x86/kvm: Add stack frame dependency to fastop() inline asm

The kbuild test robot reported this objtool warning [1]:

  arch/x86/kvm/emulate.o: warning: objtool: fastop()+0x69: call without frame pointer save/setup

The issue seems to be caused by CONFIG_PROFILE_ALL_BRANCHES.  With that
option, for some reason gcc decides not to create a stack frame in
fastop() before doing the inline asm call, which can result in a bad
stack trace.

Force a stack frame to be created if CONFIG_FRAME_POINTER is enabled by
listing the stack pointer as an output operand for the inline asm
statement.

This change has no effect for !CONFIG_PROFILE_ALL_BRANCHES.

[1] https://lists.01.org/pipermail/kbuild-all/2016-March/018249.htmlReported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/emulate.c

@@ -5110,13 +5110,17 @@ static void fetch_possible_mmx_operand(struct x86_emulate_ctxt *ctxt,
 
 static int fastop(struct x86_emulate_ctxt *ctxt, void (*fop)(struct fastop *))
 {
+	register void *__sp asm(_ASM_SP);
 	ulong flags = (ctxt->eflags & EFLAGS_MASK) | X86_EFLAGS_IF;
+
 	if (!(ctxt->d & ByteOp))
 		fop += __ffs(ctxt->dst.bytes) * FASTOP_SIZE;
+
 	asm("push %[flags]; popf; call *%[fastop]; pushf; pop %[flags]\n"
 	    : "+a"(ctxt->dst.val), "+d"(ctxt->src.val), [flags]"+D"(flags),
-	      [fastop]"+S"(fop)
+	      [fastop]"+S"(fop), "+r"(__sp)
 	    : "c"(ctxt->src2.val));
+
 	ctxt->eflags = (ctxt->eflags & ~EFLAGS_MASK) | (flags & EFLAGS_MASK);
 	if (!fop) /* exception is returned in fop variable */
 		return emulate_de(ctxt);