[libvirt] [PATCH] allow memballoon type of none to desactivate it

[Daniel P. Berrange]

On Tue, Aug 10, 2010 at 12:09:47AM +0200, Daniel Veillard wrote:
>   Mail disapeared when trying to send it... sending again ...
> 
> On Mon, Aug 09, 2010 at 07:38:32PM +0100, Daniel P. Berrange wrote:
> > On Mon, Aug 09, 2010 at 08:34:23PM +0200, Daniel Veillard wrote:
> > >   Grumpf ... that mean at the internal stucture level we need to add an
> > > extra field, that is detected at a completely different time in parsing
> > > too ... more complex in general, but I can understand the purity POV.
> > 
> > I don't see this as a real problem. It is no  different from the way that
> > we automatically add <controller> devices at the end of parsing, if we saw
> > any <disk> or <channel> devices previously.
> 
>   Huh ??? You can have a controller without a disk and you don't
> automagically add a controller even if nothing was suggested by the
> user, it is widely different. First you need to keep a tristate for
> def->balloonable, was that attribute defined, or not, then add the
> error case if it's not "yes" or "no", then you need to care for
> all the different cases of the tristate when you actually parse
> the devices section, how do you handle <memballon> definition when
> balloonable="no" was found, discard, error ?

We'd raise an error becasue the requested config doesn't make sense.

It doesn't actually need to be a tri-state - ballooning is either
enabled or not. If a user hasn't specified which in the XML, then
we'd pick the hypervisor default for that.

> I find this different because you provide the information in 2 places
> at different levels and you ned to cope with conflict ... while still
> maintaining the current behaviour of autoadding.
> This then breaks nearly all the qemu XML tests due to the extra
> balloonable="yes" added automatically to the output since the qemu
> driver makes that a default.

Yep, that's an unavoidable side-effect of adding more default data
to our XML output, but we've hit this many times in the past. eg
adding disk controllers, required us to change every single test
datafile.

> And now you have to explain in documentation all the various cases
> instead of a simple one liner about using "none".
> 
> When data is defined at 2 different places by design, well you designed
> a mess. And in that case that's cerainly true. And after looking at the
> situation in detail, no I don't by the "purity" point of view, this
> actually make things *harder* both for user and from a implementation
> point of view.

>   I firmly think that in that case the proper way to do this is to
> keep the information in one place and that place is the <memballoon>
> device, type='none' may look ugly to you, but at least the behaviour
> will be predictable, users will know where to look and that can be
> explained to the in a single line/sentence.

Neither option is keeping all the information in one place.
We already have a four-way split of memory information with
<memory> vs <currentMemory>, <memoryBacking> and now <memballoon>.
The first three are defining attributes of the VM itself, while
the latter is defining information about the device - specifically
we added it to provide info about PCI device IDs & the device alias.
Whether a guest domain supports ballooning or not is an attribute 
of the domain itself. If there is no balloon, then there is no
device to provide information on, so none of the data that appears
under <memballoon> even exists, nor can you pass this device
element to the hotplug APIs, because there's no actal device
behind it. This makes the <memballoon> element non-sensical to
me.

>   Current patch provided, I strongly suggest to not continue down this
> line and apply the previous patch instead.

To be fair, I don't actually like either patch, I'm just going for
what I think is the least worst option long term. If we could go 
back in time, I'd never have enabled the balloon device by default
in the first place and required an explicit <memballoon>.

I'd actually prefer to just ignore this problem completely, and
declare that all KVM guest must always have a balloon and not 
provide any way to disable it. The issue of limited PCI slots is
really a QEMU flaw - it should be providing a PCI bridge support
so you can have a much less limited number of PCI slots.

Regards,
Daniel
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