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

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Nov 23, 2012 at 03:13:07PM +1100, David Gibson wrote:
> On Thu, Nov 22, 2012 at 09:23:03AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Nov 22, 2012 at 01:27:18PM +1100, David Gibson wrote:
> > > On Wed, Nov 21, 2012 at 05:27:37PM +0200, Michael S. Tsirkin wrote:
> > > > On Wed, Nov 21, 2012 at 02:27:08PM +0100, Alexander Graf wrote:
> > > > > 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.
> > > > 
> > > > I think users don't normally create PHBs. They request a disk, a network
> > > > device, a pass-through device ... In this case if this requires
> > > > more PHBs create them internally but I imagine this doesn't
> > > > require any allocation scheme - simply set some address
> > > > for virtual devices, some other one for assigned devices ... no?
> > > 
> > > No.  One PHB for passthrough and one for emulated is the minimum.
> > > Since we don't emulated p2p bridges,
> > 
> > Actually qemu does emulate p2p bridges.
> > 
> > > each PHB can only support a small
> > > number of PCI devices, so if enough PCI devices are requested, we will
> > > still need to create - and assign numbers to - additional PHBs.
> > 
> > Each PHB can support up to 32 slots right? This seems ample for
> > a typical use. If you want many tens of devices you need to
> > supply addresses manually, this looks reasonable to me.
> > 
> > Allocating PHBs on the fly seems unencessarily tricky.
> 
> I still think you're thinking far too much in terms of x86-like
> guests.  But, I guess, emulated PCI devices will be pretty rare, since
> they're so slow anyway.

They are not very slow.  My point is the reverse. Device assignment is
pretty rare, assigning multiple devices even more rare. It should work
but I don't see why we should increase maintainance burden to save the user
ruynning qemu from CLI the trouble of specifying a single extra property
in this advanced case.

> But, what Alex was getting at is another factor I forgot.  Because of
> the way the PAPR PCI paravirtualized interfaces work, host devices
> which are in different "partitionable endpoints" (roughly the minimum
> granularity for safe isolation) must appear in different virtual PHBs
> in the guest.  So if we pass through a lot of PCI devices, we will
> almost certainly need a lot of guest PHBs.

One further point: physical systems normally don't let you hotplug PHBs.
So if for non hotplug you auto-add PHBs, you end up treating hotplug
and non hotplug devices differently from UI point of view this is nasty.

Look, even if solution using a required property is less elegant for CLI
use, it will work, won't it?
So how about we merge it so that things work, and then we can discuss a
patch on top that auto-generates this property?

> -- 
> 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