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

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On 08/12/2015 01:46 PM, Peter Zijlstra wrote:
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...

No, it produces the same error. This is Fedora 13, btw, uses gcc 4.4.4.

-boris



---
  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:

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