[libvirt] [Qemu-ppc] [RFC PATCH qemu] spapr_pci: Create PCI-express root bus by default

[David Gibson]

On Fri, Nov 18, 2016 at 09:17:22AM +0100, Andrea Bolognani wrote:
> On Thu, 2016-11-17 at 13:02 +1100, Alexey Kardashevskiy wrote:
> > > That said, considering that a big part of the PCI address
> > > allocation logic is based off whether the specific machine
> > > type exposes a legay PCI Root Bus or a PCI Express Root Bus,
> > > libvirt will need a way to be able to tell which one is which.
> > > 
> > > Version checks are pretty much out of the question, as they
> > > fail as soon as downstream releases enter the picture. A
> > > few ways we could deal with the situation:
> > > 
> > >   1) switch to PCI Express on newer machine types, and
> > >      expose some sort of capability through QMP so that
> > >      libvirt can know about the switch
> 
> [...]
> > > Option 1) would break horribly with existing libvirt
> > > versions, and so would Option 2) if we default to using
> > 
> > How exactly 1) will break libvirt? Migrating from pseries-2.7 to
> > pseries-2.8 does not work anyway, and machines are allowed to behave
> > different from version to version, what distinct difference will using
> > "pseries-pcie-X.Y" make?
> 
> Existing libvirt versions assume that pseries guests have
> a legacy PCI root bus, and will base their PCI address
> allocation / PCI topology decisions on that fact: they
> will, for example, use legacy PCI bridges.

Um.. yeah.. trouble is libvirt's PCI-E address allocation probably
won't work for spapr PCI-E either, because of the weird PCI-E without
root complex presentation we get in PAPR.

> So if you used a new QEMU binary with a libvirt version
> that doesn't know about the change, new guests would end up
> using the wrong controllers. Existing guests would not be
> affected as they would stick with the older machine types,
> of course.
> 
> > I believe after we introduced the very first
> > pseries-pcie-X.Y, we will just stop adding new pseries-X.Y.
> 
> Isn't i440fx still being updated despite the fact that q35
> exists? Granted, there are a lot more differences between
> those two machine types than just the root bus type.

Right, there are heaps of differences between i440fx and q35, and
reasons to keep both updated.  For pseries we have neither the impetus
nor the resources to maintain two different machine type variant,
where the only difference is between legacy PCI and weirdly presented
PCI-E.

> Even if no newer pseries-x.y were to be added after
> introducing pseries-pcie, you could still easily create
> guests that use either root bus, so no loss in functionality.
> 
> -- 
> Andrea Bolognani / Red Hat / Virtualization
> 

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson
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