Subject: Re: linux-next: manual merge of the xen-tip tree with the tip tree

From: Peter Zijlstra <[email-masked]>
Date: Wed, 12 Aug 2015 19:46:25 +0200

On Wed, Aug 12, 2015 at 07:21:05PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 12, 2015 at 09:27:38AM -0400, Boris Ostrovsky wrote:
> 
> > Incidentally, 11276d53 ("locking/static_keys: Add a new static_key
> > interface") breaks old-ish compilers (gcc version 4.4.4 20100503 (Red Hat
> > 4.4.4-2) (GCC)):
> > 
> > 
> > 
> >   CC      arch/x86/kernel/nmi.o
> > In file included from
> > /home/build/linux-boris/include/linux/jump_label.h:109,
> >                  from
> > /home/build/linux-boris/arch/x86/include/asm/spinlock.h:5,
> >                  from /home/build/linux-boris/include/linux/spinlock.h:88,
> >                  from /home/build/linux-boris/arch/x86/kernel/nmi.c:14:
> > /home/build/linux-boris/arch/x86/include/asm/jump_label.h: In function
> > ‘nmi_handle’:
> > /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: warning: asm
> > operand 0 probably doesn’t match constraints
> > /home/build/linux-boris/arch/x86/include/asm/jump_label.h:21: error:
> > impossible constraint in ‘asm’
> > make[3]: *** [arch/x86/kernel/nmi.o] Error 1
> > make[2]: *** [arch/x86/kernel] Error 2
> > make[1]: *** [arch/x86] Error 2
> 
> Ugh bugger.
> 
> I bet its that: &((char *)key)[branch] business, an earlier variant
> thereof tripped up more recent GCCs too.
> 
> So its an __always_inline function, and both argument are always compile
> time constants, @key is the address of an object in static storage (a
> global) and @branch is a simple 0/1 at the call site.
> 
> Now we wish to compute (unsigned long)key + branch at compile/link time
> to feed to the assembler as an immediate, which should be possible,
> given its all 'constants'.
> 
> It just appears GCC is having a hard time with this.
> 
> Let me see if I have a sufficiently old GCC around to play with.

Could you feed the below to your compiler? Its a bit cumbersome, but
its the next best I could come up with...

---
 arch/x86/include/asm/jump_label.h | 56 ++++++++++++++++++++++++++++-----------
 1 file changed, 41 insertions(+), 15 deletions(-)

diff --git a/arch/x86/include/asm/jump_label.h b/arch/x86/include/asm/jump_label.h
index 28d7a857f9d1..76c769ae4200 100644
--- a/arch/x86/include/asm/jump_label.h
+++ b/arch/x86/include/asm/jump_label.h
@@ -16,15 +16,30 @@
 # define STATIC_KEY_INIT_NOP GENERIC_NOP5_ATOMIC
 #endif
 
+struct foo {
+	u8 zero;
+	u8 one;
+};
+
 static __always_inline bool arch_static_branch(struct static_key *key, bool branch)
 {
-	asm_volatile_goto("1:"
-		".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
-		".pushsection __jump_table,  \"aw\" \n\t"
-		_ASM_ALIGN "\n\t"
-		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
-		".popsection \n\t"
-		: :  "i" (&((char *)key)[branch]) : : l_yes);
+	if (!branch) {
+		asm_volatile_goto("1:"
+				".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
+				".pushsection __jump_table,  \"aw\" \n\t"
+				_ASM_ALIGN "\n\t"
+				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+				".popsection \n\t"
+				: :  "i" (&((struct foo *)key)->zero) : : l_yes);
+	} else {
+		asm_volatile_goto("1:"
+				".byte " __stringify(STATIC_KEY_INIT_NOP) "\n\t"
+				".pushsection __jump_table,  \"aw\" \n\t"
+				_ASM_ALIGN "\n\t"
+				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+				".popsection \n\t"
+				: :  "i" (&((struct foo *)key)->one) : : l_yes);
+	}
 
 	return false;
 l_yes:
@@ -33,14 +48,25 @@ static __always_inline bool arch_static_branch(struct static_key *key, bool bran
 
 static __always_inline bool arch_static_branch_jump(struct static_key *key, bool branch)
 {
-	asm_volatile_goto("1:"
-		".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
-		"2:\n\t"
-		".pushsection __jump_table,  \"aw\" \n\t"
-		_ASM_ALIGN "\n\t"
-		_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
-		".popsection \n\t"
-		: :  "i" (&((char *)key)[branch]) : : l_yes);
+	if (!branch) {
+		asm_volatile_goto("1:"
+				".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
+				"2:\n\t"
+				".pushsection __jump_table,  \"aw\" \n\t"
+				_ASM_ALIGN "\n\t"
+				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+				".popsection \n\t"
+				: :  "i" (&((struct foo *)key)->zero) : : l_yes);
+	} else {
+		asm_volatile_goto("1:"
+				".byte 0xe9\n\t .long %l[l_yes] - 2f\n\t"
+				"2:\n\t"
+				".pushsection __jump_table,  \"aw\" \n\t"
+				_ASM_ALIGN "\n\t"
+				_ASM_PTR "1b, %l[l_yes], %c0 \n\t"
+				".popsection \n\t"
+				: :  "i" (&((struct foo *)key)->one) : : l_yes);
+	}
 
 	return false;
 l_yes: