On 6/12/09, Mark McLoughlin <markmc@xxxxxxxxxx> wrote: > On Fri, 2009-06-12 at 12:00 -0500, Anthony Liguori wrote: > > Mark McLoughlin wrote: > > > So, when libvirt creates a guest for the first time, it makes a copy of > > > the device tree and continues to use that even if qemu is upgraded. > > > That's enough to ensure compat is retained for all built-in devices. > > > > > > However, in order to retain compat for that SCSI device (e.g. ensuring > > > the PCI address doesn't change as other devices are added an removed), > > > we're back to the same problem ... either: > > > > > > 1) Use '-drive file=foo.img,if=scsi,pci_addr=foo'; in order to figure > > > out what address to use, libvirt would need to query qemu for what > > > address was originally allocated to device or it would do all the > > > PCI address allocation itself ... or: > > > > > > 2) Don't use the command line, instead get a dump of the entire > > > device tree (including the SCSI device) - if the device is to be > > > removed or modified in future, libvirt would need to modify the > > > device tree > > > > > > The basic problem would be that the command line config would have very > > > limited ability to override the device tree config. > > > > > > > After libvirt has done -drive file=foo... it should dump the machine > > config and use that from then on. > > Right - libvirt then wouldn't be able to avoid the complexity of merging > any future changes into the dumped machine config. > > > To combined to a single thread... > > > How do you add a new attribute to the device tree and, when a supplied > > > device tree lacking said attribute, distinguish between a device tree > > > from an old version of qemu (i.e. use the old default) and a partial > > > device tree from the VM manager (i.e. use the new default) ? > > > > > > > Please define "attribute". I don't follow what you're asking. > > e.g. a per-device "enable MSI support" flag. > > If qemu is supplied with a device tree that lacks that flag, does it > enable or disable MSI? > > Enable by default is bad - it could be a device tree dumped from an old > version of qemu, so compat would be broken. > > Disable by default is bad - it could be a simple device tree supplied by > the user, and the latest features are wanted. > > Maybe we want a per-device "this is a complete device description" flag > and if anything is missing from a supposedly complete description, the > old defaults would be used. A config dumped from qemu would have this > flag set, a config generated by libvirt would not have the flag. If the device has different behavior or different properties from guest perspective compared to the old device, it should get a new device type so that you could specify in the device tree either the old device or the new one. Flags won't help in the long term. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
