Re: [XEN PATCH v5 08/16] build: Introduce $(cpp_flags)

[Jan Beulich <jbeulich@xxxxxxxx>]

On 21.04.2020 18:12, Anthony PERARD wrote:
> --- a/xen/Rules.mk
> +++ b/xen/Rules.mk
> @@ -123,6 +123,7 @@ $(obj-bin-y): XEN_CFLAGS := $(filter-out 
> -flto,$(XEN_CFLAGS))
>  
>  c_flags = -MMD -MP -MF $(@D)/.$(@F).d $(XEN_CFLAGS) '-D__OBJECT_FILE__="$@"'
>  a_flags = -MMD -MP -MF $(@D)/.$(@F).d $(XEN_AFLAGS)
> +cpp_flags = $(filter-out -Wa$(comma)%,$(a_flags))

I can see this happening to be this way right now, but in principle
I could see a_flags to hold items applicable to assembly files only,
but not to (the preprocessing of) C files. Hence while this is fine
for now, ...

> @@ -207,7 +208,7 @@ quiet_cmd_cc_s_c = CC      $@
>  cmd_cc_s_c = $(CC) $(filter-out -Wa$(comma)%,$(c_flags)) -S $< -o $@
>  
>  quiet_cmd_s_S = CPP     $@
> -cmd_s_S = $(CPP) $(filter-out -Wa$(comma)%,$(a_flags)) $< -o $@
> +cmd_s_S = $(CPP) $(cpp_flags) $< -o $@

... this one is a trap waiting for someone to fall in imo. Instead
where I'd expect this patch to use $(cpp_flags) is e.g. in
xen/arch/x86/mm/Makefile:

guest_walk_%.i: guest_walk.c Makefile
        $(CPP) $(cpp_flags) -DGUEST_PAGING_LEVELS=$* -c $< -o $@

And note how this currently uses $(c_flags), not $(a_flags), which
suggests that your deriving from $(a_flags) isn't correct either.

Jan