Only compile the hypervisor with -Wdeclaration-after-statement

Right now, all tools and hypervisor will be complied with the option
-Wdeclaration-after-statement. While most of the code in the hypervisor
is controlled by us, for tools we may import external libraries.

The build will fail if one of them are using the construct we are
trying to prevent. This is the case when building against Python 3.12
and Yocto:

| In file included from /srv/storage/alex/yocto/build-virt/tmp/work/core2-64-poky-linux/xen-tools/4.17+stable/recipe-sysroot/usr/include/python3.12/Python.h:44,
|                  from xen/lowlevel/xc/xc.c:8:
| /srv/storage/alex/yocto/build-virt/tmp/work/core2-64-poky-linux/xen-tools/4.17+stable/recipe-sysroot/usr/include/python3.12/object.h: In function 'Py_SIZE':
| /srv/storage/alex/yocto/build-virt/tmp/work/core2-64-poky-linux/xen-tools/4.17+stable/recipe-sysroot/usr/include/python3.12/object.h:233:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
|   233 |     PyVarObject *var_ob = _PyVarObject_CAST(ob);
|       |     ^~~~~~~~~~~
| In file included from /srv/storage/alex/yocto/build-virt/tmp/work/core2-64-poky-linux/xen-tools/4.17+stable/recipe-sysroot/usr/include/python3.12/Python.h:53:
| /srv/storage/alex/yocto/build-virt/tmp/work/core2-64-poky-linux/xen-tools/4.17+stable/recipe-sysroot/usr/include/python3.12/cpython/longintrepr.h: In function '_PyLong_CompactValue':
| /srv/storage/alex/yocto/build-virt/tmp/work/core2-64-poky-linux/xen-tools/4.17+stable/recipe-sysroot/usr/include/python3.12/cpython/longintrepr.h:121:5: error: ISO C90 forbids mixed declarations and code [-Werror=declaration-after-statement]
|   121 |     Py_ssize_t sign = 1 - (op->long_value.lv_tag & _PyLong_SIGN_MASK);
|       |     ^~~~~~~~~~
| cc1: all warnings being treated as errors

Looking at the tools directory, a fair few directory already add
-Wno-declaration-after-statement to inhibit the default behavior.

We have always build the hypervisor with the flag, so for now remove
only the flag for anything but the hypervisor. We can decide at later
time whether we want to relax.

Also remove the -Wno-declaration-after-statement in some subdirectory
as the flag is now unnecessary.

Part of the commit message was take from Alexander's first proposal:

Link: https://lore.kernel.org/xen-devel/20231128174729.3880113-1-alex@linutronix.de/
Reported-by: Alexander Kanavin <alex@linutronix.de>
Acked-by: Anthony PERARD <anthony.perard@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>
Tested-by: Jason Andryuk <jandryuk@gmail.com>
Signed-off-by: Julien Grall <jgrall@amazon.com>

Author: Julien Grall

Config.mk

@@ -177,8 +177,6 @@ CFLAGS += -std=gnu99
 
 CFLAGS += -Wall -Wstrict-prototypes
 
-$(call cc-option-add,HOSTCFLAGS,HOSTCC,-Wdeclaration-after-statement)
-$(call cc-option-add,CFLAGS,CC,-Wdeclaration-after-statement)
 $(call cc-option-add,CFLAGS,CC,-Wno-unused-but-set-variable)
 $(call cc-option-add,CFLAGS,CC,-Wno-unused-local-typedefs)
 

stubdom/Makefile

@@ -245,7 +245,7 @@ tpm_emulator-$(XEN_TARGET_ARCH): tpm_emulator-$(TPMEMU_VERSION).tar.gz
 	patch -d $@ -p1 < vtpm-command-duration.patch
 	patch -d $@ -p1 < vtpm-tpm_bn_t-addr.patch
 	mkdir $@/build
-	cd $@/build; CC=${CC} $(CMAKE) .. -DCMAKE_C_FLAGS:STRING="-std=c99 -DTPM_NO_EXTERN $(TARGET_CPPFLAGS) $(TARGET_CFLAGS) -Wno-declaration-after-statement"
+	cd $@/build; CC=${CC} $(CMAKE) .. -DCMAKE_C_FLAGS:STRING="-std=c99 -DTPM_NO_EXTERN $(TARGET_CPPFLAGS) $(TARGET_CFLAGS)"
 	touch $@
 
