Re: [XEN PATCH v5 04/16] xen/build: have the root Makefile generates the CFLAGS

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.04.2020 18:11, Anthony PERARD wrote:
> Instead of generating the CFLAGS in Rules.mk everytime we enter a new
> subdirectory, we are going to generate most of them a single time, and
> export the result in the environment so that Rules.mk can use it.  The
> only flags left to be generated are the ones that depend on the
> targets, but the variable $(c_flags) takes care of that.
> 
> Arch specific CFLAGS are generated by a new file "arch/*/arch.mk"
> which is included by the root Makefile.
> 
> We export the *FLAGS via the environment variables XEN_*FLAGS because
> Rules.mk still includes Config.mk and would add duplicated flags to
> CFLAGS.
> 
> When running Rules.mk in the root directory (xen/), the variable
> `root-make-done' is set, so `need-config' will remain undef and so the
> root Makefile will not generate the cflags again.
> 
> We can't use CFLAGS in subdirectories to add flags to particular
> targets, instead start to use CFLAGS-y. Idem for AFLAGS.
> So there are two different CFLAGS-y, the one in xen/Makefile (and
> arch.mk), and the one in subdirs that Rules.mk is going to use.
> We can't add to XEN_CFLAGS because it is exported, so making change to
> it might be propagated to subdirectory which isn't intended.
> 
> Some style change are introduced in this patch:
>     when LDFLAGS_DIRECT is included in LDFLAGS
>     use of CFLAGS-$(CONFIG_INDIRECT_THUNK) instead of ifeq().
> 
> The LTO change hasn't been tested properly, as LTO is marked as
> broken.
> 
> Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
with one further question:

