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Date: Wed, 23 May 2018 00:44:22 +0300
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: Re: [Qemu-devel] [RFC 3/3] acpi-build: allocate mcfg for multiple
 host bridges
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To: Alex Williamson <alex.williamson@redhat.com>
Cc: Laszlo Ersek <lersek@redhat.com>, Marcel Apfelbaum <marcel.apfelbaum@gmail.com>, Zihan Yang <whois.zihan.yang@gmail.com>, qemu-devel@nongnu.org, Igor Mammedov <imammedo@redhat.com>, Eric Auger <eauger@redhat.com>, Drew Jones <drjones@redhat.com>, Wei Huang <wei@redhat.com>

On Tue, May 22, 2018 at 03:36:59PM -0600, Alex Williamson wrote:
> On Tue, 22 May 2018 23:58:30 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote: 
> >
> > It's not hard to think of a use-case where >256 devices
> > are helpful, for example a nested virt scenario where
> > each device is passed on to a different nested guest.
> >
> > But I think the main feature this is needed for is numa modeling.
> > Guests seem to assume a numa node per PCI root, ergo we need more PCI
> > roots.
> 
> But even if we have NUMA affinity per PCI host bridge, a PCI host
> bridge does not necessarily imply a new PCIe domain.

What are you calling a PCIe domain?

>  Nearly any Intel
> multi-socket system proves this.  Don't get me wrong, I'd like to see
> PCIe domain support and I'm surprised edk2 is so far from supporting
> it, but other than >256 hotpluggable slots, I'm having trouble coming
> up with use cases.  Maybe hotpluggable PCI root hierarchies are easier
> with separate domains?  Thanks,
> 
> Alex