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

[Julien Grall <julien.grall@xxxxxxxxxx>]

Hi Stefano,

On 30/04/2015 11:02, Stefano Stabellini wrote:
> 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.
As said earlier, the number of DevBits implemented by the ITS can be 
limited (see GITS_TYPER.Devbits).
If the devid is not within this range, the ITS won't recognize the value 
and won't be able to send the interrupt.
So this is clearly not the right value.

Regards,

--
Julien Grall

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