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

[Mark McLoughlin <markmc@redhat.com>]

On Wed, 2009-06-10 at 20:27 +0100, Jamie Lokier wrote:
> Michael S. Tsirkin wrote:
> > > I think the right long term answer to all this is a way to get QEMU to
> > > dump it's current machine configuration in glorious detail as a file
> > > which can be reloaded as a machine configuration.
> > 
> > And then we'll have the same set of problems there.
> 
> We will, and the solution will be the same: options to create devices
> as they were in older versions of QEMU.  It only needs to cover device
> features which matter to guests, not every bug fix.
> 
> However with a machine configuration which is generated by QEMU,
> there's less worry about proliferation of obscure options, compared
> with the command line.  You don't necessarily have to document every
> backward-compatibility option in any detail, you just have to make
> sure it's written and read properly, which is much the same thing as
> the snapshot code does.

This is a sensible plan, but I don't think we should mix these compat
options in with the VM manager supplied configuration.

There are two problems with that approach.

= Problem 1 - VM manager needs to parse qemu config =

Your proposal implies:

  - VM manager supplies a basic configuration to qemu

  - It then immediately asks qemu for a dump of the machine 
    configuration in all its glorious detail and retains that
    config

  - If the VM manager wishes to add a new device it needs to parse the 
    qemu config and add it, rather than just generate an entirely new 
    config

= Problem 2 - We can't predict the future =

If a VM manager supplies a configuration which is missing any given
option, qemu cannot tell the difference between:

  - This is a basic config, the VM manager wants whatever the default 
    of the current qemu version

  - This is a complete config dumped using an old version of qemu, the 
    VM manager wants the old default

= Solution - Separate configuration from compat hints =

As I suggested before:

  - Allow the VM manager to dump compat hints; this would be an opaque 
    file format, more like the savevm format than a config file

  - Use defaults where compat hints are not available; e.g. if the VM 
    manager specifies a device config, but no compat hints are 
    supplied for it, then just use default values

  - Make the config override compat hints; e.g. if there are compat 
    hints specified for a device not included in the machine config, 
    just ignore those hints

Cheers,
Mark.