[PATCH RFC v1 06/10] x86: use sched_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 sched_clock() would be preferable, because
that always needs to return _something_, even falling back to jiffies
eventually. It's not as though sched_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: Arnd Bergmann <arnd@xxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Jason A. Donenfeld <Jason@xxxxxxxxx>
---
 arch/x86/include/asm/tsc.h | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 01a300a9700b..b0c0b2b9e0f7 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -5,6 +5,7 @@
 #ifndef _ASM_X86_TSC_H
 #define _ASM_X86_TSC_H
 
+#include <linux/sched/clock.h>
 #include <asm/processor.h>
 #include <asm/cpufeature.h>
 
@@ -28,6 +29,16 @@ 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 sched_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