[patch 19/38] kcsan: Replace get_cycles() usage

[Thomas Gleixner]

KCSAN uses get_cycles() for two purposes:

  1) Seeding the random state with get_cycles() is a historical leftover.

  2) The microbenchmark uses get_cycles(), which provides an unit less
     counter value and is not guaranteed to be functional on all
     systems/platforms.

Use random_get_entropy() for seeding the random state and ktime_get() which
is universaly functional and provides at least a comprehensible unit.

This is part of a larger effort to remove get_cycles() usage from
non-architecture code.

Signed-off-by: Thomas Gleixner <tglx at kernel.org>
Cc: Marco Elver <elver at google.com>
Cc: Dmitry Vyukov <dvyukov at google.com>
Cc: kasan-dev at googlegroups.com
---
 kernel/kcsan/core.c    |    2 +-
 kernel/kcsan/debugfs.c |    8 ++++----
 2 files changed, 5 insertions(+), 5 deletions(-)

--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -798,7 +798,7 @@ void __init kcsan_init(void)
 	BUG_ON(!in_task());
 
 	for_each_possible_cpu(cpu)
-		per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles();
+		per_cpu(kcsan_rand_state, cpu) = (u32)random_get_entropy();
 
 	/*
 	 * We are in the init task, and no other tasks should be running;
--- a/kernel/kcsan/debugfs.c
+++ b/kernel/kcsan/debugfs.c
@@ -58,7 +58,7 @@ static noinline void microbenchmark(unsi
 {
 	const struct kcsan_ctx ctx_save = current->kcsan_ctx;
 	const bool was_enabled = READ_ONCE(kcsan_enabled);
-	u64 cycles;
+	ktime_t nsecs;
 
 	/* We may have been called from an atomic region; reset context. */
 	memset(&current->kcsan_ctx, 0, sizeof(current->kcsan_ctx));
@@ -70,16 +70,16 @@ static noinline void microbenchmark(unsi
 
 	pr_info("%s begin | iters: %lu\n", __func__, iters);
 
-	cycles = get_cycles();
+	nsecs = ktime_get();
 	while (iters--) {
 		unsigned long addr = iters & ((PAGE_SIZE << 8) - 1);
 		int type = !(iters & 0x7f) ? KCSAN_ACCESS_ATOMIC :
 				(!(iters & 0xf) ? KCSAN_ACCESS_WRITE : 0);
 		__kcsan_check_access((void *)addr, sizeof(long), type);
 	}
-	cycles = get_cycles() - cycles;
+	nsecs = ktime_get() - nsecs;
 
-	pr_info("%s end   | cycles: %llu\n", __func__, cycles);
+	pr_info("%s end   | nsecs: %llu\n", __func__, nsecs);
 
 	WRITE_ONCE(kcsan_enabled, was_enabled);
 	/* restore context */