Re: [libvirt] [PATCH 0/12] Standardized device addressing & SCSI controller/disk hotplug

On Fri, Dec 18, 2009 at 11:17:59AM +0000, Mark McLoughlin wrote: > On Thu, 2009-12-17 at 09:39 +0000, Daniel P. Berrange wrote: > > On Thu, Dec 10, 2009 at 10:22:20PM +0000, Daniel P. Berrange wrote: > > > The XML for each address type looks like > > > > > > <address type='pci' mode='static' domain='0x0000' bus='0x1e' slot='0x07' function='0x0'/> > > > <address type='usb' mode='dynamic' bus='007' dev='003'/> > > > <address type='drive' mode='dynamic' controller='1' bus='0' unit='5'/> > > > > > > > > > The 'mode' attribute for any of them is allowed to be either > > > 'static' or 'dynamic'. A static address is one specified by > > > the end user when defining the XML, while a dynamic address is > > > one automatically chosen by libvirt/QEMU every time a guest > > > boots. The idea of static addresses is to allow management > > > apps to guarentee that PCI device & drive numbering never > > > changes. This series does not actually implement static > > > addressing for PCI yet, because it requires that we change > > > the way we generate QEMU command line arguments. It does > > > do static addressing for disks. > > > > I'm now wondering whether we actually truely need the 'dynamic' > > option. It already doesn't really make sense for disks. For PCI > > though I'm not sure anyone would ever want the dynamic mode, > > where addresses can change at every boot. I think I should just > > remove the 'mode' attribute here, and just have libvirt detect > > PCI addresses the first time, and keep them the same thereafter. > > Just reading the proposal now and this is the bit that jumps out at me > most. I don't think we need the dynamic/static mode split. > > For PCI, we really want to just automatically remember assigned PCI > addresses and use them forever after - I don't see any obvious use case > where you wouldn't want this. > > Also, it strikes me that we've done this before - e.g. if you define a > guest with an interface with no explicit MAC address, we generate one > and save it in the config. Which is pretty much the same thing from the > users point of view. The thing I'm trying to avoid is having libvirt update the persistent config files outside the context of the existing virDefineXML API,

and updating the live state file only helps for the duration the that VM is running. Hence I came up with the static vs dynamic idea, where the app would see the dynamic address, dump the XML and re-define it with static addresses. The only other option I see is to explicitly assign all PCI addresses at time of virDefineXML, and then always pass an address on the command line to QEMU for all devices, but this doesn't work for older QEMU where we can't set PCI addresses.