[dpdk-dev] [PATCH v2] eal: choose initial PRNG seed source at runtime

[Dan Gora <dg@adax.com>]

Instead of choosing to use getentropy() or the rdseed instruction for
the random number generator entropy source using compilation flags,
determine the best source at run time.

This is accomplished by defining a weak symbol for getentropy(),
checking that the compiler can generate the rdseed instruction even if
the compilation platform does not natively support it, and checking for
the availability of the rdseed instruction on the execution platform
using rte_cpu_get_flag_enabled().

If neither getentropy() or rdseed is available, rte_get_timer_cycles()
will be continue to be used as the entropy source.

This also allows non-meson builds to use getentropy().

Signed-off-by: Dan Gora <dg@adax.com>
---
v2:
* Rebase to latest master.
* Fix spelling of "meson".

 config/x86/meson.build             |  7 +++++++
 lib/librte_eal/common/rte_random.c | 29 ++++++++++++++++++++++++-----
 lib/librte_eal/meson.build         |  3 ---
 mk/rte.cpuflags.mk                 |  8 ++++++++
 4 files changed, 39 insertions(+), 8 deletions(-)

diff --git a/config/x86/meson.build b/config/x86/meson.build
index adc857ba2..214b16f2a 100644
--- a/config/x86/meson.build
+++ b/config/x86/meson.build
@@ -20,6 +20,13 @@ if cc.get_define('__SSE4_2__', args: machine_args) == ''
 	machine_args += '-msse4'
 endif
 
+# set -mrdseed if necessary so _rdseed32_step compiles if the
+# compilation host does not support the RDSEED instruction.
+if cc.get_define('__RDSEED__', args: machine_args) == '' and cc.has_argument('-mrdseed')
+	machine_args += '-mrdseed'
+	message('RDSEED not enabled by default, explicitly setting -mrdseed')
+endif
+
 base_flags = ['SSE', 'SSE2', 'SSE3','SSSE3', 'SSE4_1', 'SSE4_2']
 foreach f:base_flags
 	dpdk_conf.set('RTE_MACHINE_CPUFLAG_' + f, 1)
diff --git a/lib/librte_eal/common/rte_random.c b/lib/librte_eal/common/rte_random.c
index 57ec8fb2b..40f8b5aab 100644
--- a/lib/librte_eal/common/rte_random.c
+++ b/lib/librte_eal/common/rte_random.c
@@ -25,6 +25,8 @@ struct rte_rand_state {
 
 static struct rte_rand_state rand_states[RTE_MAX_LCORE];
 
+__rte_weak int getentropy(void *__buffer, size_t __length);
+
 static uint32_t
 __rte_rand_lcg32(uint32_t *seed)
 {
@@ -176,10 +178,24 @@ rte_rand_max(uint64_t upper_bound)
 	return res;
 }
 
+/* Use rte_get_timer_cycles() if the system does not have
+ * genentropy() or the rdseed instruction.
+ */
+__rte_weak int
+getentropy(void *__buffer, size_t __length __rte_unused)
+{
+	uint64_t *ge_seed = __buffer;
+#ifdef RTE_MACHINE_CPUFLAG_RDSEED
+	if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_RDSEED))
+		return -1;
+#endif
+	*ge_seed = rte_get_timer_cycles();
+	return 0;
+}
+
 static uint64_t
 __rte_random_initial_seed(void)
 {
-#ifdef RTE_LIBEAL_USE_GETENTROPY
 	int ge_rc;
 	uint64_t ge_seed;
 
@@ -187,15 +203,18 @@ __rte_random_initial_seed(void)
 
 	if (ge_rc == 0)
 		return ge_seed;
-#endif
+
 #ifdef RTE_MACHINE_CPUFLAG_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);
+	if (rte_cpu_get_flag_enabled(RTE_CPUFLAG_RDSEED)) {
+		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 */
 	return rte_get_timer_cycles();
diff --git a/lib/librte_eal/meson.build b/lib/librte_eal/meson.build
index 0267c3b9d..748359b8c 100644
--- a/lib/librte_eal/meson.build
+++ b/lib/librte_eal/meson.build
@@ -15,9 +15,6 @@ deps += 'kvargs'
 if dpdk_conf.has('RTE_USE_LIBBSD')
 	ext_deps += libbsd
 endif
-if cc.has_function('getentropy', prefix : '#include <unistd.h>')
-	cflags += '-DRTE_LIBEAL_USE_GETENTROPY'
-endif
 if cc.has_header('getopt.h')
 	cflags += ['-DHAVE_GETOPT_H', '-DHAVE_GETOPT', '-DHAVE_GETOPT_LONG']
 endif
diff --git a/mk/rte.cpuflags.mk b/mk/rte.cpuflags.mk
index fa8753531..fb7bf8a53 100644
--- a/mk/rte.cpuflags.mk
+++ b/mk/rte.cpuflags.mk
@@ -53,6 +53,14 @@ endif
 
 ifneq ($(filter $(AUTO_CPUFLAGS),__RDSEED__),)
 CPUFLAGS += RDSEED
+else
+# If the native environment doesn't define __RDSEED__, see
+# if the compiler supports -mrdseed.
+RDSEED_CPUFLAGS := $(shell $(CC) $(MACHINE_CFLAGS) $(WERROR_FLAGS) $(EXTRA_CFLAGS) -mrdseed -dM -E - < /dev/null)
+ifneq ($(filter $(RDSEED_CPUFLAGS),__RDSEED__),)
+CPUFLAGS += RDSEED
+MACHINE_CFLAGS += -mrdseed
+endif
 endif
 
 ifneq ($(filter $(AUTO_CPUFLAGS),__FSGSBASE__),)
-- 
2.24.1.425.g7034cd094b