Re: [Xen-devel] [PATCH v3 1/8] x86: determine HAVE_AS_* just once

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.01.2020 17:41, Andrew Cooper wrote:
> On 06/01/2020 16:34, Jan Beulich wrote:
>> With the exception of HAVE_AS_QUOTED_SYM, populate the results into a
>> generated header instead of (at least once per [sub]directory) into
>> CFLAGS. This results in proper rebuilds (via make dependencies) in case
>> the compiler used changes between builds. It additionally eases
>> inspection of which assembler features were actually found usable.
>>
>> Some trickery is needed to avoid header generation itself to try to
>> include the to-be/not-yet-generated header.
>>
>> Since the definitions in generated/config.h, previously having been
>> command line options, might even affect xen/config.h or its descendants,
>> move adding of the -include option for the latter after inclusion of the
>> per-arch Rules.mk. Use the occasion to also move the most general -I
>> option to the common Rules.mk.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> v4: New.
> 
> Is it v3 or v4?

v4, as said in reply to the cover letter.

> Also, is it intended for just backport?  It is largely redundant with
> Anthony's Kconfig/Kbuild efforts, as moving these (and other checks)
> into the Kconfig step is the ultimate goal.

Is it? Looking at current Linux I still see e.g.

# do binutils support CFI?
cfi := $(call as-instr,.cfi_startproc\n.cfi_rel_offset 
$(sp-y)$(comma)0\n.cfi_endproc,-DCONFIG_AS_CFI=1)
# is .cfi_signal_frame supported too?
cfi-sigframe := $(call 
as-instr,.cfi_startproc\n.cfi_signal_frame\n.cfi_endproc,-DCONFIG_AS_CFI_SIGNAL_FRAME=1)
cfi-sections := $(call as-instr,.cfi_sections 
.debug_frame,-DCONFIG_AS_CFI_SECTIONS=1)

# does binutils support specific instructions?
asinstr += $(call as-instr,pshufb %xmm0$(comma)%xmm0,-DCONFIG_AS_SSSE3=1)
avx_instr := $(call as-instr,vxorps 
%ymm0$(comma)%ymm1$(comma)%ymm2,-DCONFIG_AS_AVX=1)
avx2_instr :=$(call as-instr,vpbroadcastb %xmm0$(comma)%ymm1,-DCONFIG_AS_AVX2=1)
avx512_instr :=$(call as-instr,vpmovm2b %k1$(comma)%zmm5,-DCONFIG_AS_AVX512=1)
sha1_ni_instr :=$(call as-instr,sha1msg1 
%xmm0$(comma)%xmm1,-DCONFIG_AS_SHA1_NI=1)
sha256_ni_instr :=$(call as-instr,sha256msg1 
%xmm0$(comma)%xmm1,-DCONFIG_AS_SHA256_NI=1)

KBUILD_AFLAGS += $(cfi) $(cfi-sigframe) $(cfi-sections) $(asinstr) $(avx_instr) 
$(avx2_instr) $(avx512_instr) $(sha1_ni_instr) $(sha256_ni_instr)
KBUILD_CFLAGS += $(cfi) $(cfi-sigframe) $(cfi-sections) $(asinstr) $(avx_instr) 
$(avx2_instr) $(avx512_instr) $(sha1_ni_instr) $(sha256_ni_instr)

in arch/x86/Makefile. I am, btw, also unconvinced that such belongs
into .config in the first place: The configuration doesn't change
if I swap my tool chain. Of course I realized there's the grey area
of user visible options depending on tool chain capabilities (if
one means to allow such).

Jan

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