[kvm-unit-tests PATCH] configure: Test if compiler supports -m16 on x86

[Roman Bolshakov <r.bolshakov@yadro.com>]

-m16 option is available only since GCC 4.9.0 [1]. That causes a build
failure on centos-7 [2] that has GCC 4.8.5.

Fallback to -m32 if -m16 is not available.

1. http://gcc.gnu.org/bugzilla/show_bug.cgi?id=59672
2. https://gitlab.com/bonzini/kvm-unit-tests/-/jobs/755368387

Fixes: 2616ad934e2 ("x86: realmode: Workaround clang issues")
Signed-off-by: Roman Bolshakov <r.bolshakov@yadro.com>
---
 configure           | 11 +++++++++++
 x86/Makefile.common |  4 ++++
 2 files changed, 15 insertions(+)

diff --git a/configure b/configure
index f930543..7dc2e3b 100755
--- a/configure
+++ b/configure
@@ -16,6 +16,7 @@ pretty_print_stacks=yes
 environ_default=yes
 u32_long=
 wa_divide=
+m16_support=
 vmm="qemu"
 errata_force=0
 erratatxt="$srcdir/errata.txt"
@@ -167,6 +168,15 @@ EOF
   rm -f lib-test.{o,S}
 fi
 
+# check if -m16 is supported
+if [ "$arch" = "i386" ] || [ "$arch" = "x86_64" ]; then
+  cat << EOF > lib-test.c
+int f(int a, int b) { return a + b; }
+EOF
+  m16_support=$("$cross_prefix$cc" -m16 -c lib-test.c >/dev/null 2>&1 && echo yes)
+  rm -f lib-test.{o,c}
+fi
+
 # require enhanced getopt
 getopt -T > /dev/null
 if [ $? -ne 4 ]; then
@@ -224,6 +234,7 @@ ENVIRON_DEFAULT=$environ_default
 ERRATATXT=$erratatxt
 U32_LONG_FMT=$u32_long
 WA_DIVIDE=$wa_divide
+M16_SUPPORT=$m16_support
 EOF
 
 cat <<EOF > lib/config.h
diff --git a/x86/Makefile.common b/x86/Makefile.common
index 5567d66..553bf49 100644
--- a/x86/Makefile.common
+++ b/x86/Makefile.common
@@ -72,7 +72,11 @@ $(TEST_DIR)/realmode.elf: $(TEST_DIR)/realmode.o
 	$(CC) -m32 -nostdlib -o $@ -Wl,-m,elf_i386 \
 	      -Wl,-T,$(SRCDIR)/$(TEST_DIR)/realmode.lds $^
 
+ifeq ($(M16_SUPPORT),yes)
 $(TEST_DIR)/realmode.o: bits = 16
+else
+$(TEST_DIR)/realmode.o: bits = 32
+endif
 
 $(TEST_DIR)/kvmclock_test.elf: $(TEST_DIR)/kvmclock.o
 
-- 
2.28.0