Re: [Qemu-devel] [PATCH 1/4] configure: Add --enable-migration-from-qemu-kvm

[Paolo Bonzini]

Il 20/02/2013 22:03, Anthony Liguori ha scritto:
> Paolo Bonzini <address@hidden> writes:
> 
>> Il 20/02/2013 21:26, Anthony Liguori ha scritto:
>>> Cole Robinson <address@hidden> writes:
>>>
>>>> This switch will turn on all the migration compat bits needed to
>>>> perform migration from qemu-kvm to qemu. It's just a stub for now.
>>>>
>>>> This compat will break incoming migration from qemu < 1.3, but for
>>>> distros where qemu-kvm was the only shipped package for years it's
>>>> not a big loss (and I don't know any way to avoid it).
>>>>
>>>> Signed-off-by: Cole Robinson <address@hidden>
>>>
>>> This can't be a build time option.  It's ugly and just reintroduces a fork.
>>
>> It is not a fork; it does not change the migration format of QEMU
>> itself, only the parsing of old VMState versions.  The importance of the
>> fork will dwindle over time, as the distros EOL the releases that used
>> qemu-kvm.
> 
> Once a distro enables CONFIG_QEMU_KVM, it must keep it enabled forever.
> So the distros will contineut o do CONFIG_QEMU_KVM whereas upstream does
> not set it.  It's a fork all within the same tree.

They will set it for a while.  For example, F18 is the last Fedora
release that had qemu-kvm < 1.3.0.  As soon as F18 goes end-of-life
(which is with F21), Fedora will not need to set the configure option
anymore.

Of course other distros will keep it for years rather than months, but
the idea is the same.

> What about having something processed by readconfig that wasn't disabled
> by -nodefaults?  Then we could make it a runtime option, but that's
> configurable globally.  Then the distros can choose the default value in
> the config file.
> 
> At least then, we can write unit tests with the option at run time,
> don't need to sprinkle #ifdefs everywhere, etc.

That would make sense, but it shouldn't block this patch.  What would
happen otherwise is that distros (who care about migration
compatibility) will just patch their code without even the #ifdefs.
It's already happening, these patches are in F18.

Paolo