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

[David Gibson]

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 <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?
> > > 
> > > 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.

-- 
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