Re: Configuration vs. compat hints [was Re: [Qemu-devel] [PATCHv3 03/13] qemu: add routines to manage PCI capabilities]

On 6/12/09, Mark McLoughlin [off-list ref] 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.