Re: [RFC] KVM: Balloon support for device assignment

[Muli Ben-Yehuda <muli@il.ibm.com>]

On Wed, Feb 17, 2010 at 12:27:09PM +0200, Avi Kivity wrote:
> On 02/17/2010 11:43 AM, borove@il.ibm.com wrote:
> >From: Eran Borovik<borove@il.ibm.com>
> >
> >This patch adds modifications to allow correct
> >balloon operation when a virtual guest uses a direct assigned device.
> >The modifications include a new interface between qemu and kvm to allow
> >mapping and unmapping the pages from the IOMMU as well as pinning and unpinning as needed.
> 
> The plan for iommu support is to push it into uio.  Instead of kvm
> managing the iommu directly, I'd like qemu to open a uio device and
> set up an iommu mapping there, which will just happen to match the
> kvm memory slots.  Similarly, interrupts will be forwarded using
> irqfds.  This will allow using the iommu without kvm, and reduce the
> amount of special purpose kvm code.
> 
> These patches make the transition more difficult which worries me.

That's a fair point, but they also address a real short-coming of the
current device assignment code, which pins all of the guest's memory
unconditionally. Unless the uio effort is in progress and expected to
complete shortly, I would think the benefit of these simple patches
trumps the cost.

Cheers,
Muli