Re: [Qemu-devel] [PATCH] q35: Remove old machine versions

[Eric Blake <eblake@redhat.com>]

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On 09/13/2015 03:28 AM, Michael S. Tsirkin wrote:
> On Fri, Sep 11, 2015 at 03:44:47PM -0300, Eduardo Habkost wrote:
>> Ping?
>>
>> So, what's the reason we are still keeping those old machines in the
>> code?
> 
> Victor also wanted to clean out some very old machine types for
> the PIIX, too.
> 
> But if someone created a machine with libvirt, these machine types
> are now written in the XML. Failing to start guests isn't nice.

New qemu with old libvirt isn't always guaranteed to work. And new
libvirt can be patched to automatically update any old hard-coded
machine name to a newer safe alternative, _when such update is proven
safe_ (we've done it in the past for Fedora: if I recall correctly,
Fedora 13 branched its own machine type, then in Fedora 15 qemu decided
to quit supporting the branch, so libvirt was patched downstream in
Fedora 15-17 to rewrite the old name into its safe upstream counterpart.
 The downstream patch was dropped in Fedora 18 since F15 was EOL by then
so no more new machines would have been created with the old spelling,
and since a year was deemed long enough for people to have either run
their guest to pick up the automatic update, or that their guest was so
infrequently run that they could read the error message and act on it
themselves).

Failing to start guests isn't nice, but it also isn't the end of the
world, when there is no choice but to break ABI.  An explicit ABI break
(by making the user rewrite machine type) is better than silent change
in behavior with a potential for broken guests.

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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