selftests, x86: Rework x86 target architecture detection

We currently fail to build on a non-multilib x86_64 target.  We
print a helpful error, but it's nicer to allow the build to succeed.
Fix it and improve cross-compilation support by detecting
architecture support directly and building only the relevant tests.
Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/x86/Makefile

-.PHONY: all all_32 all_64 check_build32 clean
+all:
+
+include ../lib.mk
+
+.PHONY: all all_32 all_64 warn_32bit_failure clean
 
 TARGETS_C_BOTHBITS := sigreturn single_step_syscall
 
@@ -7,29 +11,24 @@ BINARIES_64 := $(TARGETS_C_BOTHBITS:%=%_64)
 
 CFLAGS := -O2 -g -std=gnu99 -pthread -Wall
 
-all:
-
 UNAME_M := $(shell uname -m)
+CAN_BUILD_I386 := $(shell ./check_cc.sh $(CC) trivial_32bit_program.c -m32)
+CAN_BUILD_X86_64 := $(shell ./check_cc.sh $(CC) trivial_64bit_program.c)
 
-ifeq ($(CROSS_COMPILE),)
-# Always build 32-bit tests
+ifeq ($(CAN_BUILD_I386),1)
 all: all_32
-# Install 32-bit tests
 TEST_PROGS += $(BINARIES_32)
-# If we're on a 64-bit host, build 64-bit tests as well
-ifeq ($(UNAME_M),x86_64)
+endif
+
+ifeq ($(CAN_BUILD_X86_64),1)
 all: all_64
-# Install 64-bit tests
 TEST_PROGS += $(BINARIES_64)
 endif
-endif
 
-all_32: check_build32 $(BINARIES_32)
+all_32: $(BINARIES_32)
 
 all_64: $(BINARIES_64)
 
-include ../lib.mk
-
 clean:
 	$(RM) $(BINARIES_32) $(BINARIES_64)
 
@@ -39,16 +38,20 @@ $(TARGETS_C_BOTHBITS:%=%_32): %_32: %.c
 $(TARGETS_C_BOTHBITS:%=%_64): %_64: %.c
 	$(CC) -m64 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl
 
-check_build32:
-	@if ! $(CC) -m32 -o /dev/null trivial_32bit_program.c; then	\
-	  echo "Warning: you seem to have a broken 32-bit build" 2>&1; 	\
-	  echo "environment.  If you are using a Debian-like";		\
-	  echo " distribution, try:"; 					\
-	  echo "";							\
-	  echo "  apt-get install gcc-multilib libc6-i386 libc6-dev-i386"; \
-	  echo "";							\
-	  echo "If you are using a Fedora-like distribution, try:";	\
-	  echo "";							\
-	  echo "  yum install glibc-devel.*i686";			\
-	  exit 1;							\
-	fi
+# x86_64 users should be encouraged to install 32-bit libraries
+ifeq ($(CAN_BUILD_I386)$(CAN_BUILD_X86_64),01)
+all: warn_32bit_failure
+
+warn_32bit_failure:
+	@echo "Warning: you seem to have a broken 32-bit build" 2>&1; 	\
+	echo "environment.  This will reduce test coverage of 64-bit" 2>&1; \
+	echo "kernels.  If you are using a Debian-like distribution," 2>&1; \
+	echo "try:"; 2>&1; \
+	echo "";							\
+	echo "  apt-get install gcc-multilib libc6-i386 libc6-dev-i386"; \
+	echo "";							\
+	echo "If you are using a Fedora-like distribution, try:";	\
+	echo "";							\
+	echo "  yum install glibc-devel.*i686";				\
+	exit 0;
+endif

tools/testing/selftests/x86/check_cc.sh

+#!/bin/sh
+# check_cc.sh - Helper to test userspace compilation support
+# Copyright (c) 2015 Andrew Lutomirski
+# GPL v2
+
+CC="$1"
+TESTPROG="$2"
+shift 2
+
+if "$CC" -o /dev/null "$TESTPROG" -O0 "$@" 2>/dev/null; then
+    echo 1
+else
+    echo 0
+fi
+
+exit 0

tools/testing/selftests/x86/trivial_32bit_program.c

@@ -4,6 +4,10 @@
  * GPL v2
  */
 
+#ifndef __i386__
+# error wrong architecture
+#endif
+
 #include <stdio.h>
 
 int main()

tools/testing/selftests/x86/trivial_64bit_program.c

+/*
+ * Trivial program to check that we have a valid 32-bit build environment.
+ * Copyright (c) 2015 Andy Lutomirski
+ * GPL v2
+ */
+
+#ifndef __x86_64__
+# error wrong architecture
+#endif
+
+#include <stdio.h>
+
+int main()
+{
+	printf("\n");
+
+	return 0;
+}