kcsan: Expose core configuration parameters as module params

This adds early_boot, udelay_{task,interrupt}, and skip_watch as module
params. The latter parameters are useful to modify at runtime to tune
KCSAN's performance on new systems. This will also permit auto-tuning
these parameters to maximize overall system performance and KCSAN's race
detection ability.

None of the parameters are used in the fast-path and referring to them
via static variables instead of CONFIG constants will not affect
performance.
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Qian Cai <cai@lca.pw>

kernel/kcsan/core.c

@@ -6,6 +6,7 @@
 #include <linux/export.h>
 #include <linux/init.h>
 #include <linux/kernel.h>
+#include <linux/moduleparam.h>
 #include <linux/percpu.h>
 #include <linux/preempt.h>
 #include <linux/random.h>
@@ -16,6 +17,20 @@
 #include "encoding.h"
 #include "kcsan.h"
 
+static bool kcsan_early_enable = IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE);
+static unsigned int kcsan_udelay_task = CONFIG_KCSAN_UDELAY_TASK;
+static unsigned int kcsan_udelay_interrupt = CONFIG_KCSAN_UDELAY_INTERRUPT;
+static long kcsan_skip_watch = CONFIG_KCSAN_SKIP_WATCH;
+
+#ifdef MODULE_PARAM_PREFIX
+#undef MODULE_PARAM_PREFIX
+#endif
+#define MODULE_PARAM_PREFIX "kcsan."
+module_param_named(early_enable, kcsan_early_enable, bool, 0);
+module_param_named(udelay_task, kcsan_udelay_task, uint, 0644);
+module_param_named(udelay_interrupt, kcsan_udelay_interrupt, uint, 0644);
+module_param_named(skip_watch, kcsan_skip_watch, long, 0644);
+
 bool kcsan_enabled;
 
 /* Per-CPU kcsan_ctx for interrupts */
@@ -239,9 +254,9 @@ should_watch(const volatile void *ptr, size_t size, int type)
 
 static inline void reset_kcsan_skip(void)
 {
-	long skip_count = CONFIG_KCSAN_SKIP_WATCH -
+	long skip_count = kcsan_skip_watch -
 			  (IS_ENABLED(CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE) ?
-				   prandom_u32_max(CONFIG_KCSAN_SKIP_WATCH) :
+				   prandom_u32_max(kcsan_skip_watch) :
 				   0);
 	this_cpu_write(kcsan_skip, skip_count);
 }
@@ -253,8 +268,7 @@ static __always_inline bool kcsan_is_enabled(void)
 
 static inline unsigned int get_delay(void)
 {
-	unsigned int delay = in_task() ? CONFIG_KCSAN_UDELAY_TASK :
-					 CONFIG_KCSAN_UDELAY_INTERRUPT;
+	unsigned int delay = in_task() ? kcsan_udelay_task : kcsan_udelay_interrupt;
 	return delay - (IS_ENABLED(CONFIG_KCSAN_DELAY_RANDOMIZE) ?
 				prandom_u32_max(delay) :
 				0);
@@ -527,7 +541,7 @@ void __init kcsan_init(void)
 	 * We are in the init task, and no other tasks should be running;
 	 * WRITE_ONCE without memory barrier is sufficient.
 	 */
-	if (IS_ENABLED(CONFIG_KCSAN_EARLY_ENABLE))
+	if (kcsan_early_enable)
 		WRITE_ONCE(kcsan_enabled, true);
 }