 TPMEMU_STAMPFILE=$(CROSS_ROOT)/$(GNU_TARGET_ARCH)-xen-elf/lib/libtpm.a

stubdom/vtpmmgr/Makefile

@@ -17,7 +17,7 @@ OBJS += vtpm_disk.o disk_tpm.o disk_io.o disk_crypto.o disk_read.o disk_write.o
 OBJS += mgmt_authority.o
 
 CFLAGS+=-Werror -Iutil -Icrypto -Itcs
-CFLAGS+=-Wno-declaration-after-statement -Wno-unused-label
+CFLAGS+=-Wno-unused-label
 
 build: $(TARGET)
 $(TARGET): $(OBJS)

tools/libs/light/Makefile

@@ -38,8 +38,7 @@ vpath static_tables.c $(ACPI_PATH)/
 
 OBJS-$(CONFIG_X86) += $(ACPI_OBJS)
 
-CFLAGS += -Wno-format-zero-length -Wmissing-declarations \
-	-Wno-declaration-after-statement -Wformat-nonliteral
+CFLAGS += -Wno-format-zero-length -Wmissing-declarations -Wformat-nonliteral
 
 CFLAGS-$(CONFIG_X86) += -DCONFIG_PCI_SUPP_LEGACY_IRQ
 

tools/libs/util/Makefile

@@ -9,8 +9,7 @@ OBJS-y += libxlu_disk.o
 OBJS-y += libxlu_vif.o
 OBJS-y += libxlu_pci.o
 
-CFLAGS += -Wno-format-zero-length -Wmissing-declarations \
-	-Wno-declaration-after-statement -Wformat-nonliteral
+CFLAGS += -Wno-format-zero-length -Wmissing-declarations -Wformat-nonliteral
 CFLAGS += $(CFLAGS_libxenctrl)
 
 CFLAGS += $(PTHREAD_CFLAGS)

tools/tests/depriv/Makefile

@@ -1,8 +1,6 @@
 XEN_ROOT=$(CURDIR)/../../..
 include $(XEN_ROOT)/tools/Rules.mk
 
-CFLAGS += -Wno-declaration-after-statement
-
 CFLAGS += $(CFLAGS_xeninclude)
 CFLAGS += $(CFLAGS_libxenctrl)
 CFLAGS += $(CFLAGS_libxencall)

tools/xl/Makefile

@@ -5,8 +5,7 @@
 XEN_ROOT = $(CURDIR)/../..
 include $(XEN_ROOT)/tools/Rules.mk
 
-CFLAGS += -Wno-format-zero-length -Wmissing-declarations \
-	-Wno-declaration-after-statement -Wformat-nonliteral
+CFLAGS += -Wno-format-zero-length -Wmissing-declarations -Wformat-nonliteral
 CFLAGS += -fPIC
 
 CFLAGS += $(PTHREAD_CFLAGS)

xen/Makefile

@@ -393,6 +393,7 @@ CFLAGS-$(CONFIG_CC_SPLIT_SECTIONS) += -ffunction-sections -fdata-sections
 
 CFLAGS += -nostdinc -fno-builtin -fno-common
 CFLAGS += -Werror -Wredundant-decls -Wwrite-strings -Wno-pointer-arith
+$(call cc-option-add,CFLAGS,CC,-Wdeclaration-after-statement)
 $(call cc-option-add,CFLAGS,CC,-Wvla)
 CFLAGS += -pipe -D__XEN__ -include $(srctree)/include/xen/config.h
 CFLAGS-$(CONFIG_DEBUG_INFO) += -g