Re: [Qemu-devel] [Qemu-ppc] [PATCH 12/12] pseries: Generate unique LIOBNs for PCI host bridges

[Alexander Graf <agraf@suse.de>]

On 11/21/2012 02:21 PM, David Gibson wrote:
> On Wed, Nov 21, 2012 at 03:13:39PM +0200, Michael S. Tsirkin wrote:
>> On Wed, Nov 21, 2012 at 11:36:00PM +1100, David Gibson wrote:
>>> On Wed, Nov 21, 2012 at 01:34:48PM +0200, Michael S. Tsirkin wrote:
>>>> On Wed, Nov 21, 2012 at 11:57:05AM +1100, David Gibson wrote:
>>>>> On Tue, Nov 20, 2012 at 02:26:09PM +0200, Michael S. Tsirkin wrote:
>>>>>> On Tue, Nov 20, 2012 at 10:27:11AM +0100, Alexander Graf wrote:
>>>>>>> On 19.11.2012, at 23:51, David Gibson wrote:
>>>>>>>
>>>>>>>> On Mon, Nov 19, 2012 at 05:34:12PM +0100, Alexander Graf wrote:
>>>>>>>>> On 13.11.2012, at 03:47, David Gibson wrote:
>>>>>>>>>
>>>>>>>>>> From: Alexey Kardashevskiy<aik@ozlabs.ru>
>>>>>>>>>>
>>>>>>>>>> In future (with VFIO) we will have multiple PCI host bridges on
>>>>>>>>>> pseries.  Each one needs a unique LIOBN (IOMMU id).  At the moment we
>>>>>>>>>> derive these from the pci domain number, but the whole notion of
>>>>>>>>>> domain numbers on the qemu side is bogus and in any case they're not
>>>>>>>>>> actually uniquely allocated at this point.
>>>>>>>>>>
>>>>>>>>>> This patch, therefore uses a simple sequence counter to generate
>>>>>>>>>> unique LIOBNs for PCI host bridges.
>>>>>>>>>>
>>>>>>>>>> Signed-off-by: Alexey Kardashevskiy<aik@ozlabs.ru>
>>>>>>>>>> Signed-off-by: David Gibson<david@gibson.dropbear.id.au>
>>>>>>>>> I don't really like the idea of having a global variable just
>>>>>>>>> because our domain ID generation seems to not work as
>>>>>>>>> expected. Michael, any comments here?
>>>>>>>> Well, the patch I sent which changed domain id generation was
>>>>>>>> ignored.  In any case, as I said, the whole concept of domain numbers
>>>>>>> Michael?
>>>>>> This is user visible, right?
>>>>>> So IMHO we should have the user specify LIOBN through a property,
>>>>>> rather than assign what's essentially a random value.
>>>>> Well, I can implement an override through a property, which could be
>>>>> useful in some circumstances.  But we still need to have qemu generate
>>>>> unique defaults, rather than forcing it to be specified in every case.
>>>> I don't see why.
>>>> And if you want automatic defaults then they need to be generated in a
>>>> way that does not depend on implementation detail such as order of
>>>> device initialization.
>>> Because requiring explicit unique liobns to be supplied whenever there
>>> is more than one PHB is horrible for usability.
>> We should make simple things simple and complex things possible.
>> More than one PHB seems like an advanced feature
> Not for pseries.  On real hardware of this type, dozens of PHBs is
> routine.  Plus, vfio passthrough is coming, we need at minimum one PHB
> for emulated devices and one for passthrough devices.

Yeah, I second Davids opinion here. We need to make this easy for users.

How do we assign PCI slot IDs today when all you do is a -device? This 
should probably follow the same scheme.


Alex