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). -- libvir-list mailing list libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list