Re: [Xen-devel] [RFC PATCH v2 13/22] xen/arm: its: Add virtual ITS command support

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 29 Apr 2015, Julien Grall wrote:
> On 29/04/15 17:30, Vijay Kilari wrote:
> > On Wed, Apr 29, 2015 at 9:56 PM, Vijay Kilari <vijay.kilari@xxxxxxxxx> 
> > wrote:
> >> On Wed, Apr 29, 2015 at 7:05 PM, Julien Grall <julien.grall@xxxxxxxxxx> 
> >> wrote:
> >>> On 29/04/15 12:56, Julien Grall wrote:
> >>>> As the 2 suggested approach don't seem to fit our usage, we need to find
> >>>> another approach.
> >>>
> >>> I think I have another approach which doesn't require interrupt neither
> >>> polling in EL2.
> >>
> >> I could resolve all the issues around approach 1
> >> only concern is generating dummy/fake device id.
> 
> This is a big concern. We can't hardcode the devID because a real device
> could use it later. Having an ID generating at the boot time wouldn't be
> better because it could be broken with device hotplug.
> 
> It's very unfortunate that the ITS doesn't have a reserved interrupt.

Indeed, but it is an issue that can be overcome. We should just use an
out-of-range devid. One that cannot be hot-plugged later.

The one proposed by Vijay is actually not a bad idea (segment number
0xff). Segment numbers correspond to MCFG config spaces. There is
usually one per root complex, but theoretically I think one root complex
could have more.

I don't think is possible to hot-plug a root complex, so if one is spare
at boot time, we should be safe. Even if it was possible, we could still
return error from PHYSDEVOP_pci_host_bridge_add if the segment number
overlaps (see http://marc.info/?l=xen-devel&m=142495489932714).

So I think we should just go ahead and use PLAT_DUMMY_SEG 0xff.

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