netdev - Re: [PATCH] bpf: call get_random

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Date:   Mon, 5 Dec 2022 23:47:44 +0100
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, Andrii Nakryiko <andrii@...nel.org>,
        Martin KaFai Lau <martin.lau@...ux.dev>
Subject: Re: [PATCH] bpf: call get_random_u32() for random integers

On Mon, Dec 05, 2022 at 11:21:51PM +0100, Daniel Borkmann wrote:
> On 12/5/22 7:15 PM, Jason A. Donenfeld wrote:
> > Since BPF's bpf_user_rnd_u32() was introduced, there have been three
> > significant developments in the RNG: 1) get_random_u32() returns the
> > same types of bytes as /dev/urandom, eliminating the distinction between
> > "kernel random bytes" and "userspace random bytes", 2) get_random_u32()
> > operates mostly locklessly over percpu state, 3) get_random_u32() has
> > become quite fast.
> 
> Wrt "quite fast", do you have a comparison between the two? Asking as its
> often used in networking worst case on per packet basis (e.g. via XDP), would
> be useful to state concrete numbers for the two on a given machine.

Median of 25 cycles vs median of 38, on my Tiger Lake machine. So a
little slower, but too small of a difference to matter.