[dpdk-dev] [PATCH v2 1/2] eal: check for rdseed at run time for random seed

[Dan Gora <dg@adax.com>]

Instead of enabling the rdseed instruction for the random number generator
entropy source at compilation time, determine if the instruction can be
used at run time.

The DPDK build is updated to check that the compiler can generate the
rdseed instruction even if the compilation platform does not natively
support it.  If so, the -mrdseed flag is explicitly added.  This allows
binaries compiled on systems which do not support the rdseed instruction
to use the instruction if the execution platform supports it.

At run-time on x86, __rte_random_initial_seed() will check for the
availability of the rdseed instruction on the execution platform using
rte_cpu_get_flag_enabled().  This allows binaries which were compiled on
systems which support the rdseed instruction to run on x86 CPUs which do
not support the rdseed instruction.

RTE_CPUFLAG_RDSEED is removed from the list of RTE_COMPILE_TIME_CPUFLAGS
which are checked in rte_eal_init() at run time because it is no longer
required to match the compilation system.

Signed-off-by: Dan Gora <dg@adax.com>
---
 config/x86/meson.build             | 11 ++++++++---
 lib/librte_eal/common/rte_random.c | 19 +++++++++++--------
 mk/rte.cpuflags.mk                 |  9 +++++++--
 3 files changed, 26 insertions(+), 13 deletions(-)

diff --git a/config/x86/meson.build b/config/x86/meson.build
index adc857ba2..9491fad3a 100644
--- a/config/x86/meson.build
+++ b/config/x86/meson.build
@@ -20,15 +20,20 @@ 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)
 	compile_time_cpuflags += ['RTE_CPUFLAG_' + f]
 endforeach
 
-optional_flags = ['AES', 'PCLMUL',
-		'AVX', 'AVX2', 'AVX512F',
-		'RDRND', 'RDSEED']
+optional_flags = ['AES', 'PCLMUL', 'AVX', 'AVX2', 'AVX512F', 'RDRND']
 foreach f:optional_flags
 	if cc.get_define('__@0@__'.format(f), args: machine_args) == '1'
 		if f == 'PCLMUL' # special case flags with different defines
diff --git a/lib/librte_eal/common/rte_random.c b/lib/librte_eal/common/rte_random.c
index b7a089ac4..2c84c8527 100644
--- a/lib/librte_eal/common/rte_random.c
+++ b/lib/librte_eal/common/rte_random.c
@@ -2,7 +2,7 @@
  * Copyright(c) 2019 Ericsson AB
  */
 
-#ifdef RTE_MACHINE_CPUFLAG_RDSEED
+#if defined(RTE_ARCH_X86)
 #include <x86intrin.h>
 #endif
 #include <stdlib.h>
@@ -188,14 +188,17 @@ __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;
-
+#if defined(RTE_ARCH_X86)
 	/* 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)) {
+		unsigned int rdseed_low;
+		unsigned int rdseed_high;
+
+		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_tsc_cycles();
diff --git a/mk/rte.cpuflags.mk b/mk/rte.cpuflags.mk
index fa8753531..a114e9c02 100644
--- a/mk/rte.cpuflags.mk
+++ b/mk/rte.cpuflags.mk
@@ -51,8 +51,13 @@ ifneq ($(filter $(AUTO_CPUFLAGS),__RDRND__),)
 CPUFLAGS += RDRAND
 endif
 
-ifneq ($(filter $(AUTO_CPUFLAGS),__RDSEED__),)
-CPUFLAGS += RDSEED
+ifeq ($(filter $(AUTO_CPUFLAGS),__RDSEED__),)
+# 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__),)
+MACHINE_CFLAGS += -mrdseed
+endif
 endif
 
 ifneq ($(filter $(AUTO_CPUFLAGS),__FSGSBASE__),)
-- 
2.24.1.425.g7034cd094b