Re: [PATCH v2 4/5] arm64: Add support for SMCCC TRNG entropy source

From: Mark Rutland <mark.rutland@arm.com>
To: Ard Biesheuvel <ardb@kernel.org>
Cc: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Linus Walleij <linus.walleij@linaro.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	Russell King <linux@armlinux.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Andre Przywara <andre.przywara@arm.com>,
	Mark Brown <broonie@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	kvmarm <kvmarm@lists.cs.columbia.edu>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v2 4/5] arm64: Add support for SMCCC TRNG entropy source
Date: Thu, 5 Nov 2020 14:45:30 +0000	[thread overview]
Message-ID: <20201105144530.GK82102@C02TD0UTHF1T.local> (raw)
In-Reply-To: <CAMj1kXEoot13bFKywPRVf4BFhA5tSp+Nate=+Z+JsQdLW1oRrg@mail.gmail.com>

On Thu, Nov 05, 2020 at 03:34:01PM +0100, Ard Biesheuvel wrote:
> On Thu, 5 Nov 2020 at 15:30, Mark Rutland <mark.rutland@arm.com> wrote:
> > On Thu, Nov 05, 2020 at 03:04:57PM +0100, Ard Biesheuvel wrote:
> > > On Thu, 5 Nov 2020 at 15:03, Mark Rutland <mark.rutland@arm.com> wrote:
> >
> > > > That said, I'm not sure it's great to plumb this under the
> > > > arch_get_random*() interfaces, e.g. given this measn that
> > > > add_interrupt_randomness() will end up trapping to the host all the time
> > > > when it calls arch_get_random_seed_long().
> > >
> > > As it turns out, add_interrupt_randomness() isn't actually used on ARM.
> >
> > It's certainly called on arm64, per a warning I just hacked in:

[...]

> > ... and I couldn't immediately spot why 32-bit arm  would be different.
> 
> Hmm, I actually meant both arm64 and ARM.
> 
> Marc looked into this at my request a while ago, and I had a look
> myself as well at the time, and IIRC, we both concluded that we don't
> hit that code path. Darn.
> 
> In any case, the way add_interrupt_randomness() calls
> arch_get_random_seed_long() is absolutely insane, so we should try to
> fix that in any case.

I have no strong opinion there, and I'm happy with that getting cleaned
up.

Regardless, I do think it's reasonable for the common code to expect
that arch_get_random_*() to be roughly as expensive as "most other
instructions" (since even RNDR* is expensive the CPU might be able to do
useful speculative work in the mean time), whereas a trap to the host is
always liable to be expensive as no useful work can be done while the
host is handling it, so I think it makes sense to distinguish the two.

Thanks,
Mark.

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