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

From: Julien Grall <julien.grall@citrix.com>
To: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Cc: Ian Campbell <ian.campbell@citrix.com>,
	Vijay Kilari <vijay.kilari@gmail.com>,
	Prasun Kapoor <Prasun.Kapoor@caviumnetworks.com>,
	Vijaya Kumar K <vijaya.kumar@caviumnetworks.com>,
	Julien Grall <julien.grall@linaro.org>, Tim Deegan <tim@xen.org>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>,
	Stefano Stabellini <stefano.stabellini@citrix.com>,
	manish.jaggi@caviumnetworks.com
Subject: Re: [RFC PATCH v2 13/22] xen/arm: its: Add virtual ITS command support
Date: Thu, 30 Apr 2015 11:09:21 +0100	[thread overview]
Message-ID: <5541FF51.6060907@citrix.com> (raw)
In-Reply-To: <alpine.DEB.2.02.1504301031260.2640@kaball.uk.xensource.com>

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@gmail.com> wrote:
>>>> On Wed, Apr 29, 2015 at 7:05 PM, Julien Grall <julien.grall@citrix.com> 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