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

[Jerin Jacob <jerinjacobk@gmail.com>]

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
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()