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From: Peter Zijlstra <peterz@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "Theodore Y. Ts'o" <tytso@mit.edu>,
	Thomas Gleixner <tglx@linutronix.de>,
	"Ahmed S. Darwish" <darwish.07@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>,
	Nicholas Mc Guire <hofrat@opentech.at>,
	the arch/x86 maintainers <x86@kernel.org>,
	Andy Lutomirski <luto@kernel.org>,
	Kees Cook <keescook@chromium.org>
Subject: Re: x86/random: Speculation to the rescue
Date: Mon, 30 Sep 2019 18:32:15 +0200	[thread overview]
Message-ID: <20190930163215.GH4519@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAHk-=wg7YAx_+CDe6fUqApPD_ghP18H9sPnJWWUg32pQ4pU82g@mail.gmail.com>

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@mit.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.


  reply	other threads:[~2019-09-30 16:32 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-09-28 22:24 Thomas Gleixner
2019-09-28 23:53 ` Linus Torvalds
2019-09-29  7:40   ` Thomas Gleixner
2019-09-29  8:05   ` Alexander E. Patrakov
2019-09-30  1:16   ` Linus Torvalds
2019-09-30  2:59     ` Linus Torvalds
2019-09-30  6:10       ` Borislav Petkov
2019-09-30 16:06         ` Linus Torvalds
2019-10-01 13:51           ` Borislav Petkov
2019-10-01 17:14             ` Linus Torvalds
2019-10-01 17:50               ` [PATCH] char/random: Add a newline at the end of the file Borislav Petkov
2019-09-30 18:05         ` x86/random: Speculation to the rescue Kees Cook
2019-09-30  3:37     ` Theodore Y. Ts'o
2019-09-30 13:16       ` Theodore Y. Ts'o
2019-09-30 16:15         ` Linus Torvalds
2019-09-30 16:32           ` Peter Zijlstra [this message]
2019-09-30 17:03             ` Linus Torvalds
2019-10-01 10:28           ` David Laight
2019-10-15 21:50             ` Thomas Gleixner
2019-10-01 16:15   ` Ahmed S. Darwish
2019-10-01 16:37     ` Kees Cook
2019-10-01 17:18       ` Ahmed S. Darwish
2019-10-01 17:25     ` Linus Torvalds
2019-10-06 12:07       ` Pavel Machek
2019-10-02 12:01     ` Theodore Y. Ts'o
2019-10-06 11:41   ` Pavel Machek
2019-10-06 17:26     ` Linus Torvalds
2019-10-06 17:35       ` Pavel Machek
2019-10-06 18:06         ` Linus Torvalds
2019-10-06 18:21           ` Pavel Machek
2019-10-06 18:26             ` Linus Torvalds
2019-10-07 11:47             ` Theodore Y. Ts'o
2019-10-07 22:18               ` Pavel Machek
2019-10-08 11:33                 ` David Laight
2019-10-09  8:02                   ` Pavel Machek
2019-10-09  9:37                     ` David Laight
2019-10-01  2:14 hgntkwis

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