Re: [PATCH 00/23] kconfig: move compiler capability tests to Kconfig

[Masahiro Yamada]

2018-02-21 18:56 GMT+09:00 Arnd Bergmann <arnd@xxxxxxxx>:
> On Wed, Feb 21, 2018 at 8:38 AM, Masahiro Yamada
> <yamada.masahiro@xxxxxxxxxxxxx> wrote:
>> 2018-02-20 0:18 GMT+09:00 Ulf Magnusson <ulfalizer@xxxxxxxxx>:
>>
>>>>
>>>> I'm not happy that we in one context can reference CONFIG variables
>>>> directly, but inside the $(call ...) and $(shell ...) needs the $ prefix.
>>>> But I could not come up with something un-ambigious where this could be avoided.
>>>
>>> I think we should be careful about allowing references to config
>>> symbols. It mixes up the parsing and evaluation phases, since $() is
>>> expanded during parsing (which I consider a feature and think is
>>> needed to retain sanity).
>>>
>>> Patch 06/23 removes the last existing instance of symbol references in
>>> strings by getting rid of 'option env'. That's an improvement to me.
>>> We shouldn't add it back.
>>
>>
>> This is really important design decision,
>> so I'd like to hear a little more from experts.
>>
>>
>> For example, x86 allows users to choose sub-arch, either 'i386' or 'x86_64'.
>>
>> https://github.com/torvalds/linux/blob/v4.16-rc2/arch/x86/Kconfig#L4
>>
>>
>>
>> If the user toggles CONFIG_64BIT,
>> the bi-arch compiler will work in a slightly different mode
>> (at least, back-end parts)
>>
>> So, my question is, is there a case,
>>
>> $(cc-option, -m32 -foo) is y, but
>> $(cc-option, -m64 -foo) is n  ?
>> (or vice versa)
>>
>>
>> If the answer is yes, $(cc-option -foo) would have to be re-calculated
>> every time CONFIG_64BIT is toggled.
>>
>> This is what I'd like to avoid, though.
>
> The -m32/-m64 trick (and -mbig-endian/-mlittle-endian on other architectures
> as well as a couple of other flags) only works if the compiler is configured to
> support it. In other cases (e.g. big-endian xtensa), the kernel always
> detects what the compiler does and silently configures itself to match
> using Makefile logic.
>
> On x86, compilers are usually built as bi-arch, but you can build one that
> only allows one of them.
>
> I can see two reasonable ways out:
>
> - we don't use  $(cc-option -foo) in a case like this, and instead require the
>   user to have a matching toolchain.
> - we could make the 32/64 selection on x86 a 'choice' statement where
>   each option depends on both the ARCH= variable and the
>   $(cc-option, -m32)/ $(cc-option, -m64) output.
>
>        Arnd



Let me clarify my concern.

When we test the compiler flag, is there a case
where a particular flag depends on -m{32,64} ?

For example, is there a compiler that supports -fstack-protector
for 64bit mode, but unsupports it for 32bit mode?

  $(cc-option -m32)                     ->  y
  $(cc-option -m64)                     ->  y
  $(cc-option -fstack-protector)        ->  y
  $(cc-option -m32 -fstack-protector)   ->  n
  $(cc-option -m64 -fstack-protector)   ->  y

I guess this is unlikely to happen,
but I am not whether it is zero possibility.

If this could happen,
$(cc-option ) must be evaluated together with
correct bi-arch option (either -m32 or -m64).


Currently, -m32/-m64 is specified in Makefile,
but we are moving compiler tests to Kconfig
and, CONFIG_64BIT can be dynamically toggled in Kconfig.




-- 
Best Regards
Masahiro Yamada