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Subject[PATCH v2 08/11] x86: use ktime_read_raw_clock() for random_get_entropy() instead of zero
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 <>
Cc: Arnd Bergmann <>
Signed-off-by: Jason A. Donenfeld <>
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();
+ 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);

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