[RFC arm64] samples/bpf: explicitly exclude sysreg sections with asm macros

[andy@greyhouse.net (Andy Gospodarek)]

The previous fix to workaround compilation issues for samples/bpf on arm64 was
to prevent inclusion of code in sysregs.h.  This is not sustainable at this
point since other header files need access to sysregs.h for multiple
defintions.  The fact that the bpf samples cannot be compiled on arm64 is
fairly well documented on the iovisor-dev mailing and other places without a
clear-cut solution other than waiting for the llvm fix.  This attempts to
resolve that by dropping the fix to define _ASM_SYSREG_H on the clang command
line, and replace it with one that is wrapped around the offending code.  I
recognize this feels a bit fragile, but the current situation is not great
either and it seems that we are more likely to see users if the current code
actually compiles and runs.

Despite now being able to compile and run bpf programs on arm64 there
are still known warnings when building:

./include/net/sock.h:2322:35: warning: value size does not match
register size specified by the constraint and modifier
[-Wasm-operand-widths]
        smp_store_release(&sk->sk_state, newstate);
                                         ^
./include/net/sock.h:2322:2: note: use constraint modifier "w"
        smp_store_release(&sk->sk_state, newstate);
        ^
./include/asm-generic/barrier.h:157:33: note: expanded from macro
                                ^
./arch/arm64/include/asm/barrier.h:62:23: note: expanded from macro
'__smp_store_release'
                asm volatile ("stlr %1, %0"                     \
                                    ^
Discussed here: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63359

Fixes: 30b50aa61201 ("bpf: samples: exclude asm/sysreg.h for arm64")
Signed-off-by: Andy Gospodarek <gospo@broadcom.com>
---
 arch/arm64/include/asm/sysreg.h | 3 ++-
 samples/bpf/Makefile            | 2 +-
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/sysreg.h b/arch/arm64/include/asm/sysreg.h
index ac24b6e..91ee2db 100644
--- a/arch/arm64/include/asm/sysreg.h
+++ b/arch/arm64/include/asm/sysreg.h
@@ -287,6 +287,7 @@
 #else
 
 #include <linux/types.h>
+#ifndef __AVOID_ASM_MACROS__
 
 asm(
 "	.irp	num,0,1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30\n"
@@ -302,7 +303,7 @@ asm(
 	__emit_inst(0xd5000000|(\\sreg)|(.L__reg_num_\\rt))
 "	.endm\n"
 );
-
+#endif
 /*
  * Unlike read_cpuid, calls to read_sysreg are never expected to be
  * optimized away or replaced with synthetic values.
diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
index 09e9d53..fca2c13 100644
--- a/samples/bpf/Makefile
+++ b/samples/bpf/Makefile
@@ -178,7 +178,7 @@ $(src)/*.c: verify_target_bpf
 # useless for BPF samples.
 $(obj)/%.o: $(src)/%.c
 	$(CLANG) $(NOSTDINC_FLAGS) $(LINUXINCLUDE) $(EXTRA_CFLAGS) \
-		-D__KERNEL__ -D__ASM_SYSREG_H -Wno-unused-value -Wno-pointer-sign \
+		-D__KERNEL__ -D__AVOID_ASM_MACROS__ -Wno-unused-value -Wno-pointer-sign \
 		-Wno-compare-distinct-pointer-types \
 		-Wno-gnu-variable-sized-type-not-at-end \
 		-Wno-address-of-packed-member -Wno-tautological-compare \
-- 
2.7.4