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From: David Laight <David.Laight@ACULAB.COM>
To: 'Segher Boessenkool' <segher@kernel.crashing.org>
Subject: RE: remove the last set_fs() in common code, and remove it for x86
 and powerpc v3
Thread-Topic: remove the last set_fs() in common code, and remove it for x86
 and powerpc v3
Thread-Index: AQHWhvDyD2c/lZfV3kC0Ftay5UVebqlhgjnw///zuACAABnIEIAAOoMggAAi9ACAABJI8A==
Date: Thu, 10 Sep 2020 15:31:53 +0000
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> -----Original Message-----
> From: Segher Boessenkool <segher@kernel.crashing.org>
> Sent: 10 September 2020 16:21
> To: David Laight <David.Laight@ACULAB.COM>
> Cc: 'Christophe Leroy' <christophe.leroy@csgroup.eu>; 'Linus Torvalds' <t=
orvalds@linux-
> foundation.org>; linux-arch <linux-arch@vger.kernel.org>; Kees Cook <kees=
cook@chromium.org>; the
> arch/x86 maintainers <x86@kernel.org>; Nick Desaulniers <ndesaulniers@goo=
gle.com>; Linux Kernel
> Mailing List <linux-kernel@vger.kernel.org>; Alexey Dobriyan <adobriyan@g=
mail.com>; Luis Chamberlain
> <mcgrof@kernel.org>; Al Viro <viro@zeniv.linux.org.uk>; linux-fsdevel <li=
nux-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 x=
86 and powerpc v3
>=20
> On Thu, Sep 10, 2020 at 12:26:53PM +0000, David Laight wrote:
> > Actually this is pretty sound:
> > =09__label__ label;
> > =09register int eax asm ("eax");
> > =09// Ensure eax can't be reloaded from anywhere
> > =09// In particular it can't be reloaded after the asm goto line
> > =09asm volatile ("" : "=3Dr" (eax));
>=20
> This asm is fine.  It says it writes the "eax" variable, which lives in
> the eax register *in that asm* (so *not* guaranteed after it!).
>=20
> > =09// Provided gcc doesn't save eax here...
> > =09asm volatile goto ("xxxxx" ::: "eax" : label);
>=20
> So this is incorrect.

>From the other email:

> 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.

Ok, so adding '"r" (eax)' to the input section helps a bit.

> > =09// ... and reload the saved value here.
> > =09// The input value here will be that modified by the 'asm goto'.
> > =09// Since this modifies eax it can't be moved before the 'asm goto'.
> > =09asm volatile ("" : "+r" (eax));
> > =09// So here eax must contain the value set by the "xxxxx" instruction=
s.
>=20
> 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.

=09David

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