xen/pcpu: Xen physical cpus online/offline sys interface

This patch provide Xen physical cpus online/offline sys interface.
User can use it for their own purpose, like power saving:
by offlining some cpus when light workload it save power greatly.

Its basic workflow is, user online/offline cpu via sys interface,
then hypercall xen to implement, after done xen inject virq back to dom0,
and then dom0 sync cpu status.

Signed-off-by: Jiang, Yunhong <yunhong.jiang@intel.com>
Signed-off-by: Liu, Jinsong <jinsong.liu@intel.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>

Documentation/ABI/testing/sysfs-devices-system-xen_cpu

@@ -0,0 +1,20 @@
+What:		/sys/devices/system/xen_cpu/
+Date:		May 2012
+Contact:	Liu, Jinsong <jinsong.liu@intel.com>
+Description:
+		A collection of global/individual Xen physical cpu attributes
+
+		Individual physical cpu attributes are contained in
+		subdirectories named by the Xen's logical cpu number, e.g.:
+		/sys/devices/system/xen_cpu/xen_cpu#/
+
+
+What:		/sys/devices/system/xen_cpu/xen_cpu#/online
+Date:		May 2012
+Contact:	Liu, Jinsong <jinsong.liu@intel.com>
+Description:
+		Interface to online/offline Xen physical cpus
+
+		When running under Xen platform, it provide user interface
+		to online/offline physical cpus, except cpu0 due to several
+		logic restrictions and assumptions.

drivers/xen/Makefile

@@ -17,6 +17,7 @@ obj-$(CONFIG_XEN_SYS_HYPERVISOR)	+= sys-hypervisor.o
 obj-$(CONFIG_XEN_PVHVM)			+= platform-pci.o
 obj-$(CONFIG_XEN_TMEM)			+= tmem.o
 obj-$(CONFIG_SWIOTLB_XEN)		+= swiotlb-xen.o
+obj-$(CONFIG_XEN_DOM0)			+= pcpu.o
 obj-$(CONFIG_XEN_DOM0)			+= pci.o acpi.o
 obj-$(CONFIG_XEN_MCE_LOG)		+= mcelog.o
 obj-$(CONFIG_XEN_PCIDEV_BACKEND)	+= xen-pciback/