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Date:   Wed, 13 Jul 2022 16:52:20 +0100
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Subject: Re: [PATCH] random: cap jitter samples per bit to factor of HZ
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To:     "Jason A. Donenfeld" <Jason@zx2c4.com>,
        linux-kernel@vger.kernel.org
Cc:     linux-crypto@vger.kernel.org, ebiggers@google.com, tytso@mit.edu,
        torvalds@linux-foundation.org
References: <CAHmME9rrQVm72P6cLL4dUnSw+9nnXszDbQXRd3epRaQgKTy8BQ@mail.gmail.com>
 <20220713151115.1014188-1-Jason@zx2c4.com>
From:   Vladimir Murzin <vladimir.murzin@arm.com>
In-Reply-To: <20220713151115.1014188-1-Jason@zx2c4.com>
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On 7/13/22 16:11, Jason A. Donenfeld wrote:
> Currently the jitter mechanism will require two timer ticks per
> iteration, and it requires N iterations per bit. This N is determined
> with a small measurement, and if it's too big, it won't waste time with
> jitter entropy because it'd take too long or not have sufficient entropy
> anyway.
> 
> With the current max N of 32, there are large timeouts on systems with a
> small CONFIG_HZ. Rather than set that maximum to 32, instead choose a
> factor of CONFIG_HZ. In this case, 1/30 seems to yield sane values for
> different configurations of CONFIG_HZ.
> 
> Reported-by: Vladimir Murzin <vladimir.murzin@arm.com>
> Fixes: 78c768e619fb ("random: vary jitter iterations based on cycle counter speed")
> Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
> ---
> Vladimir - Can you let me know if this appears to fix the issue you're
> seeing? -Jason

Works for me, thanks! :) 

> 
>  drivers/char/random.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/drivers/char/random.c b/drivers/char/random.c
> index e3dd1dd3dd22..a1af90bacc9f 100644
> --- a/drivers/char/random.c
> +++ b/drivers/char/random.c
> @@ -1174,7 +1174,7 @@ static void __cold entropy_timer(struct timer_list *timer)
>   */
>  static void __cold try_to_generate_entropy(void)
>  {
> -	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = 32 };
> +	enum { NUM_TRIAL_SAMPLES = 8192, MAX_SAMPLES_PER_BIT = HZ / 30 };
>  	struct entropy_timer_state stack;
>  	unsigned int i, num_different = 0;
>  	unsigned long last = random_get_entropy();

FWIW

Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>

Cheers
Vladimir