Re: [patch 08/20] genirq/msi: Make MSI descriptor iterators device domain aware

[Jason Gunthorpe]

On Wed, Nov 16, 2022 at 11:32:15PM +0100, Thomas Gleixner wrote:
> On Wed, Nov 16 2022 at 14:36, Jason Gunthorpe wrote:
> > On Fri, Nov 11, 2022 at 02:56:50PM +0100, Thomas Gleixner wrote:
> >> To support multiple MSI interrupt domains per device it is necessary to
> >> segment the xarray MSI descriptor storage. Each domain gets up to
> >> MSI_MAX_INDEX entries.
> >
> > This kinds of suggests that the new per-device MSI domains should hold
> > this storage instead of per-device xarray?
> 
> No, really not. This would create random storage in random driver places
> instead of having a central storage place which is managed by the core
> code. We've had that back in the days when every architecture had it's
> own magic place to store and manage interrupt descriptors. Seen that,
> mopped it up and never want to go back.

I don't mean shift it into the msi_domain driver logic, I just mean
stick an xarray in the struct msi_domain that the core code, and only
the core code, manages.

But I suppose, on reflection, the strong reason not to do this is that
the msi_descriptor array is per-device, and while it would work OK
with per-device msi_domains we still have the legacy of global msi
domains and thus still need a per-device place to store the global msi
domain's per-device descriptors.

> > At least, I'd like to understand a bit better the motivation for using
> > a domain ID instead of a pointer.
> 
> The main motivation was to avoid device specific storage for the irq
> domain pointers. It would have started with PCI/MSI[X]: I'd had to add a
> irqdomain pointer to struct pci_dev and then have the PCI core care
> about it. So we'd add that to everything and the world which utilizes
> per device MSI domains which is quite a few places outside of PCI in the
> ARM64 world and growing.

I was thinking more that the "default" domain (eg the domain ID 0 as
this series has it) would remain as a domain pointer in the device
data as it is here, but any secondary domains would be handled with a
pointer that the driver owns.

You could have as many secondary domains as is required this way. Few
drivers would ever use a secondary domain, so it not really a big deal
for them to hold the pointer lifetime.

> So what are you concerned about?

Mostly API clarity, I find it very un-kernly to swap a clear pointer
for an ID #. We loose typing, the APIs become less clear and we now
have to worry about ID allocation policy if we ever need more than 2.

Thanks,
Jason