[PATCH v2 08/11] x86: use ktime_read_raw_clock() for random_get_entropy() instead of zero

[Jason A. Donenfeld]

In the event that random_get_entropy() can't access a cycle counter or
similar, falling back to returning 0 is really not the best we can do.
Instead, at least calling ktime_read_raw_clock() would be preferable,
because that always needs to return _something_, even falling back to
jiffies eventually. It's not as though ktime_read_raw_clock() is super
high precision or guaranteed to be entropic, but basically anything
that's not zero all the time is better than returning zero all the time.

If CONFIG_X86_TSC=n, then it's possible that we're running on a 486 with
no RDTSC, so we only need the fallback code for that case.

Cc: Thomas Gleixner <tglx at linutronix.de>
Cc: Arnd Bergmann <arnd at arndb.de>
Cc: x86 at kernel.org
Signed-off-by: Jason A. Donenfeld <Jason at zx2c4.com>
---
 arch/x86/include/asm/tsc.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 01a300a9700b..dad3027bf6a2 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -28,6 +28,16 @@ static inline cycles_t get_cycles(void)
 	return rdtsc();
 }
 
+static inline unsigned long random_get_entropy(void)
+{
+#ifndef CONFIG_X86_TSC
+	if (!boot_cpu_has(X86_FEATURE_TSC))
+		return ktime_read_raw_clock();
+#endif
+	return rdtsc();
+}
+#define random_get_entropy random_get_entropy
+
 extern struct system_counterval_t convert_art_to_tsc(u64 art);
 extern struct system_counterval_t convert_art_ns_to_tsc(u64 art_ns);
 
-- 
2.35.1