xen-scsifront: Add Xen PV SCSI frontend driver

Introduces the Xen pvSCSI frontend. With pvSCSI it is possible for a
Xen domU to issue SCSI commands to a SCSI LUN assigned to that
domU. The SCSI commands are passed to the pvSCSI backend in a driver
domain (usually Dom0) which is owner of the physical device. This
allows e.g. to use SCSI tape drives in a Xen domU.

The code is taken from the pvSCSI implementation in Xen done by
Fujitsu based on Linux kernel 2.6.18.

Changes from the original version are:
- port to upstream kernel
- put all code in just one source file
- move module to appropriate location in kernel tree
- adapt to Linux style guide
- some minor code simplifications
- replace constants with defines
- remove not used defines
- add support for larger SG lists by putting them in a granted page

Signed-off-by: Juergen Gross <jgross@suse.com>
Acked-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

drivers/scsi/Kconfig

@@ -576,6 +576,15 @@ config VMWARE_PVSCSI
 	  To compile this driver as a module, choose M here: the
 	  module will be called vmw_pvscsi.
 
+config XEN_SCSI_FRONTEND
+	tristate "XEN SCSI frontend driver"
+	depends on SCSI && XEN
+	help
+	  The XEN SCSI frontend driver allows the kernel to access SCSI Devices
+	  within another guest OS (usually Dom0).
+	  Only needed if the kernel is running in a XEN guest and generic
+	  SCSI access to a device is needed.
+
 config HYPERV_STORAGE
 	tristate "Microsoft Hyper-V virtual storage driver"
 	depends on SCSI && HYPERV

drivers/scsi/Makefile

@@ -141,6 +141,7 @@ obj-$(CONFIG_SCSI_ESAS2R)	+= esas2r/
 obj-$(CONFIG_SCSI_PMCRAID)	+= pmcraid.o
 obj-$(CONFIG_SCSI_VIRTIO)	+= virtio_scsi.o
 obj-$(CONFIG_VMWARE_PVSCSI)	+= vmw_pvscsi.o
+obj-$(CONFIG_XEN_SCSI_FRONTEND)	+= xen-scsifront.o
 obj-$(CONFIG_HYPERV_STORAGE)	+= hv_storvsc.o
 
 obj-$(CONFIG_ARM)		+= arm/