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

[Jerin Jacob <jerinjacobk@gmail.com>]

On Thu, Apr 16, 2020 at 5:06 PM Mattias Rönnblom
<mattias.ronnblom@ericsson.com> wrote:
>
> 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?

Probably none.

>
>
> > 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()