Re: [PATCH v5 04/11] mips: use fallback for random_get_entropy() instead of just c0 random

From: "Jason A. Donenfeld" <Jason@zx2c4.com>
To: "Maciej W. Rozycki" <macro@orcam.me.uk>,
	Thomas Bogendoerfer <tsbogend@alpha.franken.de>
Cc: LKML <linux-kernel@vger.kernel.org>,
	Linux Crypto Mailing List <linux-crypto@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Arnd Bergmann <arnd@arndb.de>, "Theodore Ts'o" <tytso@mit.edu>,
	Dominik Brodowski <linux@dominikbrodowski.net>,
	Russell King <linux@armlinux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Geert Uytterhoeven <geert@linux-m68k.org>,
	Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	"David S . Miller" <davem@davemloft.net>,
	Richard Weinberger <richard@nod.at>,
	Anton Ivanov <anton.ivanov@cambridgegreys.com>,
	Johannes Berg <johannes@sipsolutions.net>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
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	"H . Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH v5 04/11] mips: use fallback for random_get_entropy() instead of just c0 random
Date: Thu, 21 Apr 2022 22:01:03 +0200	[thread overview]
Message-ID: <CAHmME9o5D93M3P_xcewOBfPisbC2d-x3HYvJPFGUoC6oG_RkgA@mail.gmail.com> (raw)
In-Reply-To: <alpine.DEB.2.21.2204212018540.9383@angie.orcam.me.uk>

Hi Maciej,

On Thu, Apr 21, 2022 at 9:25 PM Maciej W. Rozycki <macro@orcam.me.uk> wrote:
>
> On Tue, 19 Apr 2022, Jason A. Donenfeld wrote:
>
> > For situations in which we don't have a c0 counter register available,
> > we've been falling back to reading the c0 "random" register, which is
> > usually bounded by the amount of TLB entries and changes every other
> > cycle or so. This means it wraps extremely often. We can do better by
> > combining this fast-changing counter with a potentially slower-changing
> > counter from random_get_entropy_fallback() in the more significant bits.
> > This commit combines the two, taking into account that the changing bits
> > are in a different bit position depending on the CPU model. In addition,
> > we previously were falling back to 0 for ancient CPUs that Linux does
> > not support anyway; remove that dead path entirely.
>
> Tested-by: Maciej W. Rozycki <macro@orcam.me.uk>
>
>  I've pushed the algorithm through testing with a number of suitable
> systems:
>
> - an R2000A and an R3000A with no timer of any kind, only jiffies,
>
> - an R3400 with a chipset timer only,
>
> - an R4400SC with a usable buggy CP0 counter and a chipset timer,
>
> - a 5Kc with a good CP0 counter only,
>
> with no obvious issues spotted.  Thank you for working on this!

Thanks for all the testing!

ThomasB - I think maybe you can re-"Acked-by" this now if you're on
board with the strategy here?

Jason