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

[Anthony Liguori <anthony@codemonkey.ws>]

Mark McLoughlin wrote:
> On Mon, 2009-06-15 at 07:48 -0500, Anthony Liguori wrote:
>   
>>> Eventually the 
>>> default configuration becomes increasingly unusable and you need a new 
>>> baseline.  You must still be able to fall back to the old baseline for 
>>> older guests.  I don't think games with configuration files can hide 
>>> that.
>>>       
>> -M pc1
>> -M pc2
>>
>> etc.
>>
>> This is pretty easy to maintain with config files.
>>     
>
> I think this would be reasonable, but it is essentially just a version
> number which you objected to on the basis that it would make
> cherry-picking harder for distros.
>   

It doesn't have to be pc1, pc2.  It could be pc-with-usb or 
pc-with-balloon.  If a distro cherry picks in such a way that their pc 
is not a standard QEMU pc, they would add a new PC type that's specific 
to their distro.

> One thing that would be nice with this '-M pc1' thing would be to retain
> '-M pc' as a symlink to the latest version. We'd also need a way to read
> the symlink too, so that you can query what the current latest version
> is and use that in future.
>   

Another option is an explicit -M default which always uses the default 
machine for the architecture.  Likewise, we would need a way to query 
what the default machine was for an architecture.

> How would this machine type version relate to e.g. changing the default
> PCI class of virtio-blk? Would we bump the version number of all machine
> types can use virtio-blk?
>   
You would introduce a new machine type.  For instance, 
pc-virtio-class-other.  The names don't have to look like that, I'm just 
doing that to make a point.  This may mean that you end up with dozens 
of machine types but you preserve compatibility, which is a good thing.

Of course, the flip side is that you make preserving the machine config 
the duty of the user and we don't maintain compatible machine types.  
This won't work without a proper config file though so for now, we're 
stuck maintaining machine types.

Regards,

Anthony Liguori