xen: asm-generic support

Some headers are shared between individual architectures or are empty.
To avoid duplication of these headers, asm-generic is introduced.

With the following patch, an architecture uses generic headers
mentioned in the file arch/$(ARCH)/include/asm/Makefile

To use a generic header is needed to add to
arch/$(ARCH)/include/asm/Makefile :
	generic-y += <name-of-header-file.h>

For each mentioned header in arch/$(ARCH)/include/asm/Makefile,
the necessary wrapper in arch/$(ARCH)/include/generated/asm will be
generated.

As the base Makefile.asm-generic from Linux kernel was taken.
( 06c2afb862f9da8 "Linux 6.5-rc1" ).

Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

.gitignore

@@ -267,6 +267,7 @@ xen/arch/*/efi/efi.h
 xen/arch/*/efi/pe.c
 xen/arch/*/efi/runtime.c
 xen/arch/*/include/asm/asm-offsets.h
+xen/arch/*/include/generated
 xen/build-dir-cppcheck/
 xen/common/config_data.S
 xen/common/config.gz

xen/Makefile

@@ -438,6 +438,7 @@ ifdef building_out_of_srctree
 endif
 CFLAGS += -I$(srctree)/include
 CFLAGS += -I$(srctree)/arch/$(SRCARCH)/include
+CFLAGS += -I$(objtree)/arch/$(SRCARCH)/include/generated
 
 # Note that link order matters!
 ALL_OBJS-y                := common/built_in.o
@@ -580,16 +581,22 @@ _clean:
 	rm -f $(TARGET).efi $(TARGET).efi.map $(TARGET).efi.elf $(TARGET).efi.stripped
 	rm -f asm-offsets.s arch/*/include/asm/asm-offsets.h
 	rm -f .banner .allconfig.tmp include/xen/compile.h
+	rm -rf $(objtree)/arch/*/include/generated
 
 .PHONY: _distclean
 _distclean: clean
 	rm -f tags TAGS cscope.files cscope.in.out cscope.out cscope.po.out GTAGS GPATH GRTAGS GSYMS .config source
 
+# Support for using generic headers in asm-generic
+PHONY += asm-generic
+asm-generic:
+	$(Q)$(MAKE) $(asm-generic)=arch/$(SRCARCH)/include/generated/asm
+
 $(TARGET).gz: $(TARGET)
 	gzip -n -f -9 < $< > $@.new
 	mv $@.new $@
 
-$(TARGET): outputmakefile FORCE
+$(TARGET): outputmakefile asm-generic FORCE
 	$(Q)$(MAKE) $(build)=tools
 	$(Q)$(MAKE) $(build)=. include/xen/compile.h
 	$(Q)$(MAKE) $(build)=include all

xen/scripts/Kbuild.include

@@ -91,6 +91,12 @@ cc-ifversion = $(shell [ $(CONFIG_GCC_VERSION)0 $(1) $(2)000 ] && echo $(3) || e
 
 clang-ifversion = $(shell [ $(CONFIG_CLANG_VERSION)0 $(1) $(2)000 ] && echo $(3) || echo $(4))
 
+###
+# Shorthand for $(Q)$(MAKE) -f $(srctree)/scripts/Makefile.asm-generic obj
+# Usage:
+# $(Q)$(MAKE) $(asm-generic)=dir
+asm-generic := -f $(srctree)/scripts/Makefile.asm-generic obj
+
 ###
 # Shorthand for $(Q)$(MAKE) -f scripts/Makefile.build obj=
 # Usage:

xen/scripts/Makefile.asm-generic

@@ -0,0 +1,53 @@
+# SPDX-License-Identifier: GPL-2.0-only
+# include/asm-generic contains a lot of files that are used
+# verbatim by several architectures.
+#
+# This Makefile reads the file arch/$(SRCARCH)/include/asm/Makefile
+# and for each file listed in this file with generic-y creates
+# a small wrapper file in arch/$(SRCARCH)/include/generated/asm.
+
+PHONY := all
+all:
+
+src := $(subst /generated,,$(obj))
+
+include $(srctree)/scripts/Kbuild.include
+-include $(src)/Makefile
+
+redundant := $(filter $(mandatory-y) $(generated-y), $(generic-y))
+redundant += $(foreach f, $(generic-y), $(if $(wildcard $(srctree)/$(src)/$(f)),$(f)))
+redundant := $(sort $(redundant))
+$(if $(redundant),\
+	$(warning redundant generic-y found in $(src)/Makefile: $(redundant)))
+
+# If arch does not implement mandatory headers, fallback to asm-generic ones.
+mandatory-y := $(filter-out $(generated-y), $(mandatory-y))
+generic-y   += $(foreach f, $(mandatory-y), $(if $(wildcard $(srctree)/$(src)/$(f)),,$(f)))
+
+generic-y   := $(addprefix $(obj)/, $(generic-y))
+generated-y := $(addprefix $(obj)/, $(generated-y))
+
+# Remove stale wrappers when the corresponding files are removed from generic-y
+old-headers := $(wildcard $(obj)/*.h)
+unwanted    := $(filter-out $(generic-y) $(generated-y),$(old-headers))
+
+quiet_cmd_wrap = WRAP    $@
+      cmd_wrap = echo "\#include <asm-generic/$*.h>" > $@
+
+quiet_cmd_remove = REMOVE  $(unwanted)
+      cmd_remove = rm -f $(unwanted)
+
+all: $(generic-y)
+	$(if $(unwanted),$(call cmd,remove))
+	@:
+
+$(obj)/%.h:
+	$(call cmd,wrap)
+
+# Create output directory. Skip it if at least one old header exists
+# since we know the output directory already exists.
+ifeq ($(old-headers),)
+$(shell mkdir -p $(obj))
+endif
+
+.PHONY: $(PHONY)