"error: expected writable system register or pstate" in arch/arm64/lib/delay.c

[nathanchance]

After llvm/llvm-project@b115dd1:

arch/arm64/lib/delay.c:52:3: error: expected writable system register or pstate
   52 |                 wfit(end);
      |                 ^
arch/arm64/include/asm/barrier.h:24:32: note: expanded from macro 'wfit'
   24 | #define wfit(val)       asm volatile("msr s0_3_c1_c0_1, %0"     \
      |                                      ^
<inline asm>:1:6: note: instantiated into assembly here
    1 |         msr s0_3_c1_c0_1, x26
      |             ^
arch/arm64/lib/delay.c:54:4: error: expected writable system register or pstate
   54 |                         wfet(end);
      |                         ^
arch/arm64/include/asm/barrier.h:21:32: note: expanded from macro 'wfet'
   21 | #define wfet(val)       asm volatile("msr s0_3_c1_c0_0, %0"     \
      |                                      ^
<inline asm>:1:6: note: instantiated into assembly here
    1 |         msr s0_3_c1_c0_0, x26
      |             ^
2 errors generated.

I have asked the author if this is intentional (which it seems like it is). If so, we will probably need something like

diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
index fe60738e5943..0e21f8030adc 100644
--- a/arch/arm64/Kconfig
+++ b/arch/arm64/Kconfig
@@ -2135,6 +2135,10 @@ config ARM64_EPAN
 	  if the cpu does not implement the feature.
 endmenu # "ARMv8.7 architectural features"
 
+config AS_HAS_WFXT
+	# supported by binutils 2.36+ and LLVM 13+ (17+ for working .arch)
+	def_bool $(as-instr,.arch armv8.7-a\nwfit x1)
+
 config AS_HAS_MOPS
 	def_bool $(as-instr,.arch_extension mops)
 
diff --git a/arch/arm64/include/asm/barrier.h b/arch/arm64/include/asm/barrier.h
index 9495c4441a46..37725391179d 100644
--- a/arch/arm64/include/asm/barrier.h
+++ b/arch/arm64/include/asm/barrier.h
@@ -16,13 +16,19 @@
 #define __nops(n)	".rept	" #n "\nnop\n.endr\n"
 #define nops(n)		asm volatile(__nops(n))
 
+#ifdef CONFIG_AS_HAS_WFXT
+#define __wfet_asm ".arch armv8.7-a\nwfet"
+#define __wfit_asm ".arch armv8.7-a\nwfit"
+#else
+#define __wfet_asm "msr s0_3_c1_c0_0,"
+#define __wfit_asm "msr s0_3_c1_c0_1,"
+#endif
+
 #define sev()		asm volatile("sev" : : : "memory")
 #define wfe()		asm volatile("wfe" : : : "memory")
-#define wfet(val)	asm volatile("msr s0_3_c1_c0_0, %0"	\
-				     : : "r" (val) : "memory")
+#define wfet(val)	asm volatile(__wfet_asm " %0" : : "r" (val) : "memory")
 #define wfi()		asm volatile("wfi" : : : "memory")
-#define wfit(val)	asm volatile("msr s0_3_c1_c0_1, %0"	\
-				     : : "r" (val) : "memory")
+#define wfit(val)	asm volatile(__wfit_asm " %0" : : "r" (val) : "memory")
 
 #define isb()		asm volatile("isb" : : : "memory")
 #define dmb(opt)	asm volatile("dmb " #opt : : : "memory")