linux-kernel - Re: jitterentropy: `jent_mod

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Date:   Tue, 10 Nov 2020 10:25:42 +0100
From:   Stephan Mueller <smueller@...onox.de>
To:     Herbert Xu <herbert@...dor.apana.org.au>,
        "David S. Miller" <davem@...emloft.net>,
        Paul Menzel <pmenzel@...gen.mpg.de>
Cc:     linux-crypto@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: jitterentropy: `jent_mod_init()` takes 17 ms

Am Montag, 9. November 2020, 20:31:02 CET schrieb Paul Menzel:

Hi Paul,

> Dear Linux folks,
> 
> 
> 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?

> 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.

Ciao
Stephan