[PATCH] arm64: Work around broken GCC 4.9 handling of "S" constraint

[Marc Zyngier]

GCC 4.9 seems to have a problem with the "S" asm constraint
when the symbol lives in the same compilation unit, and pretends
the constraint is impossible:

$ cat x.c
void *foo(void)
{
	static int x;
	int *addr;
	asm("adrp %0, %1" : "=r" (addr) : "S" (&x));
	return addr;
}

$ ~/Work/gcc-linaro-aarch64-linux-gnu-4.9-2014.09_linux/bin/aarch64-linux-gnu-gcc -S -x c -O2 x.c
x.c: In function ‘foo’:
x.c:5:2: error: impossible constraint in ‘asm’
  asm("adrp %0, %1" : "=r" (addr) : "S" (&x));
  ^

Boo. Following revisions of the compiler work just fine, though.

We can fallback to the "i" constraint for GCC version prior to 5.0,
which *seems* to do the right thing. Hopefully we will be able to
remove this at some point, but in the meantime this gets us going.

Signed-off-by: Marc Zyngier <maz at kernel.org>
---
* From v1: Dropped the detection hack and rely on GCC_VERSION

 arch/arm64/include/asm/kvm_asm.h | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 7ccf770c53d9..8a33d83ea843 100644
--- a/arch/arm64/include/asm/kvm_asm.h
+++ b/arch/arm64/include/asm/kvm_asm.h
@@ -199,6 +199,12 @@ extern void __vgic_v3_init_lrs(void);
 
 extern u32 __kvm_get_mdcr_el2(void);
 
+#if defined(GCC_VERSION) && GCC_VERSION < 50000
+#define SYM_CONSTRAINT	"i"
+#else
+#define SYM_CONSTRAINT	"S"
+#endif
+
 /*
  * Obtain the PC-relative address of a kernel symbol
  * s: symbol
@@ -215,7 +221,7 @@ extern u32 __kvm_get_mdcr_el2(void);
 		typeof(s) *addr;					\
 		asm("adrp	%0, %1\n"				\
 		    "add	%0, %0, :lo12:%1\n"			\
-		    : "=r" (addr) : "S" (&s));				\
+		    : "=r" (addr) : SYM_CONSTRAINT (&s));		\
 		addr;							\
 	})
 
-- 
2.29.2