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References: <20190925161255.1871-1-ard.biesheuvel@linaro.org> <20190925161255.1871-12-ard.biesheuvel@linaro.org>
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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Wed, 25 Sep 2019 14:01:33 -0700
Message-ID: <CAHk-=wi8+MHz8xGtx_mUZPBsRT6qkptGW7a_pOrK=SnTRAiecA@mail.gmail.com>
Subject: Re: [RFC PATCH 11/18] int128: move __uint128_t compiler test to Kconfig
To:     Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc:     Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
        Linux ARM <linux-arm-kernel@lists.infradead.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
        David Miller <davem@davemloft.net>,
        Greg KH <gregkh@linuxfoundation.org>,
        "Jason A . Donenfeld" <Jason@zx2c4.com>,
        Samuel Neves <sneves@dei.uc.pt>,
        Dan Carpenter <dan.carpenter@oracle.com>,
        Arnd Bergmann <arnd@arndb.de>,
        Eric Biggers <ebiggers@google.com>,
        Andy Lutomirski <luto@kernel.org>,
        Will Deacon <will@kernel.org>, Marc Zyngier <maz@kernel.org>,
        Catalin Marinas <catalin.marinas@arm.com>
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On Wed, Sep 25, 2019 at 9:14 AM Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
>
>  config ARCH_SUPPORTS_INT128
>         bool
> +       depends on !$(cc-option,-D__SIZEOF_INT128__=0)

Hmm. Does this actually work?

If that "depends on" now ends up being 'n', afaik the people who
_enable_ it just do a

       select ARCH_SUPPORTS_INT128

and now you'll end up with the Kconfig erroring out with

   WARNING: unmet direct dependencies detected for ARCH_SUPPORTS_INT128

and then you end up with CONFIG_ARCH_SUPPORTS_INT128 anyway, instead
of the behavior you _want_ to get, which is to not get that CONFIG
defined at all.

So I heartily agree with your intent, but I don't think that model
works. I think you need to change the cases that currently do

       select ARCH_SUPPORTS_INT128

to instead have that cc-option test.

And take all the above with a pinch of salt. Maybe what you are doing
works, and I am just missing some piece of the puzzle. But I _think_
it's broken, and you didn't test with a compiler that doesn't support
that thing properly.

             Linus