Re: [RFC PATCH v3 4/7] bus/cdx: add cdx-MSI domain with gic-its domain as parent

[Robin Murphy]

On 2022-09-07 12:17, Marc Zyngier wrote:
> On Tue, 06 Sep 2022 18:19:06 +0100,
> Jason Gunthorpe <jgg@xxxxxxxxxx> wrote:
> > On Tue, Sep 06, 2022 at 07:17:58PM +0530, Nipun Gupta wrote:
> >
> > > +static void cdx_msi_write_msg(struct irq_data *irq_data,
> > > +			      struct msi_msg *msg)
> > > +{
> > > +	/*
> > > +	 * Do nothing as CDX devices have these pre-populated
> > > +	 * in the hardware itself.
> > > +	 */
> > > +}
> >
> > Huh?
> >
> > There is no way it can be pre-populated, the addr/data pair,
> > especially on ARM, is completely under SW control.
>
> There is nothing in the GIC spec that says that.
>
> > There is some commonly used IOVA base in Linux for the ITS page, but
> > no HW should hardwire that.
>
> That's not strictly true. It really depends on how this block is
> integrated, and there is a number of existing blocks that know *in HW*
> how to signal an LPI.
>
> See, as the canonical example, how the mbigen driver doesn't need to
> know about the address of GITS_TRANSLATER.
>
> Yes, this messes with translation (the access is downstream of the
> SMMU) if you relied on it to have some isolation, and it has a "black
> hole" effect as nobody can have an IOVA that overlaps with the
> physical address of the GITS_TRANSLATER register.
>
> But is it illegal as per the architecture? No. It's just stupid.

If that were the case, then we'd also need a platform quirk so the SMMU 
driver knows about it. Yuck.

But even then, are you suggesting there is some way to convince the ITS 
driver to allocate a specific predetermined EventID when a driver 
requests an MSI? Asking for a friend...

Cheers,
Robin.