[PATCH v2] selftest/powerpc/benchmark: remove requirement libc-dev

[Madhavan Srinivasan <maddy@linux.ibm.com>]

Currently exec-target.c file is linked as static and this
post a requirement to install libc dev package to build.
Without it, build-break when compiling selftest/powerpc/benchmark.

  CC       exec_target
/usr/bin/ld: cannot find -lc: No such file or directory
collect2: error: ld returned 1 exit status

exec_target.c is using "syscall" library function which
could be replaced with a inline assembly and the same is
proposed as a fix here.

Suggested-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
---
Chnagelog v1:
- Add comment for clobber register and proper list of
  clobber registers as suggested by Michael Ellerman and
  Christophe Leroy

 .../selftests/powerpc/benchmarks/Makefile        |  2 +-
 .../selftests/powerpc/benchmarks/exec_target.c   | 16 ++++++++++++++--
 2 files changed, 15 insertions(+), 3 deletions(-)

diff --git a/tools/testing/selftests/powerpc/benchmarks/Makefile b/tools/testing/selftests/powerpc/benchmarks/Makefile
index 1321922038d0..ca4483c238b9 100644
--- a/tools/testing/selftests/powerpc/benchmarks/Makefile
+++ b/tools/testing/selftests/powerpc/benchmarks/Makefile
@@ -18,4 +18,4 @@ $(OUTPUT)/context_switch: LDLIBS += -lpthread
 
 $(OUTPUT)/fork: LDLIBS += -lpthread
 
-$(OUTPUT)/exec_target: CFLAGS += -static -nostartfiles
+$(OUTPUT)/exec_target: CFLAGS += -nostartfiles
diff --git a/tools/testing/selftests/powerpc/benchmarks/exec_target.c b/tools/testing/selftests/powerpc/benchmarks/exec_target.c
index c14b0fc1edde..a6408d3f26cd 100644
--- a/tools/testing/selftests/powerpc/benchmarks/exec_target.c
+++ b/tools/testing/selftests/powerpc/benchmarks/exec_target.c
@@ -7,10 +7,22 @@
  */
 
 #define _GNU_SOURCE
-#include <unistd.h>
 #include <sys/syscall.h>
 
 void _start(void)
 {
-	syscall(SYS_exit, 0);
+	asm volatile (
+		"li %%r0, %[sys_exit];"
+		"li %%r3, 0;"
+		"sc;"
+		:
+		: [sys_exit] "i" (SYS_exit)
+		/*
+		 * "sc" will clobber r0, r3-r13, cr0, ctr, xer and memory.
+		 * Even though sys_exit never returns, handle clobber
+		 * registers.
+		 */
+		: "r0", "r3", "r4", "r5", "r6", "r7", "r8", "r9", "r10",
+		  "r11", "r12", "r13", "cr0", "ctr", "xer", "memory"
+	);
 }
-- 
2.45.2