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

[Eric Blake <eblake@redhat.com>]

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On 10/23/2017 10:13 AM, Vadim Galitsyn 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that once configuration was changed during live migration -- it might be an issue because ‘writeconfig’ data became outdated (and might be make sense to think about to embed this data into migration stream itself).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 think we should expose 'writeconfig' via QMP as long as there is
still the chance of inconsistent data being written.  And I think we
have a lot more issues where existing code abuses QemuOpts in ways that
current configuration does not match the original command line, but
where you cannot easily expose the current configuration in a way that
would be reparsed by the command line into the current state.
Therefore, I'm not sure this series is worthwhile.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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