linux-kernel - Re: [PATCH 6/6] s390: introduce execute-trampolines for branches

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Date:   Thu, 8 Feb 2018 08:44:54 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Martin Schwidefsky <schwidefsky@...ibm.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Pavel Machek <pavel@....cz>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Jon Masters <jcm@...hat.com>,
        Marcus Meissner <meissner@...e.de>,
        Jiri Kosina <jkosina@...e.cz>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Alan Cox <gnomes@...rguk.ukuu.org.uk>,
        Kees Cook <keescook@...omium.org>,
        Ulf Magnusson <ulfalizer@...il.com>
Subject: Re: [PATCH 6/6] s390: introduce execute-trampolines for branches

2018-02-08 2:55 GMT+09:00 Linus Torvalds <torvalds@...ux-foundation.org>:
> On Wed, Feb 7, 2018 at 4:17 AM, Martin Schwidefsky
> <schwidefsky@...ibm.com> wrote:
>>> That isn't, though. Linus asked us to drop the $(warning) part.
>>>
>>> ... and then spent a week building with a non-retpoline compiler and
>>> not noticing, so he might have changed his mind ;)
>>
>> I found the warning to have some value, it helps for the case where my
>> fingers are faster than my brain and I type "make" instead of "smake"
>> which uses the alternative compiler with the required support.
>>
>> @Linus: do you want a warning or prefer not to have one ?
>
> Honestly, I think I'd be much happier with the warning as part of the
> "make config" phase.
>
> What really annoyed me was that it showed up at every build.
>
> What I would really want - and this is entirely unrelated to this
> particular case - is to have those damn compiler option tests as part
> of the config phase in general. We now have about a million of these
> crazy things, where we have config options that simply depend on which
> compiler we have, and we have no sane way to show them at
> configuration time.
>
> Though Andrew's tree I got yet another ugly hack
> (CONFIG_CC_STACKPROTECTOR_AUTO) that handles just _one_ special case
> by turning it into a special magic Kconfig entry in the main Makefile.
> See commit 44c6dc940b19 ("Makefile: introduce
> CONFIG_CC_STACKPROTECTOR_AUTO"). I wasn't sure if I really wanted it,
> and honestly, I'm still thinking of just reverting it, because it's
> _so_ ugly and _so_ wrong.
>
> What we need is an extension to the Kconfig language itself so that we can do
>
>    config CC_HAS_RETPOLINE
>         cc_option "-mindirect-branch=thunk -mindirect-branch-table"
>
> or something. And then we can make sane _conditional_ dependencies at
> Kconfig time, and our makefiles would be much cleaner too when you
> could just do
>
>      cflags-$(USE_RETPOLINE) += -mfunction-return=thunk -mindirect-branch-table
>
> because the validity of the C compiler flag has been tested when configuring.
>
> And then we could add that warning at configure time (or just disable
> the option there thanks to "depends on CC_HAS_xyz" logic).
>
> All our compiler option handling right now is just nasty nasty nasty crud.
>
> Adding more people in the hopes that somebody gets motivated.. I've
> talked about this before, so far we haven't made any progress.


Sorry for slow progress.

I agreed this before, and still motivated.
(because I also motivated to remove kbuild cache.
This turned out not so clever as I first thought)

I was trying to do this, but in this development cycle
I spent most of my time to flush out lots of piled up Kconfig patches.
Sorry.

Unless somebody is working, I will.



-- 
Best Regards
Masahiro Yamada