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

[Greg Kurz <groug@kaod.org>]

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On Fri, 13 Jan 2017 09:57:36 +1100
David Gibson <david@gibson.dropbear.id.au> wrote:

> 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.
> 

Well, there might be someone now :)

Michael Roth had posted a RFC patchset back in 2015:

https://lists.gnu.org/archive/html/qemu-ppc/2015-04/msg00275.html

I'll start from here.

Cheers.

--
Greg

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