linux-kernel - Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3

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Message-ID: <186a62fc-042c-d6ab-e7dc-e61b18945498@csgroup.eu>
Date:   Thu, 10 Sep 2020 10:13:42 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     David Laight <David.Laight@...LAB.COM>,
        'Linus Torvalds' <torvalds@...ux-foundation.org>,
        Segher Boessenkool <segher@...nel.crashing.org>
Cc:     linux-arch <linux-arch@...r.kernel.org>,
        Kees Cook <keescook@...omium.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Christoph Hellwig <hch@....de>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Alexey Dobriyan <adobriyan@...il.com>
Subject: Re: remove the last set_fs() in common code, and remove it for x86
 and powerpc v3



Le 10/09/2020 à 10:04, David Laight a écrit :
> From: Linus Torvalds
>> Sent: 09 September 2020 22:34
>> On Wed, Sep 9, 2020 at 11:42 AM Segher Boessenkool
>> <segher@...nel.crashing.org> wrote:
>>>
>>> It will not work like this in GCC, no.  The LLVM people know about that.
>>> I do not know why they insist on pushing this, being incompatible and
>>> everything.
>>
>> Umm. Since they'd be the ones supporting this, *gcc* would be the
>> incompatible one, not clang.
> 
> I had an 'interesting' idea.
> 
> Can you use a local asm register variable as an input and output to
> an 'asm volatile goto' statement?
> 
> Well you can - but is it guaranteed to work :-)
> 

With gcc at least it should work according to 
https://gcc.gnu.org/onlinedocs/gcc/Local-Register-Variables.html

They even explicitely tell: "The only supported use for this feature is 
to specify registers for input and output operands when calling Extended 
asm "

Christophe