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Re: [Qemu-devel] [PATCH 0/3] balloon: error if guest driver is not initi

From: Daniel P. Berrange
Subject: Re: [Qemu-devel] [PATCH 0/3] balloon: error if guest driver is not initialised
Date: Fri, 9 Dec 2011 11:55:21 +0000
User-agent: Mutt/1.5.21 (2010-09-15)

On Fri, Dec 09, 2011 at 05:19:35PM +0530, Amit Shah wrote:
> Hello,
> These patches make qemu generate an error on failure in setting a
> balloon value:
> (qemu) balloon 400
> Device 'balloon' has not been activated
> This can happen when the guest hasn't yet loaded a balloon driver.

This is a pretty significant change in behaviour/semantics of the
balloon driver interface.

eg, when libvirt starts a guest, it launches QEMU paused, and
immediately sets a balloon target, which may be lower than the
memory limit, and then starts CPUS to let the guest boot. The
guest will utilize this target when it loads the balloon driver
during bootup.

With this proposed change, libvirt will immediately get
an error, and have to guess when the balloon driver might be
loaded by the guest, in order to set the limit post-boot.

> Previously, such a ballooning attempt didn't produce an error, but the
> virtio-balloon driver remembered the value nevertheless, causing a
> module load in the guest using the attempted balloon value
> originally.  This may not be the desired course of action.

This is *exactly* desired behaviour. Please don't change this.

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