linux-kernel - Re: jitterentropy: `jent_mod

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Date:   Tue, 10 Nov 2020 10:37:02 +0100
From:   Paul Menzel <pmenzel@...gen.mpg.de>
To:     Stephan Mueller <smueller@...onox.de>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: jitterentropy: `jent_mod_init()` takes 17 ms

Dear Stephan,


Thank you for the quick reply.

Am 10.11.20 um 10:25 schrieb Stephan Mueller:
> Am Montag, 9. November 2020, 20:31:02 CET schrieb Paul Menzel:

>> By mistake I built `XFRM_ESP` into the Linux kernel, resulting in
>>
>>       CONFIG_CRYPTO_SEQIV=y
>>       CONFIG_CRYPTO_ECHAINIV=y
>>
>> and also the Jitterentropy RNG to be built in.
>>
>>       CRYPTO_JITTERENTROPY=y
>>
>> So, on the Asus F2A85-M PRO starting Linux 4.10-rc3 with
>> `initcall_debug`, the init method is run unconditionally, and it takes
>> 17.5 ms, which is over ten percent of the overall 900 ms the Linux
> 
> Hm, 17.5 / 900 = 2%, or am I missing something?

Indeed, that is embarrassing. My bad.

>> kernel needs until loading the init process.
>>
>>       [    0.300544] calling  jent_mod_init+0x0/0x2c @ 1
>>       [    0.318438] initcall jent_mod_init+0x0/0x2c returned 0 after 17471 usecs
>>
>> Looking at the output of systemd-bootchart, it looks like, that this
>> indeed delayed the boot a little, as the other init methods seem to be
>> ordered after it.
>>
>> I am now building it as a module, but am wondering if the time can be
>> reduced to below ten milliseconds.
> 
> What you see is the test whether the Jitter RNG has a proper noise source. The
> function jent_entropy_init() is the cause of the operation. It performs 1024
> times a test to validate the appropriateness of the noise source. You can
> adjust that with the TESTLOOPCOUNT in this function. But I am not sure
> adjusting is a wise course of action.

Out of curiosity, why 1024 and not, for example, 128 or 2048? Is there 
some statistics behind it?


Kind regards,

Paul