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 12:15:42 -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>,
        Alexey Dobriyan <adobriyan@...il.com>,
        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>,
        Christoph Hellwig <hch@....de>
Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3

On Thu, Sep 10, 2020 at 03:31:53PM +0000, David Laight wrote:
> > > 	asm volatile ("" : "+r" (eax));
> > > 	// So here eax must contain the value set by the "xxxxx" instructions.
> > 
> > No, the register eax will contain the value of the eax variable.  In the
> > asm; it might well be there before or after the asm as well, but none of
> > that is guaranteed.
> 
> Perhaps not 'guaranteed', but very unlikely to be wrong.
> It doesn't give gcc much scope for not generating the desired code.

Wanna bet?  :-)

Correct is correct.  Anything else is not.


Segher