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Subject[PATCH v6 13/17] x86: use fallback 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 random_get_entropy_fallback() would be
preferable, because that always needs to return _something_, even
falling back to jiffies eventually. It's not as though
random_get_entropy_fallback() 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. While we're
at it, fix up both the new function and the get_cycles() function from
which its derived to use cpu_feature_enabled() rather than boot_cpu_has().

Cc: Thomas Gleixner <>
Cc: Arnd Bergmann <>
Cc: Borislav Petkov <>
Signed-off-by: Jason A. Donenfeld <>
arch/x86/include/asm/timex.h | 10 ++++++++++
arch/x86/include/asm/tsc.h | 4 ++--
2 files changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/timex.h b/arch/x86/include/asm/timex.h
index a4a8b1b16c0c..2b841c39d876 100644
--- a/arch/x86/include/asm/timex.h
+++ b/arch/x86/include/asm/timex.h
@@ -5,6 +5,16 @@
#include <asm/processor.h>
#include <asm/tsc.h>

+static inline unsigned long random_get_entropy(void)
+#ifndef CONFIG_X86_TSC
+ if (!cpu_feature_enabled(X86_FEATURE_TSC))
+ return random_get_entropy_fallback();
+ return rdtsc();
+#define random_get_entropy random_get_entropy
/* Assume we use the PIT time source for the clock tick */

diff --git a/arch/x86/include/asm/tsc.h b/arch/x86/include/asm/tsc.h
index 01a300a9700b..192ea48ec80b 100644
--- a/arch/x86/include/asm/tsc.h
+++ b/arch/x86/include/asm/tsc.h
@@ -21,12 +21,12 @@ extern void disable_TSC(void);
static inline cycles_t get_cycles(void)
#ifndef CONFIG_X86_TSC
- if (!boot_cpu_has(X86_FEATURE_TSC))
+ if (!cpu_feature_enabled(X86_FEATURE_TSC))
return 0;
return rdtsc();
+#define get_cycles get_cycles

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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