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

From: Marc Zyngier <[email-masked]>
Date: Mon, 7 Dec 2020 15:43:41 +0000

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 <[email-masked]>
---
 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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