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

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Date:   Thu, 10 Sep 2020 09:26:28 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christophe Leroy' <christophe.leroy@...roup.eu>,
        '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

From: Christophe Leroy
> Sent: 10 September 2020 09:14
> 
> 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 "

A quick test isn't good....

int bar(char *z)
{
        __label__ label;
        register int eax asm ("eax") = 6;
        asm volatile goto (" mov $1, %%eax" ::: "eax" : label);

label:
        return eax;
}

0000000000000040 <bar>:
  40:   b8 01 00 00 00          mov    $0x1,%eax
  45:   b8 06 00 00 00          mov    $0x6,%eax
  4a:   c3                      retq

although adding:
        asm volatile ("" : "+r" (eax));
either side of the 'asm volatile goto' does fix it.

	David

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