oprofile, x86: Add comments to IBS LVT offset initialization

Adding a comment in the code as IBS LVT setup is not obvious at all ...
Signed-off-by: Robert Richter <robert.richter@amd.com>

arch/x86/kernel/apic/apic.c

@@ -390,7 +390,8 @@ static unsigned int reserve_eilvt_offset(int offset, unsigned int new)
 
 /*
  * If mask=1, the LVT entry does not generate interrupts while mask=0
- * enables the vector. See also the BKDGs.
+ * enables the vector. See also the BKDGs. Must be called with
+ * preemption disabled.
  */
 
 int setup_APIC_eilvt(u8 offset, u8 vector, u8 msg_type, u8 mask)

arch/x86/oprofile/op_model_amd.c

@@ -609,16 +609,21 @@ static int setup_ibs_ctl(int ibs_eilvt_off)
 	return 0;
 }
 
+/*
+ * This runs only on the current cpu. We try to find an LVT offset and
+ * setup the local APIC. For this we must disable preemption. On
+ * success we initialize all nodes with this offset. This updates then
+ * the offset in the IBS_CTL per-node msr. The per-core APIC setup of
+ * the IBS interrupt vector is called from op_amd_setup_ctrs()/op_-
+ * amd_cpu_shutdown() using the new offset.
+ */
 static int force_ibs_eilvt_setup(void)
 {
 	int offset;
 	int ret;
 
-	/*
-	 * find the next free available EILVT entry, skip offset 0,
-	 * pin search to this cpu
-	 */
 	preempt_disable();
+	/* find the next free available EILVT entry, skip offset 0 */
 	for (offset = 1; offset < APIC_EILVT_NR_MAX; offset++) {
 		if (get_eilvt(offset))
 			break;