[PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

[PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

[Jason Gerecke]

When compiling an external kernel module with `-O0` or `-O1`, the following
compile error may be reported:

    ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
       25 |  asm_volatile_goto("1:"
          |  ^~~~~~~~~~~~~~~~~

It appears that these lower optimization levels prevent GCC from detecting
that the key/branch arguments can be treated as constants and used as
immediate operands. To work around this, explicitly add the `const` label.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ard Biesheuvel <ardb@kernel.org>
---
Marked RFC since I'm not familiar with this subsystem or the asm blocks that
are impacted. Extra-close inspection would be appreciated.

 arch/x86/include/asm/jump_label.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 06c3cc22a058..7f2006645d84 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -20,7 +20,7 @@
 #include <linux/stringify.h>
 #include <linux/types.h>
 
-static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
@@ -36,7 +36,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
 	return true;
 }
 
-static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
-- 
2.30.1

Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

[Steven Rostedt]

On Thu, 11 Feb 2021 13:48:48 -0800
Jason Gerecke <killertofu@gmail.com> wrote:

> When compiling an external kernel module with `-O0` or `-O1`, the following
> compile error may be reported:
> 
>     ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
>        25 |  asm_volatile_goto("1:"
>           |  ^~~~~~~~~~~~~~~~~
> 
> It appears that these lower optimization levels prevent GCC from detecting
> that the key/branch arguments can be treated as constants and used as
> immediate operands. To work around this, explicitly add the `const` label.

Yes this makes sense. The "i" constraint needs to be a constant.

Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

-- Steve

> 
> Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> ---
> Marked RFC since I'm not familiar with this subsystem or the asm blocks that
> are impacted. Extra-close inspection would be appreciated.
> 
>  arch/x86/include/asm/jump_label.h | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
> index 06c3cc22a058..7f2006645d84 100644
> --- a/arch/x86/include/asm/jump_label.h
> +++ b/arch/x86/include/asm/jump_label.h
> @@ -20,7 +20,7 @@
>  #include <linux/stringify.h>
>  #include <linux/types.h>
>  
> -static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
> +static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
>  {
>  	asm_volatile_goto("1:"
>  		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
> @@ -36,7 +36,7 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
>  	return true;
>  }
>  
> -static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
> +static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
>  {
>  	asm_volatile_goto("1:"
>  		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"

Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

[Peter Zijlstra]

On Fri, Feb 12, 2021 at 09:40:59AM -0500, Steven Rostedt wrote:
> On Thu, 11 Feb 2021 13:48:48 -0800
> Jason Gerecke <killertofu@gmail.com> wrote:
> 
> > When compiling an external kernel module with `-O0` or `-O1`, the following
> > compile error may be reported:
> > 
> >     ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
> >        25 |  asm_volatile_goto("1:"
> >           |  ^~~~~~~~~~~~~~~~~
> > 
> > It appears that these lower optimization levels prevent GCC from detecting
> > that the key/branch arguments can be treated as constants and used as
> > immediate operands. To work around this, explicitly add the `const` label.
> 
> Yes this makes sense. The "i" constraint needs to be a constant.

Right, using -O[01] seems a little daft though. But yeah, that patch is
correct and won't cause harm.

I've queued it for after the next merge window.

Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

[Jason Gerecke]

On Fri, Feb 12, 2021 at 7:27 AM Peter Zijlstra <peterz@infradead.org> wrote:
>
> On Fri, Feb 12, 2021 at 09:40:59AM -0500, Steven Rostedt wrote:
> > On Thu, 11 Feb 2021 13:48:48 -0800
> > Jason Gerecke <killertofu@gmail.com> wrote:
> >
> > > When compiling an external kernel module with `-O0` or `-O1`, the following
> > > compile error may be reported:
> > >
> > >     ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
> > >        25 |  asm_volatile_goto("1:"
> > >           |  ^~~~~~~~~~~~~~~~~
> > >
> > > It appears that these lower optimization levels prevent GCC from detecting
> > > that the key/branch arguments can be treated as constants and used as
> > > immediate operands. To work around this, explicitly add the `const` label.
> >
> > Yes this makes sense. The "i" constraint needs to be a constant.
>
> Right, using -O[01] seems a little daft though. But yeah, that patch is
> correct and won't cause harm.
>
> I've queued it for after the next merge window.

