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 10:16:19 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     David Laight <David.Laight@...lab.com>
Cc:     "'Christophe Leroy'" <christophe.leroy@...roup.eu>,
        "'Linus Torvalds'" <torvalds@...ux-foundation.org>,
        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

On Thu, Sep 10, 2020 at 09:26:28AM +0000, David Laight wrote:
> 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;
> }

It is neither input nor output operand here!  Only *then* is a local
register asm guaranteed to be in the given reg: as input or output to an
inline asm.


Segher