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Date: Fri, 8 Apr 2022 20:40:08 +0200
From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Subject: Re: [PATCH RFC v1 07/10] arm64: use sched_clock() for
 random_get_entropy() instead of zero
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To: Mark Rutland <mark.rutland@arm.com>
Cc: linux-kernel@vger.kernel.org, linux-crypto@vger.kernel.org, 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>, Thomas Bogendoerfer <tsbogend@alpha.franken.de>, 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>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>, Dave Hansen <dave.hansen@linux.intel.com>, "H . Peter Anvin" <hpa@zytor.com>, Chris Zankel <chris@zankel.net>, Max Filippov <jcmvbkbc@gmail.com>, John Stultz <john.stultz@linaro.org>, Stephen Boyd <sboyd@kernel.org>, linux-arm-kernel@lists.infradead.org, linux-m68k@lists.linux-m68k.org, linux-mips@vger.kernel.org, linux-riscv@lists.infradead.org, sparclinux@vger.kernel.org, linux-um@lists.infradead.org, x86@kernel.org, linux-xtensa@linux-xtensa.org
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Hi Mark,

On Fri, Apr 08, 2022 at 07:33:53PM +0100, Mark Rutland wrote:
> On Fri, Apr 08, 2022 at 08:21:42PM +0200, Jason A. Donenfeld wrote:
> > In the event that random_get_entropy() can't access a cycle counter or
> > similar, falling back to returning 0 is really not the best we can do.
> > Instead, at least calling sched_clock() would be preferable, because
> > that always needs to return _something_, even falling back to jiffies
> > eventually. It's not as though sched_clock() 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.
> > 
> > If CONFIG_ARM_ARCH_TIMER=n, then get_cycles() will return 0, so we only
> > need the fallback code for that case.
> 
> In arch/arm64/Kconfig we unconditionally select CONFIG_ARM_ARCH_TIMER,
> so that configuration shouldn't be possible, and I don't think this
> patch is necessary.
> 
> On arm64 we depend on the architected timer in a bunch of places, so
> anyone hacking that out has bigger problems.

Thanks for the tip. I'll drop this patch from the series.

Jason