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

[Alexander Graf]

On 21.11.2012, at 06:00, David Gibson <address@hidden> 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 <address@hidden>
>>>>> 
>>>>> 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 <address@hidden>
>>>>> Signed-off-by: David Gibson <address@hidden>
>>>> 
>>>> 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?
>> 
>>> makes no sense on the qemu side, so I don't think increasing reliance
>>> on them by using them here is a good idea.
>>> 
>>> It would be conceptually nicer to derive the liobn from the buid, but
>>> that would rely on the buid's being unique in the low 32-bits, which
>>> is true in practice, but seems risky to rely on.
>> 
>> Well, there has to be some uniqueness from the guest's POV already,
>> no?
> 
> Yes, the BUIDs are unique, but they are 64-bit, whereas the LIOBN is
> only 32-bit.

Tricky. Michael, any ideas?

Alex

> 
> -- 
> David Gibson            | I'll have my music baroque, and my code
> david AT gibson.dropbear.id.au    | minimalist, thank you.  NOT _the_ _other_
>                | _way_ _around_!
> http://www.ozlabs.org/~dgibson