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Date:   Thu, 10 Sep 2020 12:15:42 -0500
From:   Segher Boessenkool <segher@kernel.crashing.org>
To:     David Laight <David.Laight@aculab.com>
Cc:     "'Christophe Leroy'" <christophe.leroy@csgroup.eu>,
        "'Linus Torvalds'" <torvalds@linux-foundation.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linuxppc-dev <linuxppc-dev@lists.ozlabs.org>,
        Christoph Hellwig <hch@lst.de>
Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3
Message-ID: <20200910171542.GL28786@gate.crashing.org>
References: <20200903142803.GM1236603@ZenIV.linux.org.uk> <CAHk-=wgQNyeHxXfckd1WtiYnoDZP1Y_kD-tJKqWSksRoDZT=Aw@mail.gmail.com> <20200909184001.GB28786@gate.crashing.org> <CAHk-=whu19Du_rZ-zBtGsXAB-Qo7NtoJjQjd-Sa9OB5u1Cq_Zw@mail.gmail.com> <3beb8b019e4a4f7b81fdb1bc68bd1e2d@AcuMS.aculab.com> <186a62fc-042c-d6ab-e7dc-e61b18945498@csgroup.eu> <59a64e9a210847b59f70f9bd2d02b5c3@AcuMS.aculab.com> <5050b43687c84515a49b345174a98822@AcuMS.aculab.com> <20200910152030.GJ28786@gate.crashing.org> <18fdbaeacba349a0a8bf7568f709e991@AcuMS.aculab.com>
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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