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Re: [libvirt] [PATCH v4 4/4] auto-create pci-bridge controller info

On 03/04/2013 11:51 AM, Ján Tomko wrote:
> On 02/19/13 03:25, liguang wrote:
>> if some devices specify a pci bus number that
>> haven't been defined by a pci-bridge controller
>> then fill the required correct controller info
>> silently.
>> Acked-by: Daniel P. Berrange <berrange redhat com>
>> Signed-off-by: liguang <lig fnst cn fujitsu com>
>> ---
> This will only add bridges for the explictly mentioned buses, which
> would mean we could have buses 0 and 6 with no buses between them.
> Maybe we should add them the way we add disk controllers - find the
> highest index and add all bridges with indexes from 1 to max.

But each pci bridge device takes up another slot, which we may not want
to give up. (Although arguably, once we have pci-bridge devices, we can
create as many slots as we like :-)

Is there some advantage to having all the buses filled in (other than
similarity to what's done for other types of controllers)?

> It would be nice if we could add pci bridges even when there weren't any
> specified in the XML, but there are too many PCI devices. I don't know
> what would be the nicest way to do that.

If we auto-assign addresses for un-addressed devices first (your recent
patches did that, right? I forget the status of those), *then*
auto-create the required bridges (which themselves will need an
auto-assigned address), that should just happen.

Of course, if we do that, then we won't have reserved any slots on bus 0
for even a single pci-bridge device, so we'll fail. Is the proper way
out of this to always reserve one (or maybe two, for good measure) slots
on any given bus to be used only for bridges? That way no matter how out
of hand things get, we'll always have a place we can add another bus.
(In the case of *lots* of devices, I assume that a device on a nested
PCI bridge would be less efficient, and may have some other limitations,
but it would be better than nothing. And if the admin was really
concerned about that, they could modify their config to explicitly place
several bridges directly on bus 0)

This also brings up the fact that we need to deal with the possibility
of all buses being full during hotplug - when that happens I suppose we
will also want to auto-add a pci bridge.

>  Maybe Peter's patches for
> config parser defaults will be helpful:
> https://www.redhat.com/archives/libvir-list/2013-March/msg00042.html

A circular reference of data definitions led to danpb saying that this
is probably the wrong way to do it, so it looks like it will require
some rework, but definitely whatever shape that takes, this code will
want to fit into that framework.

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