reachable by the device. Can
> you please clarify?
> 
> Is your concern that the device should be able to only DMA to a subset of
> GIC Redistributor, for the MSIs? If so, in the IORT, there is "memory address
> size limit" for both device and root complex nodes. In the implementation,
> we can enforce that the GICR is within that range. And, if a device deviates
> further than that (ex: by having accessibility gaps within the GICR range),
> then that is out of scope for support.

No, please don't try to abuse the Memory Address Size Limit - that has 
far more chance of adversely affecting normal DMA operation than of 
being any use here.

I believe the point Marc was trying to make is that firmware should not 
associate a device with any one *specific* redistributor, however ACPI 
currently has no way to describe that MSIs can target redistributors *at 
all*, only ITS groups - there is no such concept as a "redistributor group".

Robin.