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 ARCH=arm64 xenconfig > /dev/null; \
	grep XEN .config > arm64-config

Since the options are already split up with a generic config and
architecture specific configs you anything on the x86 configs
are known to only work right now on x86. For instance arm64 doesn't
support MEMORY_HOTPLUG yet as such although we try to enabe it
generically arm64 doesn't have it yet, so we leave the xen
specific kconfig option XEN_BALLOON_MEMORY_HOTPLUG on x86's config
file to set expecations correctly.

Then on x86 we have differences between i386 and x86-64. The difference
between 64-bit-config and 32-bit-config is you don't get XEN_MCE_LOG as
this is only supported on 64-bit. You also do not get on i386
XEN_BALLOON_MEMORY_HOTPLUG, there does not seem to be any technical
reasons to not allow this but I gave up after a few attempts.

Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Borislav Petkov <bp@suse.de>
Cc: Pekka Enberg <penberg@kernel.org>
Cc: David Rientjes <rientjes@google.com>
Cc: Michal Marek <mmarek@suse.cz>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: penberg@kernel.org
Cc: levinsasha928@gmail.com
Cc: mtosatti@redhat.com
Cc: fengguang.wu@intel.com
Cc: David Vrabel <david.vrabel@citrix.com>
Cc: Ian Campbell <Ian.Campbell@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xenproject.org
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Michal Marek <mmarek@suse.cz>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Luis R. Rodriguez <mcgrof@suse.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>

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