linux-kernel - Re: x86/random: Speculation to the rescue

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Message-ID: <20190930163215.GH4519@hirez.programming.kicks-ass.net>
Date:   Mon, 30 Sep 2019 18:32:15 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     "Theodore Y. Ts'o" <tytso@....edu>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Ahmed S. Darwish" <darwish.07@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Nicholas Mc Guire <hofrat@...ntech.at>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...omium.org>
Subject: Re: x86/random: Speculation to the rescue

On Mon, Sep 30, 2019 at 09:15:55AM -0700, Linus Torvalds wrote:
> On Mon, Sep 30, 2019 at 6:16 AM Theodore Y. Ts'o <tytso@....edu> wrote:

> But it _also_ means that if you have a small and excessively stupid
> in-order CPU, I can almost guarantee that you will at least have cache
> misses likely all the way out to memory. So a CPU-only loop like the
> LFSR thing that Thomas reports generates entropy even on its own would
> likely generate nothing at all on a simple in-order core - but I do

In my experience LFSRs are good at defeating branch predictors, which
would make even in-order cores suffer lots of branch misses. And that
might be enough, maybe.