Re: [Qemu-devel] [PATCH v2] ahci: add -drive support

[Markus Armbruster]

Gerd Hoffmann <address@hidden> writes:

>   Hi,
>
>>> If -hda has the semantics of "create an IDE device", then no, we can't 
>>> change it. It doesn't however. IIRC on -M pseries -hda creates SCSI 
>>> devices. On s390 -hda creates virtio devices. So if on -M q35 -hda would 
>>> create if=ahci, I don't see how that breaks the CLI.
>> 
>> It doesn't per se, that is as long as you need to explicitly specify -M
>> q35. But then changing the default machine from the existing pc to q35
>> would break the command line.
>
> That will happen _anyway_.  No doubt there will be configurations which
> work with -M pc and break with -M q35.

Yup.

Without -M, you get whatever the developer picked for you.

If we took backward compatibility dogmatically serious, the default
would still be pc-0.10.  Fortunately, we don't.  We pick something we
think makes sense for most users, in particular casual users unaware of
-M.  Whether and when that will become some q35 machine will be debated
when q35 works.

If you want the best possible compatibility, use -M FOO, where FOO is
whatever machine type you've been using (machine type, not alias,
i.e. -M pc-1.2, not -M pc).