linux-kernel - Re: [PATCH v5 04/11] mips: use fallback for random_get

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Date:   Thu, 21 Apr 2022 22:01:03 +0200
From:   "Jason A. Donenfeld" <Jason@...c4.com>
To:     "Maciej W. Rozycki" <macro@...am.me.uk>,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Linux Crypto Mailing List <linux-crypto@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Arnd Bergmann <arnd@...db.de>, "Theodore Ts'o" <tytso@....edu>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Geert Uytterhoeven <geert@...ux-m68k.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        "David S . Miller" <davem@...emloft.net>,
        Richard Weinberger <richard@....at>,
        Anton Ivanov <anton.ivanov@...bridgegreys.com>,
        Johannes Berg <johannes@...solutions.net>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H . Peter Anvin" <hpa@...or.com>, Chris Zankel <chris@...kel.net>,
        Max Filippov <jcmvbkbc@...il.com>,
        Stephen Boyd <sboyd@...nel.org>,
        Dinh Nguyen <dinguyen@...nel.org>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        linux-m68k <linux-m68k@...ts.linux-m68k.org>,
        "open list:BROADCOM NVRAM DRIVER" <linux-mips@...r.kernel.org>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        sparclinux <sparclinux@...r.kernel.org>,
        linux-um@...ts.infradead.org, X86 ML <x86@...nel.org>,
        linux-xtensa@...ux-xtensa.org
Subject: Re: [PATCH v5 04/11] mips: use fallback for random_get_entropy()
 instead of just c0 random

Hi Maciej,

On Thu, Apr 21, 2022 at 9:25 PM Maciej W. Rozycki <macro@...am.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@...am.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