Re: [Qemu-devel] Proposal PCI/PCIe device placement on PAPR guests

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

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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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