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

[Eric Blake]

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 migration -- it might be an issue because ‘writeconfig’ data became outdated 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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