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Subject: Re: [virtio] New virtio balloon...

On Thu, Feb 06, 2014 at 12:12:21PM +1030, Rusty Russell wrote:
> Daniel Kiper <daniel.kiper@oracle.com> writes:
> > On Wed, Feb 05, 2014 at 01:45:38PM +1030, Rusty Russell wrote:
> >> self-ballooning?  It seems like they'll just fight over target values
> >> if they both try to act.
> >
> > Sadly, yep. I know about that issue. Do you mean that we would like to avoid
> > such situation in new VIRTIO balloon driver? Any other reasons?
> To step back a moment: I feel that you, Luiz and I are engaged in a
> search for enlightenment, and we are slowly making progress!

I have the same feeling.

> The specification needs to be cover how to implement both device and
> driver.  Simple enough to implement correctly, complex enough to be
> useful.


> I see Luiz, with his self-ballooning patch (which was basically a "gimme
> more mem please!" from the guest) heading in the same direction that
> Xen's self-balloon ended up.
> Does this mean that the current (legacy) virtio ballon is completely
> backwards, and that experience shows that the guest must drive?  Or does
> it really mean that there are cases for both, and the we need a balloon
> driver which does both?
> If both, it needs to be specified what side wins.  A feature bit would
> be the classic method.
> If we really don't know, we should leave the balloon device out of the
> standard entirely for v1.0 rather than rushing it (except to reserve id
> 13 for the future virtio balloon).

Maybe we should not built in any complexity in balloon driver itself.
Hence it should be just simple get/put pages from balloon device (with
option to hotplug memory in the future). However, host and guest (maybe
it should be configured at boot?) should provide knobs (via sysfs, ioctl,
...?) to drive balloon device/driver. This way anybody could build
separate process/module with required logic and policies. There is one but...
Such idea was once implemented in Xen by Dan Magenheimer (look for
tools/xenballoon in Xen 4.2) but later it was dropped. I do not why.
So I am CC-ing Bob and Konrad who may know more about that.


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