Re: [Qemu-devel] [PATCH v2 0/2] Add global device ID in virt machine

[Diana Madalina Craciun]

On 07/31/2017 05:06 PM, Michael S. Tsirkin wrote:
> On Mon, Jul 31, 2017 at 01:22:45PM +0000, Diana Madalina Craciun wrote:
>>>> If we are to use a value of 0 for the constant in case of PCI devices,
>>>> what happens if we have multiple PCI controllers?
>>> I guess we'd use the PCI Segment number for that?
>>>
>>>
>> Yes, we can use the PCI segment for this scenario. But this would mean
>> different solutions for the same problem. The main problem is that we
>> can have multiple entities in the system that are using MSIs (for now
>> PCI and NXP non-PCI bus infrastructure
>> (https://emea01.safelinks.protection.outlook.com/?url=https%3A%2F%2Flwn.net%2FArticles%2F635905%2F&data=01%7C01%7Cdiana.craciun%40nxp.com%7C6b0c6c879af64718a21908d4d81d534e%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0&sdata=bpYMMqajWzgzdbdQgy%2FUYR7y%2BswyvwE%2BqFzs7wdIkkA%3D&reserved=0).
>>  I guess that we may have other
>> platform devices that are using MSIs in the future.
>>
>> Thanks,
>> Diana
>>
>>
> Don't have the time to explore NXP in depth, sorry - there's
> a lot of complexity there.
> Could you maybe stick some bits to specify bus type in there?
> It just looks very wrong to push low level things like this
> that users have no interest in up the stack.
>
Let's generalize the problem a little bit, the NXP details just does not
matter much. The problem we have is the following:

The GIC-ITS, the ARM MSI controller is using deviceIDs in order to remap
the interrupts. Each device which is expected to send MSIs has a
deviceID associated with it. These deviceIDs are configured into devices
by software/firmware. There is support in the device tree to specify the
correlation between requesterID and deviceID:

"msi-map: Maps a Requester ID to an MSI controller and associated
  msi-specifier data. The property is an arbitrary number of tuples of
  (rid-base,msi-controller,msi-base,length)"
(https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/Documentation/devicetree/bindings/pci/pci-msi.txt)

Our problem is that we have to allocate these deviceIDs in QEMU as well
and we have to ensure that they are unique. Currently, for PCI, the
assumption requesterID=deviceID is made which will no longer be true in
case other devices are added. So we need a way (preferable a general
one) to allocate these IDs to different devices in the system in a
consistent way which will ensure that two devices do not share the same ID.

The reason I put this ID into the controller itself is because on real
hardware is actually programmed into the controller. It is needed (for
example) when the MSIs are sent.

Thanks,

Diana