Re: [Qemu-devel] [PATCH RFC] docs: add PCIe devices placement guidelines

["Daniel P. Berrange" <berrange@redhat.com>]

On Mon, Sep 05, 2016 at 06:24:48PM +0200, Laszlo Ersek wrote:
> On 09/01/16 15:22, Marcel Apfelbaum wrote:
> > +2.3 PCI only hierarchy
> > +======================
> > +Legacy PCI devices can be plugged into pcie.0 as Integrated Devices or
> > +into DMI-PCI bridge. PCI-PCI bridges can be plugged into DMI-PCI bridges
> > +and can be nested until a depth of 6-7. DMI-BRIDGES should be plugged
> > +only into pcie.0 bus.
> > +
> > +   pcie.0 bus
> > +   ----------------------------------------------
> > +        |                            |
> > +   -----------               ------------------
> > +   | PCI Dev |               | DMI-PCI BRIDGE |
> > +   ----------                ------------------
> > +                               |            |
> > +                        -----------    ------------------
> > +                        | PCI Dev |    | PCI-PCI Bridge |
> > +                        -----------    ------------------
> > +                                         |           |
> > +                                  -----------     -----------
> > +                                  | PCI Dev |     | PCI Dev |
> > +                                  -----------     -----------
> 
> Works for me, but I would again elaborate a little bit on keeping the
> hierarchy flat.
> 
> First, in order to preserve compatibility with libvirt's current
> behavior, let's not plug a PCI device directly in to the DMI-PCI bridge,
> even if that's possible otherwise. Let's just say
> 
> - there should be at most one DMI-PCI bridge (if a legacy PCI hierarchy
> is required),

Why do you suggest this ? If the guest has multiple NUMA nodes
and you're creating a PXB for each NUMA node, then it looks valid
to want to have a DMI-PCI bridge attached to each PXB, so you can
have legacy PCI devices on each NUMA node, instead of putting them
all on the PCI bridge without NUMA affinity.

> - only PCI-PCI bridges should be plugged into the DMI-PCI bridge,

What's the rational for that, as opposed to plugging devices directly
into the DMI-PCI bridge which seems to work ?

Regards,
Daniel
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