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 in that case, 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@xxxxxxxxxx>
---
arch/arm64/Makefile | 9 +++++++++
arch/arm64/include/asm/kvm_asm.h | 8 +++++++-
2 files changed, 16 insertions(+), 1 deletion(-)
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 5789c2d18d43..c4ee8e64ad1a 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -44,12 +44,21 @@ cc_has_k_constraint := $(call try-run,echo \
return 0; \
}' | $(CC) -S -x c -o "$$TMP" -,,-DCONFIG_CC_HAS_K_CONSTRAINT=1)
+cc_has_broken_s_constraint := $(call try-run,echo \
+ 'void *foo(void) { \
+ static int x; \
+ int *addr; \
+ asm("adrp %0, %1" : "=r" (addr) : "S" (&x)); \
+ return addr; \
+ }' | $(CC) -S -x c -c -O2 -o "$$TMP" -,,-DCONFIG_CC_HAS_BROKEN_S_CONSTRAINT=1)
+
ifeq ($(CONFIG_BROKEN_GAS_INST),y)
$(warning Detected assembler with broken .inst; disassembly will be unreliable)
endif
KBUILD_CFLAGS += -mgeneral-regs-only \
$(compat_vdso) $(cc_has_k_constraint)
+KBUILD_CFLAGS += $(cc_has_broken_s_constraint)
KBUILD_CFLAGS += $(call cc-disable-warning, psabi)
KBUILD_AFLAGS += $(compat_vdso)
diff --git a/arch/arm64/include/asm/kvm_asm.h b/arch/arm64/include/asm/kvm_asm.h
index 7ccf770c53d9..fa8e886998a3 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);
+#ifdef CONFIG_CC_HAS_BROKEN_S_CONSTRAINT
+#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
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