#define nmi_to_desc(type) (&nmi_desc[type])
-static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs)
+static int notrace __kprobes nmi_handle(unsigned int type, struct pt_regs *regs, bool b2b)
{
struct nmi_desc *desc = nmi_to_desc(type);
struct nmiaction *a;
* can be latched at any given time. Walk the whole list
* to handle those situations.
*/
- list_for_each_entry_rcu(a, &desc->head, list) {
-
+ list_for_each_entry_rcu(a, &desc->head, list)
handled += a->handler(type, regs);
- }
-
rcu_read_unlock();
/* return total number of NMI events handled */
spin_lock_irqsave(&desc->lock, flags);
/*
+ * most handlers of type NMI_UNKNOWN never return because
+ * they just assume the NMI is theirs. Just a sanity check
+ * to manage expectations
+ */
+ WARN_ON_ONCE(type == NMI_UNKNOWN && !list_empty(&desc->head));
+
+ /*
* some handlers need to be executed first otherwise a fake
* event confuses some handlers (kdump uses this flag)
*/
{
int handled;
- handled = nmi_handle(NMI_UNKNOWN, regs);
+ /*
+ * Use 'false' as back-to-back NMIs are dealt with one level up.
+ * Of course this makes having multiple 'unknown' handlers useless
+ * as only the first one is ever run (unless it can actually determine
+ * if it caused the NMI)
+ */
+ handled = nmi_handle(NMI_UNKNOWN, regs, false);
if (handled)
return;
#ifdef CONFIG_MCA
pr_emerg("Dazed and confused, but trying to continue\n");
}
+static DEFINE_PER_CPU(bool, swallow_nmi);
+static DEFINE_PER_CPU(unsigned long, last_nmi_rip);
+
static notrace __kprobes void default_do_nmi(struct pt_regs *regs)
{
unsigned char reason = 0;
int handled;
+ bool b2b = false;
/*
* CPU-specific NMI must be processed before non-CPU-specific
* NMI, otherwise we may lose it, because the CPU-specific
* NMI can not be detected/processed on other CPUs.
*/
- handled = nmi_handle(NMI_LOCAL, regs);
- if (handled)
+
+ /*
+ * Back-to-back NMIs are interesting because they can either
+ * be two NMI or more than two NMIs (any thing over two is dropped
+ * due to NMI being edge-triggered). If this is the second half
+ * of the back-to-back NMI, assume we dropped things and process
+ * more handlers. Otherwise reset the 'swallow' NMI behaviour
+ */
+ if (regs->ip == __this_cpu_read(last_nmi_rip))
+ b2b = true;
+ else
+ __this_cpu_write(swallow_nmi, false);
+
+ __this_cpu_write(last_nmi_rip, regs->ip);
+
+ handled = nmi_handle(NMI_LOCAL, regs, b2b);
+ if (handled) {
+ /*
+ * There are cases when a NMI handler handles multiple
+ * events in the current NMI. One of these events may
+ * be queued for in the next NMI. Because the event is
+ * already handled, the next NMI will result in an unknown
+ * NMI. Instead lets flag this for a potential NMI to
+ * swallow.
+ */
+ if (handled > 1)
+ __this_cpu_write(swallow_nmi, true);
return;
+ }
/* Non-CPU-specific NMI: NMI sources can be processed on any CPU */
raw_spin_lock(&nmi_reason_lock);
}
raw_spin_unlock(&nmi_reason_lock);
- unknown_nmi_error(reason, regs);
+ /*
+ * Only one NMI can be latched at a time. To handle
+ * this we may process multiple nmi handlers at once to
+ * cover the case where an NMI is dropped. The downside
+ * to this approach is we may process an NMI prematurely,
+ * while its real NMI is sitting latched. This will cause
+ * an unknown NMI on the next run of the NMI processing.
+ *
+ * We tried to flag that condition above, by setting the
+ * swallow_nmi flag when we process more than one event.
+ * This condition is also only present on the second half
+ * of a back-to-back NMI, so we flag that condition too.
+ *
+ * If both are true, we assume we already processed this
+ * NMI previously and we swallow it. Otherwise we reset
+ * the logic.
+ *
+ * There are scenarios where we may accidentally swallow
+ * a 'real' unknown NMI. For example, while processing
+ * a perf NMI another perf NMI comes in along with a
+ * 'real' unknown NMI. These two NMIs get combined into
+ * one (as descibed above). When the next NMI gets
+ * processed, it will be flagged by perf as handled, but
+ * noone will know that there was a 'real' unknown NMI sent
+ * also. As a result it gets swallowed. Or if the first
+ * perf NMI returns two events handled then the second
+ * NMI will get eaten by the logic below, again losing a
+ * 'real' unknown NMI. But this is the best we can do
+ * for now.
+ */
+ if (b2b && __this_cpu_read(swallow_nmi))
+ ;
+ else
+ unknown_nmi_error(reason, regs);
}
dotraplinkage notrace __kprobes void
{
ignore_nmis--;
}
+
+/* reset the back-to-back NMI logic */
+void local_touch_nmi(void)
+{
+ __this_cpu_write(last_nmi_rip, 0);
+}