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

On 03/05/13 11:36, Daniel P. Berrange wrote:
> On Mon, Mar 04, 2013 at 02:39:40PM -0500, Laine Stump wrote:
>> On 03/04/2013 11:51 AM, Ján Tomko wrote:
>>> 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)

Maybe we could reserve the first bus as well?

> You'd have todo a 2 pass assignment. First count the number of devices
> that need to have PCI addresses assigned. Then create the required
> number of bridges, then assign the addresses.

Which one is better? The reserved slots would make auto-adding bridges
on hotplug possible, but I fear the code would be too ugly. On the other
hand, 2-pass assignment sounds really easy to do :)


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