Re: [Xen-devel] [XEN PATCH v3 14/23] xen/build: use new $(c_flags) and $(a_flags) instead of $(CFLAGS)

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Feb 27, 2020 at 11:22:38AM +0100, Roger Pau Monné wrote:
> On Wed, Feb 26, 2020 at 11:33:46AM +0000, Anthony PERARD wrote:
> > From: Anthony PERARD <anthony.perard@xxxxxxxxx>
> > 
> > In a later patch ("xen/build: have the root Makefile generates the
> > CFLAGS), we want to generate the CFLAGS in xen/Makefile, then export
> > it and have Rules.mk use a CFLAGS from the environment variables. That
> > changes the flavor of the CFLAGS and flags intended for one target
> > (like -D__OBJECT_FILE__ and -M%) gets propagated and duplicated. So we
> > start by moving such flags out of $(CFLAGS) and into $(c_flags) which
> > is to be modified by only Rules.mk.
> > 
> > __OBJECT_FILE__ is only used by arch/x86/mm/*.c files, so having it in
> > $(c_flags) is enough, we don't need it in $(a_flags).
> 
> This seem to be used only by source files that are build multiple
> times with different parameters in order to generate different object
> files.
> 
> Is there any harm in having it also in the assembler flags? (in case
> we require such usage in the future)

Not really any harm, no, but that can be done later when needed I think.

> Or maybe we could even limit __OBJECT_FILE__ to mm/ files that require
> it only?

That's a possibility, yes. I'll be adding flags to those specific files
anyway (GUEST_PAGING_LEVELS, done in a later patch), I could add
__OBJECT_FILE__ to the list.

> > 
> > For include/Makefile and as-insn we can keep using CFLAGS, but since
> > it doesn't have -M* flags anymore there is no need to filter them out.
> > 
> > The XEN_BUILD_EFI tests in arch/x86/Makefile was filtering out
> > CFLAGS-y, but according to dd40177c1bc8 ("x86-64/EFI: add CFLAGS to
> > check compile"), it was done to filter out -MF. CFLAGS doesn't
> > have those flags anymore, so no filtering is needed.
> > 
> > This is inspired by the way Kbuild generates CFLAGS for each targets.
> > 
> > Signed-off-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>
> 
> Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Thanks,

-- 
Anthony PERARD

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