linux-kernel - Re: [PATCH v6 06/17] timekeeping: add raw clock fallback for random_get

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Date:   Mon, 25 Apr 2022 15:22:00 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, linux-crypto@...r.kernel.org,
        arnd@...db.de, Theodore Ts'o <tytso@....edu>
Subject: Re: [PATCH v6 06/17] timekeeping: add raw clock fallback for
 random_get_entropy()

Hi Thomas,

On Mon, Apr 25, 2022 at 02:37:11PM +0200, Thomas Gleixner wrote:
> On Sat, Apr 23 2022 at 23:26, Jason A. Donenfeld wrote:
> > The addition of random_get_entropy_fallback() provides access to
> > whichever time source has the highest frequency, which is useful for
> > gathering entropy on platforms without available cycle counters. It's
> > not necessarily as good as being able to quickly access a cycle counter
> > that the CPU has, but it's still something, even when it falls back to
> > being jiffies-based.
> >
> > In the event that a given arch does not define get_cycles(), falling
> > back to the get_cycles() default implementation that returns 0 is really
> > not the best we can do. Instead, at least calling
> > random_get_entropy_fallback() would be preferable, because that always
> > needs to return _something_, even falling back to jiffies eventually.
> > It's not as though random_get_entropy_fallback() is super high precision
> > or guaranteed to be entropic, but basically anything that's not zero all
> > the time is better than returning zero all the time.
> >
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > Cc: Arnd Bergmann <arnd@...db.de>
> > Cc: Theodore Ts'o <tytso@....edu>
> > Signed-off-by: Jason A. Donenfeld <Jason@...c4.com>
> 
> Not that I care much, but in general taking over authorship w/o
> attribution via Suggested-by or such is frowned upon.

Sorry about that. Usually I'm pretty good about adding those. I guess
something must have gotten lost this time through, as the v1 of this
started out using sched_clock() (Arnd's suggestion) and then moved to
using the raw ktime clock after your suggestion, and I missed the
Suggested-by. I'll add that. Meanwhile, do you want to Ack this patch?
Do the technical aspects look okay to you?

Jason