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

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