[PATCH 15/18] random: remove RDSEED fallback for initial seed

[Stephen Hemminger <stephen@networkplumber.org>]

RDSEED is an x86 instruction that returns random bits from a hardware
source. It has well known reliability problems: it can fail
persistently when many cores use it at once, and several processors
have errata affecting it. Since commit bfb3c5b9ffe7 ("random: always
use getentropy") it was only reachable if getentropy() failed, which
does not happen in practice.

Nothing is lost by removing it. The kernel already mixes RDSEED into
its own entropy pool as one of several sources, so what it provides is
present in the getentropy() result anyway, combined with other sources
rather than trusted on its own.

The remaining fallback, the timestamp counter, still logs an error so
that a low entropy seed is reported.

Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
---
 lib/eal/common/rte_random.c | 17 ++---------------
 1 file changed, 2 insertions(+), 15 deletions(-)

diff --git a/lib/eal/common/rte_random.c b/lib/eal/common/rte_random.c
index db4e19b414..c6f3136408 100644
--- a/lib/eal/common/rte_random.c
+++ b/lib/eal/common/rte_random.c
@@ -2,11 +2,6 @@
  * Copyright(c) 2019 Ericsson AB
  */
 
-#ifdef __RDSEED__
-#ifndef RTE_TOOLCHAIN_MSVC
-#include <x86intrin.h>
-#endif
-#endif
 #include <errno.h>
 #include <string.h>
 #include <unistd.h>
@@ -276,16 +271,8 @@ __rte_random_initial_seed(void)
 
 	if (ge_rc == 0)
 		return ge_seed;
-#ifdef __RDSEED__
-	unsigned int rdseed_low;
-	unsigned int rdseed_high;
-
-	/* first fallback: rdseed instruction, if available */
-	if (_rdseed32_step(&rdseed_low) == 1 &&
-	    _rdseed32_step(&rdseed_high) == 1)
-		return (uint64_t)rdseed_low | ((uint64_t)rdseed_high << 32);
-#endif
-	/* second fallback: seed using rdtsc */
+
+	/* fallback: seed using rdtsc */
 	EAL_LOG(ERR, "getentropy() failed (%s), seeding PRNG from TSC: seed has low entropy",
 		strerror(errno));
 	return rte_get_tsc_cycles();
-- 
2.53.0