[PATCH v4 14/17] watchdog/perf: Add a weak function for an arch to detect if perf can use NMIs

[Douglas Anderson <dianders@chromium.org>]

On arm64, NMI support needs to be detected at runtime. Add a weak
function to the perf hardlockup detector so that an architecture can
implement it to detect whether NMIs are available.

Signed-off-by: Douglas Anderson <dianders@chromium.org>
---
While I won't object to this patch landing, I consider it part of the
arm64 perf hardlockup effort. I would be OK with the earlier patches
in the series landing and then not landing ${SUBJECT} patch nor
anything else later.

I'll also note that, as an alternative to this, it would be nice if we
could figure out how to make perf_event_create_kernel_counter() fail
on arm64 if NMIs aren't available. Maybe we could add a "must_use_nmi"
element to "struct perf_event_attr"?

Changes in v4:
- ("Add a weak function for an arch to detect ...") new for v4.

 include/linux/nmi.h    |  1 +
 kernel/watchdog_perf.c | 12 +++++++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/include/linux/nmi.h b/include/linux/nmi.h
index 90aa33317b4c..9caea5ba494d 100644
--- a/include/linux/nmi.h
+++ b/include/linux/nmi.h
@@ -212,6 +212,7 @@ static inline bool trigger_single_cpu_backtrace(int cpu)
 
 #ifdef CONFIG_HARDLOCKUP_DETECTOR_PERF
 u64 hw_nmi_get_sample_period(int watchdog_thresh);
+bool arch_perf_nmi_is_available(void);
 #endif
 
 #if defined(CONFIG_HARDLOCKUP_CHECK_TIMESTAMP) && \
diff --git a/kernel/watchdog_perf.c b/kernel/watchdog_perf.c
index a55a6eab1b3a..0d1c292a655d 100644
--- a/kernel/watchdog_perf.c
+++ b/kernel/watchdog_perf.c
@@ -234,12 +234,22 @@ void __init hardlockup_detector_perf_restart(void)
 	}
 }
 
+bool __weak __init arch_perf_nmi_is_available(void)
+{
+	return true;
+}
+
 /**
  * watchdog_hardlockup_probe - Probe whether NMI event is available at all
  */
 int __init watchdog_hardlockup_probe(void)
 {
-	int ret = hardlockup_detector_event_create();
+	int ret;
+
+	if (!arch_perf_nmi_is_available())
+		return -ENODEV;
+
+	ret = hardlockup_detector_event_create();
 
 	if (ret) {
 		pr_info("Perf NMI watchdog permanently disabled\n");
-- 
2.40.1.521.gf1e218fcd8-goog