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Re: [libvirt] [PATCH 4/7] add pci-bridge controller type

On Fri, Apr 05, 2013 at 12:32:04PM -0400, Laine Stump wrote:
> On 04/03/2013 11:50 AM, Ján Tomko wrote:
> > From: liguang <lig fnst cn fujitsu com>
> >
> > add a new controller type, then one can
> > define a pci-bridge controller like this:
> >     <controller type='pci-bridge' index='0'/>
> In the next patch we're prohibiting exactly this config (index='0')
> because the pre-existing pci bus on the "pc-*" machinetypes is already
> named pci.0. If we don't allow it, we shouldn't include it as an example
> in the commit log :-)

NB, it isn't always named 'pci.0' - on many arches it is merely 'pci'.

> More on this - one of the things this points out is that there is no
> representation in the config of the pci.0 bus, it's just assumed to
> always be there. That is the case for pc-* machinetypes (and probably
> several others with PCI buses), but for q35, there is no pci.0 bus in
> the basic machine, only a pcie.0; if you want a pci.0 on q35 (which
> *will* be necessary in order to attach any pci devices, so I imagine we
> will always want one), you have to attach a pcie->pci bridge, which is
> the device "i82801b11-bridge", to pcie.0.

> The reason I bring this up here, is I'm wondering:
> 1) should we have some representation of the default pci.0 bus in the
> config, even though it is just "always there" for the pc machinetypes
> and there is no way to disable it, and nothing on the commandline that
> specifies its existence?

Yep, we should be aiming for the XML to fully describe the machine
hardware. So since we're adding the concept of PCI controllers/bridges
etc to the XML, we should be auto-adding the default bus to the XML.

> 2) For the q35 machinetype, should we just always add an
> i82801b11-bridge device and name it pci.0? Or should that need to be
> present in the xml?

We've been burnt before auto-adding stuff that ought to have
been optional. So I'd tend towards only having the minimal
config that is required. If the users want this, let them
explicitly ask for the bridge

Also from the apps POV the QEMU device name is irrelevant. The
XML config works off the PCI addresses. So there's no need
to force/specialcase a i82801b11-bridge to use the name

> 3) Most important - depending on the answers to (1) and (2), should we
> maybe name this device "pci", and use a different backend depending on
> index and machinetype? (or alternately explicitly specifiable with a
> <driver> subelement). To be specific, we would have:
>    <controller type='pci' index='0'/>
> which on pc machinetypes would just be a placeholder in the config (and
> always inserted if it wasn't there, for machinetypes that have a pci
> bus). On the q35 machinetype, that same line would equate to adding an
> i82801b11-bridge device (with source defaulting to
> bus=pcie.0,addr=1e.0). This would serve several purposes:
> a) on pc machinetypes, it would be a visual aid indicating that pci.0
> exists, and that index='0' isn't available for a new pci controller.
> b) it would make switching a domain config from pc to q35 simpler, since
> pci.0 would always already be in place for attaching pci devices
> (including pci.1, pci.2, etc)
> c) it would make the config a true complete description of the machine
> being created.
> (I've suggested naming the controller "pci" rather than "pci-bridge"
> because in the case of a "root" bus like pci.0 it seems to not be a
> "bridge", but maybe the name "pci-bridge" is always appropriate, even
> when it's a root bus. Maybe someone with better pci/pcie knowledge can
> provide an opinion on this)

I think "pci" is a little too generic - how about we call it  'pci-root'

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