linux-kernel - Re: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

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Message-ID: <20211108165016.kdehec32k3dw4334@treble>
Date:   Mon, 8 Nov 2021 08:50:16 -0800
From:   Josh Poimboeuf <jpoimboe@...hat.com>
To:     Vasily Gorbik <gor@...ux.ibm.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...e.de>,
        Miroslav Benes <mbenes@...e.cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] compiler.h: Avoid using inline asm operand modifiers

On Mon, Nov 08, 2021 at 04:40:38PM +0100, Vasily Gorbik wrote:
> > Does this work on s390?
> > 
> > diff --git a/include/linux/compiler.h b/include/linux/compiler.h
> > index 3d5af56337bd..42935500a712 100644
> > --- a/include/linux/compiler.h
> > +++ b/include/linux/compiler.h
> > @@ -115,24 +115,18 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
> >   * The __COUNTER__ based labels are a hack to make each instance of the macros
> >   * unique, to convince GCC not to merge duplicate inline asm statements.
> >   */
> > -#define __stringify_label(n) #n
> > -
> > -#define __annotate_reachable(c) ({					\
> > -	asm volatile(__stringify_label(c) ":\n\t"			\
> > +#define annotate_reachable() ({						\
> > +	asm volatile("%c0:\n\t"						\
> >  		     ".pushsection .discard.reachable\n\t"		\
> > -		     ".long " __stringify_label(c) "b - .\n\t"		\
> > -		     ".popsection\n\t");				\
> > +		     ".long %c0b - .\n\t"				\
> > +		     ".popsection\n\t" : : "i" (__COUNTER__ & 0x7f));	\
> >  })
> 
> hm, could we just add asm input back and not use it? and keep
> __stringify_label(c) as is? would that work as well?

Yeah, that seems to work:

diff --git a/include/linux/compiler.h b/include/linux/compiler.h
index 3d5af56337bd..429dcebe2b99 100644
--- a/include/linux/compiler.h
+++ b/include/linux/compiler.h
@@ -121,7 +121,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm volatile(__stringify_label(c) ":\n\t"			\
 		     ".pushsection .discard.reachable\n\t"		\
 		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define annotate_reachable() __annotate_reachable(__COUNTER__)
 
@@ -129,7 +129,7 @@ void ftrace_likely_update(struct ftrace_likely_data *f, int val,
 	asm volatile(__stringify_label(c) ":\n\t"			\
 		     ".pushsection .discard.unreachable\n\t"		\
 		     ".long " __stringify_label(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define annotate_unreachable() __annotate_unreachable(__COUNTER__)
 
diff --git a/include/linux/instrumentation.h b/include/linux/instrumentation.h
index fa2cd8c63dcc..24359b4a9605 100644
--- a/include/linux/instrumentation.h
+++ b/include/linux/instrumentation.h
@@ -11,7 +11,7 @@
 	asm volatile(__stringify(c) ": nop\n\t"				\
 		     ".pushsection .discard.instr_begin\n\t"		\
 		     ".long " __stringify(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define instrumentation_begin() __instrumentation_begin(__COUNTER__)
 
@@ -50,7 +50,7 @@
 	asm volatile(__stringify(c) ": nop\n\t"				\
 		     ".pushsection .discard.instr_end\n\t"		\
 		     ".long " __stringify(c) "b - .\n\t"		\
-		     ".popsection\n\t");				\
+		     ".popsection\n\t" : : "i" (c));			\
 })
 #define instrumentation_end() __instrumentation_end(__COUNTER__)
 #else