s390/bpf: Fix gcov stack space problem

When compiling the kernel for GCOV (CONFIG_GCOV_KERNEL,-fprofile-arcs),
gcc allocates a lot of stack space because of the large switch statement
in bpf_jit_insn().

This leads to the following compile warning:

 arch/s390/net/bpf_jit_comp.c: In function 'bpf_jit_prog':
 arch/s390/net/bpf_jit_comp.c:1144:1: warning: frame size of
  function 'bpf_jit_prog' is 12592 bytes which is more than
  half the stack size. The dynamic check would not be reliable.
  No check emitted for this function.

 arch/s390/net/bpf_jit_comp.c:1144:1: warning: the frame size of 12504
  bytes is larger than 1024 bytes [-Wframe-larger-than=]

And indead gcc allocates 12592 bytes of stack space:

 # objdump -d arch/s390/net/bpf_jit_comp.o
 ...
 0000000000000c60 <bpf_jit_prog>:
     c60:       eb 6f f0 48 00 24       stmg    %r6,%r15,72(%r15)
     c66:       b9 04 00 ef             lgr     %r14,%r15
     c6a:       e3 f0 fe d0 fc 71       lay     %r15,-12592(%r15)

As a workaround of that problem we now define bpf_jit_insn() as
noinline which then reduces the stack space.

 # objdump -d arch/s390/net/bpf_jit_comp.o
 ...
 0000000000000070 <bpf_jit_insn>:
      70:       eb 6f f0 48 00 24       stmg    %r6,%r15,72(%r15)
      76:       c0 d0 00 00 00 00       larl    %r13,76 <bpf_jit_insn+0x6>
      7c:       a7 f1 3f 80             tmll    %r15,16256
      80:       b9 04 00 ef             lgr     %r14,%r15
      84:       e3 f0 ff a0 ff 71       lay     %r15,-96(%r15)

Signed-off-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/net/bpf_jit_comp.c

@@ -443,8 +443,11 @@ static void bpf_jit_epilogue(struct bpf_jit *jit)
 
 /*
  * Compile one eBPF instruction into s390x code
+ *
+ * NOTE: Use noinline because for gcov (-fprofile-arcs) gcc allocates a lot of
+ * stack space for the large switch statement.
  */
-static int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp, int i)
+static noinline int bpf_jit_insn(struct bpf_jit *jit, struct bpf_prog *fp, int i)
 {
 	struct bpf_insn *insn = &fp->insnsi[i];
 	int jmp_off, last, insn_count = 1;