xen: add balloon driver

The balloon driver allows memory to be dynamically added or removed from the domain,
in order to allow host memory to be balanced between multiple domains.

This patch introduces the Xen balloon driver, though it currently only
allows a domain to be shrunk from its initial size (and re-grown back to
that size).  A later patch will add the ability to grow a domain beyond
its initial size.

Signed-off-by: Jeremy Fitzhardinge <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>

drivers/Kconfig

@@ -97,4 +97,6 @@ source "drivers/dca/Kconfig"
 source "drivers/auxdisplay/Kconfig"
 
 source "drivers/uio/Kconfig"
+
+source "drivers/xen/Kconfig"
 endmenu

drivers/xen/Kconfig

@@ -0,0 +1,19 @@
+config XEN_BALLOON
+	bool "Xen memory balloon driver"
+	depends on XEN
+	default y
+	help
+	  The balloon driver allows the Xen domain to request more memory from
+	  the system to expand the domain's memory allocation, or alternatively
+	  return unneeded memory to the system.
+
+config XEN_SCRUB_PAGES
+	bool "Scrub pages before returning them to system"
+	depends on XEN_BALLOON
+	default y
+	help
+	  Scrub pages before returning them to the system for reuse by
+	  other domains.  This makes sure that any confidential data
+	  is not accidentally visible to other domains.  Is it more
+	  secure, but slightly less efficient.
+	  If in doubt, say yes.

drivers/xen/Makefile

@@ -1,3 +1,4 @@
 obj-y	+= grant-table.o features.o events.o
 obj-y	+= xenbus/
 obj-$(CONFIG_XEN_XENCOMM)	+= xencomm.o
+obj-$(CONFIG_XEN_BALLOON)	+= balloon.o