[libvirt] Proposal PCI/PCIe device placement on PAPR guests

[David Gibson]

On Fri, Jan 06, 2017 at 06:34:29PM +0100, Andrea Bolognani wrote:
> [Added Laine to CC, fixed qemu-devel address]
> 
> On Thu, 2017-01-05 at 16:46 +1100, David Gibson wrote:
> [...]
> >   * To allow for hotplugged devices, libvirt should also add a number
> >     of additional, empty vPHBs (the PAPR spec allows for hotplug of
> >     PHBs, but this is not yet implemented in qemu).
> 
> "A number" here will have to mean "one", same number of
> empty PCIe Root Ports libvirt will add to a newly-defined
> q35 guest.

Umm.. why?

> >     When hotplugging
> >     a new device (or PE) libvirt should locate a vPHB which doesn't
> >     currently contain anything.
> 
> This will need to be a PHB-specific behavior, because at the
> moment libvirt will happily pick one of the empty slots in
> an existing PHB.

Exactly.  Well, whether it's PHB model specific or machine type
specific is up to you really.  We can only have PAPR PHBs on a PAPR
machine type, so it's kind of arbitrary.

> 
> >   * libvirt should only (automatically) add PHBs - never root ports or
> >     other PCI to PCI bridges
> > 
> > In order to handle migration, the vPHBs will need to be represented in
> > the domain XML, which will also allow the user to override this
> > topology if they want.
> 
> We'll have to decide how to represent them in the XML, but
> that's basically your average bikeshedding.

Right.  Maybe we'd best get started with it, in the hopes of finishing
it in the forseeable future.

> Overall, the plan seems entirely reasonable to me.
> 
> It's pretty clear at this point that pseries guest are
> different enough in their handling of PCI that none of
> the address allocation algorithms currently implemented
> in libvirt could be quite adapted to work with it, so
> a custom one is in order.

Yes, that was my conclusion as well.

-- 
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
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