Re: proposal: deprecate -readconfig/-writeconfig

[Paolo Bonzini <pbonzini@redhat.com>]

On 14/05/20 10:56, Daniel P. Berrangé wrote:
> On Thu, May 14, 2020 at 10:09:21AM +0200, Paolo Bonzini wrote:
>> IMHO configuration files are in general a failed experiment.  In
>> practice, they do not add much value over just a shell script because
>> they don't allow configuring all QEMU options, they are very much fixed
>> (by their nature).  I think it's more or less agreed that they are not
>> solving any problem for higher-level management stacks as well; those
>> would prefer to configure the VM via QMP or another API.
>>
>> So, any objections to deprecating -readconfig and -writeconfig?
> 
> Libvirt would like to have a config file for QEMU, but it would have
> to be one that actually covers all the config options QEMU supports,
> and ideally using a data format in common with that used for runtime
> changes. So for libvirt's needs the current readconfig is entirely
> useless.

Yes, this is what I was thinking about.

> For a less general purpose mgmt app, that targets some specific use
> cases I could imagine people might have used readconfig. [...]
> If we deprecate them, the only alternative users have right now is
> to go back to passing CLI args. [...] We'd be deciding to kill the
> feature with no direct replacement, even though it is potentially
> useful in some limited scenarios.
> 
> If we have a general strategy to eliminate QemuOpts and move entirely
> to QAPI based config, then I can see -readcofig/-writeconfig may be
> creating a burden of back compatibility on maintainers.

I don't see QemuOpts going away anytime soon, but I do see more QMP/QAPI
and less command line in the future as far as management tools are
concerned.  QemuOpts and HMP will remain for direct usage, for the
foreseeable future.

So I guess we can keep -readconfig but deprecate, or even remove,
-writeconfig.

Paolo