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Date:   Mon, 10 Aug 2020 09:31:48 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Willy Tarreau <w@....eu>
Cc:     George Spelvin <lkml@....org>, Netdev <netdev@...r.kernel.org>,
        Amit Klein <aksecurity@...il.com>,
        Eric Dumazet <edumazet@...gle.com>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        "Theodore Ts'o" <tytso@....edu>,
        Marc Plumb <lkml.mplumb@...il.com>,
        Stephen Hemminger <stephen@...workplumber.org>,
        Florian Westphal <fw@...len.de>
Subject: Re: [DRAFT PATCH] random32: make prandom_u32() output unpredictable

On Mon, Aug 10, 2020 at 4:47 AM Willy Tarreau <w@....eu> wrote:
>
> Doing testing on real hardware showed that retrieving the TSC on every
> call had a non negligible cost, causing a loss of 2.5% on the accept()
> rate and 4% on packet rate when using iptables -m statistics.

And by "real hardware" I assume you mean x86, with a fairly fast and
high-performance TSC for get_random_entropy().

Reading the TSC takes on the order of 20-50 cycles, iirc.

But it can actually be *much* more expensive. On non-x86, it can be an
IO cycle to external chips.

And on older hardware VM's in x86, it can be a vm exit etc, so
thousands of cycles. I hope nobody uses those VM's any more, but it
would be a reasonable test-case for some non-x86 implementations, so..

IOW, no. You guys are - once again - ignoring reality.

            Linus

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