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

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

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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