Re: [Qemu-devel] [PATCH] ppc/pnv: Add model for Power8 PHB3 PCIe Host bridge

[Andrea Bolognani]

On Thu, 2018-06-28 at 13:59 +1000, David Gibson wrote:
> On Wed, Jun 27, 2018 at 12:22:31PM +0200, Andrea Bolognani wrote:
> > On Tue, 2018-06-26 at 19:02 +0200, Cédric Le Goater wrote:
> > > I didn't follow that discussion but this is "another" kind of PHB.
> > > This one models the baremetal controller as found on OpenPOWER and
> > > IBM Power machines. pSeries has a virtual PHB.
> > 
> > I understand that, and of course libvirt will need to learn about
> > this new type of PHB and make sure both pSeries and PowerNV guests
> > get the correct one assigned to them.
> 
> Hmm.. does it?  I would have thought pnv could act more like x86, in
> that libvirt doesn't attempt to create PHBs at all and just use the
> ones that are built in.

AFAIK x86 guests have a single PHB and additional ones cannot be
created in any way, which means we don't have to do any additional
second-guessing when assigning IDs to additional PCI controllers.

> Though, come to that, I wouldn't think pnv support for libvirt would
> be much of a priority anyway.  The machine type is still very much in
> flux, and it's designed primarily for testing and development, not
> "real world" usage.

Can you *guarantee* that someone won't ask for PowerNV support in
libvirt at some point? Because if you can't (and I don't think you
can ;) then this is still a valuable conversation to have.

> > What I meant is that pSeries guests get a single PHB by default,
> > with additional ones being instantiable through -device; this is
> > also consistent with how PCI controllers are added to other guest
> > types including pc, q35 and aarch64/virt, so it would be really
> > nice if PowerNV behaved the same way.
> 
> Well.. sure.. but it doesn't.  pSeries is a virtual platform, so we
> have a reasonable amount of flexibility to define it as we want.
> PowerNV is an emulation of existing hardware which has a specific
> behaviour which we need to match.

Sure, that's something to keep in mind.

But the thing is, you still need to have *some* flexibility in
the number of PHBs, since there is variation among real Power8
and Power9 chips; in the current incarnation, that flexibility
is provided by the num_phbs parameter, which is an entirely new
interface that's exclusive to PowerNV.

What I'm suggesting is that the same amount of flexibility is
offered through a standard interface, namely -device, instead.

-- 
Andrea Bolognani / Red Hat / Virtualization