Re: x86/random: Speculation to the rescue

[Peter Zijlstra]

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@xxxxxxx> 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.