Re: [Qemu-devel] [RFC 3/3] acpi-build: allocate mcfg for multiple host bridges

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, May 22, 2018 at 03:47:41PM -0600, Alex Williamson wrote:
> On Wed, 23 May 2018 00:44:22 +0300
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > 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?
> 
> Domain/segment
> 
> 0000:00:00.0
> ^^^^ This

Right. So we can thinkably have PCIe root complexes share an ACPI segment.
I don't see what this buys us by itself.

> Isn't that the only reason we'd need a new MCFG section and the reason
> we're limited to 256 buses?  Thanks,
> 
> Alex

I don't know whether a single MCFG section can describe multiple roots.
I think it would be certainly unusual.

-- 
MST