[libvirt] Proposal PCI/PCIe device placement on PAPR guests
David Gibson
david at gibson.dropbear.id.au
Thu Jan 12 22:57:36 UTC 2017
On Thu, Jan 12, 2017 at 11:31:35AM +0100, Andrea Bolognani wrote:
> On Mon, 2017-01-09 at 10:46 +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?
>
> Because some applications using libvirt would inevitably
> start relying on the fact that such spare PHBs are
> available, locking us into providing at least the same
> number forever. In other words, increasing the amount at
> a later time is always possible, but decreasing it isn't.
> We did the same when we started automatically adding PCIe
> Root Ports to q35 machines.
>
> The rationale is that having a single spare hotpluggable
> slot is extremely convenient for basic usage, eg. a simple
> guest created by someone who's not necessarily very
> familiar with virtualization; on the other hand, if you
> are actually deploying in production you ought to conduct
> proper capacity planning and figure out in advance how
> many devices you're likely to need to hotplug throughout
> the guest's life.
Hm, ok. Well I guess the limitation is the same as on x86, so it
shouldn't surprise people.
> Of course this all will be moot once we can hotplug PHBs :)
Yes. Unfortunately, nobody's actually working on that at present.
--
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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