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

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).