Re: [PATCH] Add a page cache-backed balloon device driver.

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jun 27, 2012 at 08:48:55AM -0700, Frank Swiderski wrote:
> On Tue, Jun 26, 2012 at 7:56 PM, Rusty Russell <rusty@rustcorp.com.au> wrote:
> > On Wed, 27 Jun 2012 00:41:06 +0300, "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >> On Tue, Jun 26, 2012 at 01:32:58PM -0700, Frank Swiderski wrote:
> >> > This implementation of a virtio balloon driver uses the page cache to
> >> > "store" pages that have been released to the host.  The communication
> >> > (outside of target counts) is one way--the guest notifies the host when
> >> > it adds a page to the page cache, allowing the host to madvise(2) with
> >> > MADV_DONTNEED.  Reclaim in the guest is therefore automatic and implicit
> >> > (via the regular page reclaim).  This means that inflating the balloon
> >> > is similar to the existing balloon mechanism, but the deflate is
> >> > different--it re-uses existing Linux kernel functionality to
> >> > automatically reclaim.
> >> >
> >> > Signed-off-by: Frank Swiderski <fes@google.com>
> >>
> >> I'm pondering this:
> >>
> >> Should it really be a separate driver/device ID?
> >> If it behaves the same from host POV, maybe it
> >> should be up to the guest how to inflate/deflate
> >> the balloon internally?
> >
> > Well, it shouldn't steal ID 10, either way :)  Either use a completely
> > bogus number, or ask for an id.
> >
> > But AFAICT this should be a an alternate driver of for the same device:
> > it's not really a separate device, is it?
> >
> > Cheers,
> > Rusty.
> 
> Apologies, Rusty.  Asking for an ID is in the virtio spec, and I
> completely neglected that step.  Though as you and others have pointed
> out, this probably fits better as a different driver for the same
> device.  Since it changes whether or not the deflate operation is
> necessary, it also seems that how this should look is different
> behavior based on a feature bit in the device.
> 
> If that sounds reasonable, then what I'll do with this patch is merge
> it with the existing virtio balloon driver with a feature bit for
> determining which behavior to use.
> 
> I also think the idea of a generic balloon that the different balloon
> drivers use for the inflate/deflate operations is interesting and
> useful, though I think the suggestion of pending that until later is
> correct.
> 
> Sounds reasonable?
> 
> Regards,
> fes

I think a spec patch would be a good spec at this point.
You can get the spec from Rusty, or a mirror
from my git:

git://git.kernel.org/pub/scm/virt/kvm/mst/virtio-spec.git