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

["Mattias Rönnblom" <mattias.ronnblom@ericsson.com>]

On 2020-04-16 08:07, Jerin Jacob wrote:
> On Thu, Apr 16, 2020 at 4:42 AM Dan Gora <dg@adax.com> wrote:
>> 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-Mason builds to use getentropy().
>>
>> Signed-off-by: Dan Gora <dg@adax.com>
>> ---
>>   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;
> IMO, we need to create a new arch EAL abstraction to get uint64_t
> random number. Reason being:
> 1) ARMv8.5 supports similar instruction
> https://developer.arm.com/docs/ddi0595/c/aarch64-system-registers/rndr


How many ARMv8.5 system will there be running a libc older than 2.25 or 
a Linux kernel older than 3.17?


> 2) Avoid #ifdef clutter in common code.
>
> Abstraction can return a random uint64_t number. Based on the arch
> capabilities, it can hook
> to specialized instruction or rte_get_timer_cycles()