virtio: balloon driver

After discussions with Anthony Liguori, it seems that the virtio
balloon can be made even simpler.  Here's my attempt.

The device configuration tells the driver how much memory it should
take from the guest (ie. balloon size).  The guest feeds the page
numbers it has taken via one virtqueue.

A second virtqueue feeds the page numbers the driver wants back: if
the device has the VIRTIO_BALLOON_F_MUST_TELL_HOST bit, then this
queue is compulsory, otherwise it's advisory (and the guest can simply
fault the pages back in).

This driver can be enhanced later to deflate the balloon via a
shrinker, oom callback or we could even go for a complete set of
in-guest regulators.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

1 files changed, 10 insertions, 0 deletions

diff --git a/drivers/virtio/Kconfig b/drivers/virtio/Kconfig
index 833db2f36e9b..3dd6294d10b6 100644
--- a/drivers/virtio/Kconfig
+++ b/drivers/virtio/Kconfig
@@ -23,3 +23,13 @@ config VIRTIO_PCI
 
           If unsure, say M.
 
+config VIRTIO_BALLOON
+        tristate "Virtio balloon driver (EXPERIMENTAL)"
+        select VIRTIO
+        select VIRTIO_RING
+        ---help---
+         This driver supports increasing and decreasing the amount
+         of memory within a KVM guest.
+
+         If unsure, say M.
+