kconfig: add xenconfig defconfig helper

This lets you build a kernel which can support xen dom0
or xen guests on i386, x86-64 and arm64 by just using:

   make xenconfig

You can start from an allnoconfig and then switch to xenconfig.
This also splits out the options which are available currently
to be built with x86 and 'make ARCH=arm64' under a shared config.

Technically xen supports a dom0 kernel and also a guest
kernel configuration but upon review with the xen team
since we don't have many dom0 options its best to just
combine these two into one.

A few generic notes: we enable both of these:

CONFIG_INET=y
CONFIG_BINFMT_ELF=y

although technically not required given you likely will
end up with a pretty useless system otherwise.

A few architectural differences worth noting:

$ make allnoconfig; make xenconfig > /dev/null ; \
	grep XEN .config > 64-bit-config
$ make ARCH=i386 allnoconfig; make ARCH=i386 xenconfig > /dev/null; \
	grep XEN .config > 32-bit-config
$ make ARCH=arm64 allnoconfig; make AR...

arch/x86/configs/xen.config

+# global x86 required specific stuff
+# On 32-bit HIGHMEM4G is not allowed
+CONFIG_HIGHMEM64G=y
+CONFIG_64BIT=y
+
+# These enable us to allow some of the
+# not so generic stuff below
+CONFIG_HYPERVISOR_GUEST=y
+CONFIG_PCI=y
+CONFIG_PCI_MSI=y
+CONFIG_X86_MCE=y
+CONFIG_ACPI_PROCESSOR=y
+CONFIG_CPU_FREQ=y
+
+# x86 xen specific config options
+CONFIG_XEN_PVH=y
+CONFIG_XEN_MAX_DOMAIN_MEMORY=500
+CONFIG_XEN_SAVE_RESTORE=y
+# CONFIG_XEN_DEBUG_FS is not set
+CONFIG_XEN_MCE_LOG=y
+CONFIG_XEN_ACPI_PROCESSOR=m
+# x86 specific backend drivers
+CONFIG_XEN_PCIDEV_BACKEND=m
+# x86 specific frontend drivers
+CONFIG_XEN_PCIDEV_FRONTEND=m
+# depends on MEMORY_HOTPLUG, arm64 doesn't enable this yet,
+# move to generic config if it ever does.
+CONFIG_XEN_BALLOON_MEMORY_HOTPLUG=y

kernel/configs/xen.config

+# global stuff - these enable us to allow some
+# of the not so generic stuff below for xen
+CONFIG_PARAVIRT=y
+CONFIG_NET=y
+CONFIG_NET_CORE=y
+CONFIG_NETDEVICES=y
+CONFIG_BLOCK=y
+CONFIG_WATCHDOG=y
+CONFIG_TARGET_CORE=y
+CONFIG_SCSI=y
+CONFIG_FB=y
+CONFIG_INPUT_MISC=y
+CONFIG_MEMORY_HOTPLUG=y
+CONFIG_TTY=y
+# Technically not required but otherwise produces
+# pretty useless systems starting from allnoconfig
+# You want TCP/IP and ELF binaries right?
+CONFIG_INET=y
+CONFIG_BINFMT_ELF=y
+# generic config
+CONFIG_XEN=y
+CONFIG_XEN_DOM0=y
+# backend drivers
+CONFIG_XEN_BACKEND=y
+CONFIG_XEN_BLKDEV_BACKEND=m
+CONFIG_XEN_NETDEV_BACKEND=m
+CONFIG_HVC_XEN=y
+CONFIG_XEN_WDT=m
+CONFIG_XEN_SCSI_BACKEND=m
+# frontend drivers
+CONFIG_XEN_FBDEV_FRONTEND=m
+CONFIG_HVC_XEN_FRONTEND=y
+CONFIG_INPUT_XEN_KBDDEV_FRONTEND=m
+CONFIG_XEN_SCSI_FRONTEND=m
+# others
+CONFIG_XEN_BALLOON=y
+CONFIG_XEN_SCRUB_PAGES=y
+CONFIG_XEN_DEV_EVTCHN=m
+CONFIG_XEN_BLKDEV_FRONTEND=m
+CONFIG_XEN_NETDEV_FRONTEND=m
+CONFIG_XENFS=m
+CONFIG_XEN_COMPAT_XENFS=y
+CONFIG_XEN_SYS_HYPERVISOR=y
+CONFIG_XEN_XENBUS_FRONTEND=y
+CONFIG_XEN_GNTDEV=m
+CONFIG_XEN_GRANT_DEV_ALLOC=m
+CONFIG_SWIOTLB_XEN=y
+CONFIG_XEN_PRIVCMD=m

scripts/kconfig/Makefile

@@ -115,6 +115,10 @@ PHONY += kvmconfig
 kvmconfig: kvm_guest.config
 	@:
 
+PHONY += xenconfig
+xenconfig: xen.config
+	@:
+
 PHONY += tinyconfig
 tinyconfig:
 	$(Q)$(MAKE) -f $(srctree)/Makefile allnoconfig tiny.config
@@ -140,6 +144,7 @@ help:
 	@echo  '  listnewconfig   - List new options'
 	@echo  '  olddefconfig	  - Same as silentoldconfig but sets new symbols to their default value'
 	@echo  '  kvmconfig	  - Enable additional options for kvm guest kernel support'
+	@echo  '  xenconfig       - Enable additional options for xen dom0 and guest kernel support'
 	@echo  '  tinyconfig	  - Configure the tiniest possible kernel'
 
 # lxdialog stuff