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

[Anthony Liguori]

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.

>> I assume the issue with making it a run time option is that libvirt
>> would need to be updated?
>
> You cannot update libvirt, because whether to pass the flag or not
> depends on the _source_ QEMU, which may be an ancient version we cannot
> add any patch to.
>
>> Why can't qemu-kvm be a script wrapper that
>> passes a fixed flag?
>
> See Cole's answer; qemu-system-* also used to be built from qemu-kvm
> sources in Fedora.

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.

Regards,

Anthony Liguori

>
> Paolo