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

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

On 2020-04-16 13:48, Jerin Jacob wrote:
> 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.


In that case getentropy() will always available, and having ARMv8.5 rndr 
support would be pointless, as far as rte_random.c is concerned.


>>
>>> 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()