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

[David Gibson <david@gibson.dropbear.id.au>]

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
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.

-- 
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