> --- a/xen/arch/x86/Rules.mk
> +++ b/xen/arch/x86/Rules.mk
> @@ -1,89 +1,10 @@
>  ########################################
>  # x86-specific definitions
>  
> -XEN_IMG_OFFSET := 0x200000
> -
> -CFLAGS += -I$(BASEDIR)/include
> -CFLAGS += -I$(BASEDIR)/include/asm-x86/mach-generic
> -CFLAGS += -I$(BASEDIR)/include/asm-x86/mach-default
> -CFLAGS += -DXEN_IMG_OFFSET=$(XEN_IMG_OFFSET)
> -CFLAGS += '-D__OBJECT_LABEL__=$(subst /,$$,$(subst -,_,$(subst 
> $(BASEDIR)/,,$(CURDIR))/$@))'
> -
> -# Prevent floating-point variables from creeping into Xen.
> -CFLAGS += -msoft-float
> -
> -ifeq ($(CONFIG_CC_IS_CLANG),y)
> -# Note: Any test which adds -no-integrated-as will cause subsequent tests to
> -# succeed, and not trigger further additions.
> -#
> -# The tests to select whether the integrated assembler is usable need to 
> happen
> -# before testing any assembler features, or else the result of the tests 
> would
> -# be stale if the integrated assembler is not used.
> -
> -# Older clang's built-in assembler doesn't understand .skip with labels:
> -# https://bugs.llvm.org/show_bug.cgi?id=27369
> -$(call as-option-add,CFLAGS,CC,".L0: .L1: .skip (.L1 - .L0)",,\
> -                     -no-integrated-as)
> -
> -# Check whether clang asm()-s support .include.
> -$(call as-option-add,CFLAGS,CC,".include \"asm-x86/indirect_thunk_asm.h\"",,\
> -                     -no-integrated-as)
> -
> -# Check whether clang keeps .macro-s between asm()-s:
> -# https://bugs.llvm.org/show_bug.cgi?id=36110
> -$(call as-option-add,CFLAGS,CC,\
> -                     ".macro FOO;.endm"$$(close); asm volatile 
> $$(open)".macro FOO;.endm",\
> -                     -no-integrated-as)
> -endif
> -
> -$(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS))
> -$(call cc-option-add,CFLAGS,CC,-Wnested-externs)
> -$(call as-option-add,CFLAGS,CC,"vmcall",-DHAVE_AS_VMX)
> -$(call as-option-add,CFLAGS,CC,"crc32 %eax$$(comma)%eax",-DHAVE_AS_SSE4_2)
> -$(call as-option-add,CFLAGS,CC,"invept (%rax)$$(comma)%rax",-DHAVE_AS_EPT)
> -$(call as-option-add,CFLAGS,CC,"rdrand %eax",-DHAVE_AS_RDRAND)
> -$(call as-option-add,CFLAGS,CC,"rdfsbase %rax",-DHAVE_AS_FSGSBASE)
> -$(call as-option-add,CFLAGS,CC,"xsaveopt (%rax)",-DHAVE_AS_XSAVEOPT)
> -$(call as-option-add,CFLAGS,CC,"rdseed %eax",-DHAVE_AS_RDSEED)
> -$(call as-option-add,CFLAGS,CC,"clwb (%rax)",-DHAVE_AS_CLWB)
> -$(call as-option-add,CFLAGS,CC,".equ \"x\"$$(comma)1", \
> -                     -U__OBJECT_LABEL__ -DHAVE_AS_QUOTED_SYM \
> -                     '-D__OBJECT_LABEL__=$(subst 
> $(BASEDIR)/,,$(CURDIR))/$$@')
> -$(call as-option-add,CFLAGS,CC,"invpcid 
> (%rax)$$(comma)%rax",-DHAVE_AS_INVPCID)
> -
> -# GAS's idea of true is -1.  Clang's idea is 1
> -$(call as-option-add,CFLAGS,CC,\
> -    ".if ((1 > 0) < 0); .error \"\";.endif",,-DHAVE_AS_NEGATIVE_TRUE)
> -
> -# Check to see whether the assmbler supports the .nop directive.
> -$(call as-option-add,CFLAGS,CC,\
> -    ".L1: .L2: .nops (.L2 - .L1)$$(comma)9",-DHAVE_AS_NOPS_DIRECTIVE)
> -
> -CFLAGS += -mno-red-zone -fpic -fno-asynchronous-unwind-tables
> -
> -# Xen doesn't use SSE interally.  If the compiler supports it, also skip the
> -# SSE setup for variadic function calls.
> -CFLAGS += -mno-sse $(call cc-option,$(CC),-mskip-rax-setup)
> -
> -# Compile with thunk-extern, indirect-branch-register if avaiable.
> -ifeq ($(CONFIG_INDIRECT_THUNK),y)
> -CFLAGS += -mindirect-branch=thunk-extern -mindirect-branch-register
> -CFLAGS += -fno-jump-tables
> +ifneq ($(filter -DHAVE_AS_QUOTED_SYM,$(XEN_CFLAGS)),)
> +object_label_flags = '-D__OBJECT_LABEL__=$(subst $(BASEDIR)/,,$(CURDIR))/$@'
> +else
> +object_label_flags = '-D__OBJECT_LABEL__=$(subst /,$$,$(subst -,_,$(subst 
> $(BASEDIR)/,,$(CURDIR))/$@))'
>  endif
> -
> -# If supported by the compiler, reduce stack alignment to 8 bytes. But allow
> -# this to be overridden elsewhere.
> -$(call cc-option-add,CFLAGS-stack-boundary,CC,-mpreferred-stack-boundary=3)
> -CFLAGS += $(CFLAGS-stack-boundary)
> -
> -ifeq ($(CONFIG_UBSAN),y)
> -# Don't enable alignment sanitisation.  x86 has efficient unaligned accesses,
> -# and various things (ACPI tables, hypercall pages, stubs, etc) are wont-fix.
> -# It also causes an as-yet-unidentified crash on native boot before the
> -# console starts.
> -$(call cc-option-add,CFLAGS_UBSAN,CC,-fno-sanitize=alignment)
> -endif
> -
> -# Set up the assembler include path properly for older toolchains.
> -CFLAGS += -Wa,-I$(BASEDIR)/include
> -
> +c_flags += $(object_label_flags) $(CFLAGS-stack-boundary)
> +a_flags += $(object_label_flags) $(CFLAGS-stack-boundary)

Why are you also adding these to a_flags? It probably doesn't hurt,
but I'd prefer if it was removed (could be done while committing if
all acks arrive and other other need for av6 arises) if there's no
clear need.

Jan