Re: [PATCH] eal: add support for TRNG with Arm RNG feature

["Mattias Rönnblom" <hofors@lysator.liu.se>]

On 2024-07-27 17:45, Wathsala Wathawana Vithanage wrote:
> Hi Mattias,
> 
>>> The primary goal of this patch is to provide a direct interface to HW,
>>> instead of letting kernel handle it. This is not an API just for Arm
>>> CPUs, as other vendors also have similar HW features. For instance,
>>> Intel and AMD has support for x86 RDRAND and RDSEED instructions, thus
>>> can easily implement this API.
>>>
>>
>> No DPDK library (or PMD) currently needs this functionality, and no
>> application, to my knowledge, has asked for this. If an app or a DPDK library
>> would require cryptographically secure random numbers, it would most likely
>> require it on all CPU/OS platforms (and with all DPDK -march flags).
>>
> 
> I'm sorry, I'm not following this. Are you saying
> 
> (a) adding a feature someone hasn't already asked for is impossible?
> 

No, not if you can explain why this feature will be useful. You guys 
made no such attempt.

> (b) it is impossible to add support for a feature that is only available in a few CPUs
> of an architecture family?
> 

Cryptographically secure random numbers are available on all CPUs, via 
the operating system. Arguably, such random numbers are more secure than 
anything that a machine instruction can offer.

If your patch are to have non-zero chance of being accepted, it should 
include a base implementation based on getrandom() (and the Windows 
equivalent), with the proper optimizations (e.g., batching entropy 
requests to the kernel on a per-lcore basis).

You would also need to provide a rationale why ARM CPU HW random numbers 
are better than what the kernel can offer. The only potential reason I 
can think of is performance, so you would need to quantify that in some way.

In addition, reliance on CPU CSRNG would need to be a build-time option, 
and disabled by default.

Plus, what I've mentioned several times, give a rationale why DPDK 
should have this functionality.

>> RDRAND is only available on certain x86_64 CPUs, and is incredibly slow
>> - slower than getting entropy via the kernel, even with non-vDSO syscalls.
>>
>> Agner Fog lists the RDRAND latency as ~3700 cc for Zen 2. Later generations of
>> both AMD and Intel CPUs have much shorter latencies, but a reciprocal
>> throughput so low that one have to wait thousands of clock cycles before
>> issuing another RDRAND, or risk stalling the core.
>>
>> My Raptor Lake seems to require ~1000 cc retire RDRAND, which is ~11x
>> slower than getting entropy (in bulk) via getentropy().
>>
>> What is the latency for the ARM equivalent? Does it also have a reciprocal
>> throughput issue?
>>
> 
> Agree, from the numbers you are showing, it looks like they are all slow and
> unsuitable for the data plane. But aren't there multi-plane DPDK applications
> with control-plane threads that can benefit from a CSRNG, albeit slow?
> 
>