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

[Laine Stump]

On 03/07/2013 01:58 PM, Daniel P. Berrange wrote:
> On Thu, Mar 07, 2013 at 01:34:16PM -0500, Laine Stump wrote:
>> On 03/06/2013 10:23 AM, Daniel P. Berrange wrote:
>>> On Wed, Mar 06, 2013 at 04:15:28PM +0100, Ján Tomko wrote:
>>>> 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?
>> I'm guessing that putting things behind a bridge might reduce
>> performance somewhat? (or maybe it's a completely artificial concept
>> within qemu that only exists for the sake of making things look right to
>> the guest, I just don't know)
>>
>>> I don't think we should artifically reserve anything that isn't
>>> absolutely required by the underlying machine type.
>> Yeah, I don't think we should reserve an entire bus. However, when
>> auto-assigning slots, I think we should make a "best effort" to always
>> leave at least one slot open at the end. This will ensure that anyone
>> hotplugging bunches of devices with no explicit pci address will succeed
>> (although their final topology may be suboptimal).
> I worry that trying to keep one slot open is going to break existing
> users which may be filling up their slots 100%, but not have a new
> enough QEMU for bridge support. We need to be very careful not to
> regress for anyone when changing this address assignemnt

Sure. This should be one of those "post-parse postprocessing" things
that is aware of qemu's capabilities. If there's no support for the
pci-bridge device, it won't even do the 2 passes, and won't have any
reason to try and keep a slot open (except maybe in anticipation of a
future where the host's qemu *does* support pci-bridge, but I think
that's beyond the scope of "best effort" :-)

And of course any existing domain will already have had addresses
assigned to all devices, so we wouldn't be able to break them (although
I guess that means nothing for transient domains).