Re: [PATCH v8 1/8] ppc64 (le): prepare for -mprofile-kernel

On Wed, Feb 17, 2016 at 09:55:40PM +1100, Michael Ellerman wrote:

On Wed, 2016-02-10 at 17:21 +0100, Torsten Duwe wrote:

quoted

--- a/arch/powerpc/kernel/module_64.c
+++ b/arch/powerpc/kernel/module_64.c

@@ -476,17 +474,44 @@ static unsigned long stub_for_addr(Elf64_Shdr *sechdrs,
 	return (unsigned long)&stubs[i];
 }
 
+#ifdef CC_USING_MPROFILE_KERNEL
+static int is_early_mcount_callsite(u32 *instruction)
+{
+	/* -mprofile-kernel sequence starting with
+	 * mflr r0 and maybe std r0, LRSAVE(r1).
+	 */
+	if ((instruction[-3] == PPC_INST_MFLR &&
+	     instruction[-2] == PPC_INST_STD_LR) ||
+	    instruction[-2] == PPC_INST_MFLR) {
+		/* Nothing to be done here, it's an _mcount
+		 * call location and r2 will have to be
+		 * restored in the _mcount function.
+		 */
+		return 1;
+	}
+	return 0;
+}

On a kernel built with the 2 instruction version this will fault when the
function we're looking at is located at the beginning of a page. Because
instruction[-3] goes off the front of the mapping.

We can probably fix that. But it's still a bit dicey.

Not necessarily. Now that it's a separate function, it can be nested a bit deeper,
so we don't take chances on compiler optimisation:

if (instruction[-2] == PPC_INST_STD_LR) /* where should R0 come from? there must be... */
  {
    if (instruction[-3] == PPC_INST_MFLR)
      return 1;
  }
else if (instruction[-2] == PPC_INST_MFLR)
    return 1;
return 0;

I'm wondering if we want to just say we only support the 2 instruction version.
Currently that means GCC 6 only, or a distro compiler with the backport of
e95d0248dace. But we could also ask GCC to backport it to 4.9 and 5.

Thoughts?

IMHO that's a too weak reason for a too strong limitation. OTOH getting everyone
to use the 2 insn version sounds appealing...

Is e95d0248dace self-sufficient or does it depend on other improvements?

	Torsten