Re: [PATCH v4 4/5] arm64: Add support for SMCCC TRNG entropy source

From: Mark Brown <broonie@kernel.org>
To: Andre Przywara <andre.przywara@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>,
	Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>,
	Theodore Ts'o <tytso@mit.edu>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Linus Walleij <linus.walleij@linaro.org>,
	Sudeep Holla <sudeep.holla@arm.com>,
	linux-kernel@vger.kernel.org,
	Russell King <linux@armlinux.org.uk>,
	kvmarm@lists.cs.columbia.edu, Marc Zyngier <maz@kernel.org>,
	Will Deacon <will@kernel.org>, Ard Biesheuvel <ardb@kernel.org>,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH v4 4/5] arm64: Add support for SMCCC TRNG entropy source
Date: Fri, 11 Dec 2020 16:26:12 +0000	[thread overview]
Message-ID: <20201211162612.GF4929@sirena.org.uk> (raw)
In-Reply-To: <20201211160005.187336-5-andre.przywara@arm.com>

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On Fri, Dec 11, 2020 at 04:00:04PM +0000, Andre Przywara wrote:

>  static inline bool __must_check arch_get_random_seed_long(unsigned long *v)
>  {
> +	struct arm_smccc_res res;
> +
> +	/*
> +	 * We prefer the SMCCC call, since its semantics (return actual
> +	 * hardware backed entropy) is closer to the idea behind this
> +	 * function here than what even the RNDRSS register provides
> +	 * (the output of a pseudo RNG freshly seeded by a TRNG).
> +	 */

This logic...

> @@ -77,10 +117,20 @@ arch_get_random_seed_long_early(unsigned long *v)
>  {
>  	WARN_ON(system_state != SYSTEM_BOOTING);
>  
> -	if (!__early_cpu_has_rndr())
> -		return false;
> +	if (__early_cpu_has_rndr())
> +		return __arm64_rndr(v);
> +
> +	if (smccc_trng_available) {
> +		struct arm_smccc_res res;
>  
> -	return __arm64_rndr(v);
> +		arm_smccc_1_1_invoke(ARM_SMCCC_TRNG_RND64, 64, &res);
> +		if ((int)res.a0 >= 0) {
> +			*v = res.a3;
> +			return true;
> +		}
> +	}
> +
> +	return false;

...seems to also apply here but we prefer the RNDR instead of the SMCC.
We probably want to either do the same thing or add a comment saying
what's going on.

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