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

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

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.

> so I don't see why it needs to be made very easy to use.
> With an appropriate error message it should not be
> too hard for users to figure it out.

-- 
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