Re: [Qemu-devel] [PATCH 3/3 v2] Add a pc-0-10 machine type for compatibility with 0.10.x

[Gerd Hoffmann <kraxel@redhat.com>]

On 07/08/09 17:08, Mark McLoughlin wrote:
> On Wed, 2009-07-08 at 16:09 +0200, Gerd Hoffmann wrote:
>>> /usr/share/qemu/configs/pc-0-10.dts
>>> /usr/share/qemu/configs/pc-0-11.dts
>>> /usr/share/qemu/configs/pc ->  /usr/share/qemu/configs/pc-0-11.dts
>> Doesn't work.  Your virtio blk device isn't in there.
>
> Right - I had assumed Anthony meant that such a config file would also
> have details on the defaults used for devices which can be added to that
> machine.

Could probably be done that way, but that wouldn't be different from a 
global compat variable where the virtio-block-pci driver looks at.

We have different versions of devices.  We should model them that way. 
I'd start with a global compat_level.  We might switch to a per-device 
compat level later if there is a need.  With qdev it should be easy once 
the devices are converted.  I think it will be much more better 
maintainable than adding new devices because there is a standardized way 
to switch a driver to an older version instead of having to know "oh, 
that driver has these compat versions, but this one hasn't".

virtio blk+console will switch class for compat_level < 0.11.  virtio 
net will disable msi for compat_level < 0.11.  ide might adapt the 
firmware version for the virtual disk/cdrom to the compat level. 
Everybody else is free to ignore it.

Note that different machine config versions might make sense 
nevertheless in case the set of (default) devices changes.  i.e. if we 
turn on usb by default we might add a compat machine type without usb.

cheers,
   Gerd