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

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Message-ID: <20210104114814.5e1fe218@slackpad.fritz.box>
Date:   Mon, 4 Jan 2021 11:49:54 +0000
From:   Andre Przywara <andre.przywara@....com>
To:     Mark Brown <broonie@...nel.org>
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, 11 Dec 2020 16:26:12 +0000
Mark Brown <broonie@...nel.org> wrote:

> 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.

Argh, you are right, I missed to change this part as well.

Will send a fixed and rebased v5 ASAP.

Cheers,
Andre