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

[Sunil Muthuswamy]

 On Saturday, July 31, 2021 2:52 AM,
 Marc Zyngier <maz@xxxxxxxxxx> 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.

- Sunil