Re: [Qemu-devel] QMP, HMP: introduce 'writeconfig' command

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Vadim Galitsyn (vadim.galitsyn@profitbricks.com) wrote:
> Hi Guys,
> 
> This thread is a continuation of discussion started in
> http://lists.nongnu.org/archive/html/qemu-devel/2017-02/msg03182.html.
> 
> This series introduces ‘writeconfig’ command support for QMP and HMP monitors. This functionality might be useful for live migration for cases when guest configuration was modified in runtime (for example as a result of hot- plug/unplug operations) and actual Qemu command line no longer reflects setup exposed to guest.
> 
> Original series has ‘qemu_opts’ patch as well (http://lists.nongnu.org/archive/html/qemu-devel/2017-02/msg03183.html) because HMP’s ‘object_add’ result was not reflected in ‘writeconfig’ output. Later I found that QMP’s ‘object-add’ has the same issue. Anyway, I don’t include ‘qemu_opts’ patches here because Markus mentioned (here http://lists.nongnu.org/archive/html/qemu-devel/2017-02/msg03476.html) that this functionality is going to be reworked in some future and such patches might collide with the rework process.
> 
> Markus, could you please post if you have an update on this topic? Current ‘master’ branch (9993c82dc2f5ce58b41d708b765e1a717ad4281d) still has the issue.
> 
> Also, Markus mentioned that once configuration was changed during live migration -- it might be an issue because ‘writeconfig’ data became outdated (and might be make sense to think about to embed this data into migration stream itself). In the same time David said that this is another problem which is unrelated to this patch series. What is your current opinion on this topic? Can we consider these patches to be included into ‘master’ taking into account that not all configuration is dumped by ‘writeconfig’ (‘object_add’ problem), but this can be fixed later?

I don't see anything wrong with having a 'writeconfig' command - we
already have the command line equivalent - that is assuming hotplug
etc all cause the values written to be correct.

The data becoming outdated doesn't sound like a big issue to me;
it's the management layer that's doing any hotplugs, it can reissue
another 'writeconfig' command.

Dave

> 
> Best regards,
> Vadim
> 
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK