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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>,
	Christoph Hellwig <hch@lst.de>,
	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>,
	Alexey Dobriyan <adobriyan@gmail.com>
Subject: Re: remove the last set_fs() in common code, and remove it for x86 and powerpc v3
Date: Thu, 10 Sep 2020 10:16:19 -0500	[thread overview]
Message-ID: <20200910151619.GI28786@gate.crashing.org> (raw)
In-Reply-To: <59a64e9a210847b59f70f9bd2d02b5c3@AcuMS.aculab.com>

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@kernel.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

  parent reply	other threads:[~2020-09-10 15:54 UTC|newest]

Thread overview: 58+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-03 14:22 remove the last set_fs() in common code, and remove it for x86 and powerpc v3 Christoph Hellwig
2020-09-03 14:22 ` [PATCH 01/14] proc: remove a level of indentation in proc_get_inode Christoph Hellwig
2020-09-03 14:22 ` [PATCH 02/14] proc: cleanup the compat vs no compat file ops Christoph Hellwig
2020-09-03 14:22 ` [PATCH 03/14] proc: add a read_iter method to proc proc_ops Christoph Hellwig
2020-09-03 14:22 ` [PATCH 04/14] sysctl: Convert to iter interfaces Christoph Hellwig
2020-09-03 14:22 ` [PATCH 05/14] fs: don't allow kernel reads and writes without iter ops Christoph Hellwig
2020-10-01 22:38   ` Eric Biggers
2020-10-01 22:40     ` Al Viro
2020-10-02 16:27       ` Linus Torvalds
2020-10-09 22:06         ` Eric Biggers
2020-10-10  1:03           ` Linus Torvalds
2020-10-10  1:19             ` Eric Biggers
2020-10-10  1:29               ` Linus Torvalds
2020-10-10  1:55                 ` Alexander Viro
2020-10-14  5:51                   ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 06/14] fs: don't allow splice read/write without explicit ops Christoph Hellwig
2020-09-03 14:22 ` [PATCH 07/14] uaccess: add infrastructure for kernel builds with set_fs() Christoph Hellwig
2020-09-03 14:22 ` [PATCH 08/14] test_bitmap: remove user bitmap tests Christoph Hellwig
2020-09-03 14:22 ` [PATCH 09/14] lkdtm: remove set_fs-based tests Christoph Hellwig
2020-09-03 14:22 ` [PATCH 10/14] x86: move PAGE_OFFSET, TASK_SIZE & friends to page_{32,64}_types.h Christoph Hellwig
2020-09-03 14:22 ` [PATCH 11/14] x86: make TASK_SIZE_MAX usable from assembly code Christoph Hellwig
2020-09-03 14:22 ` [PATCH 12/14] x86: remove address space overrides using set_fs() Christoph Hellwig
2020-09-03 21:30   ` David Laight
2020-09-03 23:25     ` Linus Torvalds
2020-09-04  7:59       ` David Laight
2020-09-04  2:55   ` Al Viro
2020-09-04  4:41     ` Al Viro
2020-09-04  6:38     ` Christoph Hellwig
2020-09-04  7:47       ` Christoph Hellwig
2020-09-03 14:22 ` [PATCH 13/14] powerpc: use non-set_fs based maccess routines Christoph Hellwig
2020-09-03 14:22 ` [PATCH 14/14] powerpc: remove address space overrides using set_fs() Christoph Hellwig
2020-09-03 15:43   ` Christophe Leroy
2020-09-03 15:49     ` Christoph Hellwig
2020-09-03 15:56       ` Christoph Hellwig
2020-09-03 16:03         ` Christophe Leroy
2020-09-03 14:28 ` remove the last set_fs() in common code, and remove it for x86 and powerpc v3 Al Viro
2020-09-03 14:30   ` Christoph Hellwig
2020-09-03 14:36     ` Al Viro
2020-09-03 14:40       ` Christoph Hellwig
2020-09-09 17:31   ` Linus Torvalds
2020-09-09 18:40     ` Segher Boessenkool
2020-09-09 21:33       ` Linus Torvalds
2020-09-10  8:04         ` David Laight
2020-09-10  8:13           ` Christophe Leroy
2020-09-10  9:26             ` David Laight
2020-09-10 12:26               ` David Laight
2020-09-10 15:20                 ` Segher Boessenkool
2020-09-10 15:31                   ` David Laight
2020-09-10 17:15                     ` Segher Boessenkool
2020-09-10 15:16               ` Segher Boessenkool [this message]
2020-09-10 15:44         ` Segher Boessenkool
2020-09-03 15:49 ` Linus Torvalds
2020-09-04  6:00 ` Ingo Molnar
2020-09-04 17:58   ` Alexey Dobriyan
2020-09-04 18:42     ` Linus Torvalds
2020-09-04 21:01     ` David Laight
2020-09-05  7:16       ` Christophe Leroy
2020-09-05 10:13         ` David Laight

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