Thanks. Only reason I even tried compiling at those levels was to play
around with GCC's new static analyzer options. They seem to be
ineffective at more normal optimization levels.

Re: [PATCH] RFC: x86/jump_label: Mark arguments as const to satisfy asm constraints

[Josh Poimboeuf]

On Thu, Feb 11, 2021 at 01:48:48PM -0800, Jason Gerecke wrote:
> When compiling an external kernel module with `-O0` or `-O1`, the following
> compile error may be reported:
> 
>     ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
>        25 |  asm_volatile_goto("1:"
>           |  ^~~~~~~~~~~~~~~~~
> 
> It appears that these lower optimization levels prevent GCC from detecting
> that the key/branch arguments can be treated as constants and used as
> immediate operands. To work around this, explicitly add the `const` label.
> 
> Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Ard Biesheuvel <ardb@kernel.org>
> ---
> Marked RFC since I'm not familiar with this subsystem or the asm blocks that
> are impacted. Extra-close inspection would be appreciated.

Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>

-- 
Josh

[tip: locking/core] x86/jump_label: Mark arguments as const to satisfy asm constraints

[tip-bot2 for Jason Gerecke]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     566a9522381495d27b596ee3bdc9578ba02a895d
Gitweb:        https://git.kernel.org/tip/566a9522381495d27b596ee3bdc9578ba02a895d
Author:        Jason Gerecke <killertofu@gmail.com>
AuthorDate:    Thu, 11 Feb 2021 13:48:48 -08:00
Committer:     Peter Zijlstra <peterz@infradead.org>
CommitterDate: Tue, 02 Mar 2021 15:06:34 +01:00

x86/jump_label: Mark arguments as const to satisfy asm constraints

When compiling an external kernel module with `-O0` or `-O1`, the following
compile error may be reported:

    ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
       25 |  asm_volatile_goto("1:"
          |  ^~~~~~~~~~~~~~~~~

It appears that these lower optimization levels prevent GCC from detecting
that the key/branch arguments can be treated as constants and used as
immediate operands. To work around this, explicitly add the `const` label.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20210211214848.536626-1-jason.gerecke@wacom.com
---
 arch/x86/include/asm/jump_label.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 06c3cc2..7f20066 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -20,7 +20,7 @@
 #include <linux/stringify.h>
 #include <linux/types.h>
 
-static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
@@ -36,7 +36,7 @@ l_yes:
 	return true;
 }
 
-static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"

[tip: locking/core] x86/jump_label: Mark arguments as const to satisfy asm constraints

[tip-bot2 for Jason Gerecke]

The following commit has been merged into the locking/core branch of tip:

Commit-ID:     864b435514b286c0be2a38a02f487aa28d990ef8
Gitweb:        https://git.kernel.org/tip/864b435514b286c0be2a38a02f487aa28d990ef8
Author:        Jason Gerecke <killertofu@gmail.com>
AuthorDate:    Thu, 11 Feb 2021 13:48:48 -08:00
Committer:     Ingo Molnar <mingo@kernel.org>
CommitterDate: Sat, 06 Mar 2021 12:51:00 +01:00

x86/jump_label: Mark arguments as const to satisfy asm constraints

When compiling an external kernel module with `-O0` or `-O1`, the following
compile error may be reported:

    ./arch/x86/include/asm/jump_label.h:25:2: error: impossible constraint in ‘asm’
       25 |  asm_volatile_goto("1:"
          |  ^~~~~~~~~~~~~~~~~

It appears that these lower optimization levels prevent GCC from detecting
that the key/branch arguments can be treated as constants and used as
immediate operands. To work around this, explicitly add the `const` label.

Signed-off-by: Jason Gerecke <jason.gerecke@wacom.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
Acked-by: Josh Poimboeuf <jpoimboe@redhat.com>
Link: https://lkml.kernel.org/r/20210211214848.536626-1-jason.gerecke@wacom.com
---
 arch/x86/include/asm/jump_label.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 06c3cc2..7f20066 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -20,7 +20,7 @@
 #include <linux/stringify.h>
 #include <linux/types.h>
 
-static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
@@ -36,7 +36,7 @@ l_yes:
 	return true;
 }
 
-static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
+static __always_inline bool arch_static_branch_jump(struct static_key * const key, const bool branch)
 {
 	asm_volatile_goto("1:"
 		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"