linux-kernel - Re: [PATCH 2/2] arm64: Add support for SMCCC TRNG firmware interface

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Message-ID: <65057faa-d06b-6baf-4f12-9587cacbe3a9@arm.com>
Date:   Wed, 7 Oct 2020 15:16:07 +0100
From:   James Morse <james.morse@....com>
To:     Andre Przywara <andre.przywara@....com>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Lorenzo Pieralisi <lorenzo.pieralisi@....com>,
        Richard Henderson <richard.henderson@...aro.org>,
        linux-kernel@...r.kernel.org, Mark Brown <broonie@...nel.org>,
        Sudeep Holla <sudeep.holla@....com>,
        Ard Biesheuvel <ardb@...nel.org>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 2/2] arm64: Add support for SMCCC TRNG firmware interface

Hi Andre,

On 06/10/2020 21:18, Andre Przywara wrote:
> The ARM architected TRNG firmware interface, described in ARM spec
> DEN0098[1], defines an ARM SMCCC based interface to a true random number
> generator, provided by firmware.
> This can be discovered via the SMCCC >=v1.1 interface, and provides
> up to 192 bits of entropy per call.
> 
> Hook this SMC call into arm64's arch_get_random_*() implementation,
> coming to the rescue when the CPU does not implement the ARM v8.5 RNG
> system registers.
> 
> For the detection, we piggy back on the PSCI/SMCCC discovery (which gives
> us the conduit to use: hvc or smc), then try to call the
> ARM_SMCCC_TRNG_VERSION function, which returns -1 if this interface is
> not implemented.

>  arch/arm64/include/asm/archrandom.h | 83 +++++++++++++++++++++++++----
>  1 file changed, 73 insertions(+), 10 deletions(-)

> diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
> index ffb1a40d5475..b6c291c42a48 100644
> --- a/arch/arm64/include/asm/archrandom.h
> +++ b/arch/arm64/include/asm/archrandom.h
> @@ -7,6 +7,13 @@
>  #include <linux/bug.h>
>  #include <linux/kernel.h>
>  #include <asm/cpufeature.h>
> +#include <linux/arm-smccc.h>
> +
> +static enum smc_trng_status {
> +	SMC_TRNG_UNKNOWN,
> +	SMC_TRNG_NOT_SUPPORTED,
> +	SMC_TRNG_SUPPORTED
> +} smc_trng_status = SMC_TRNG_UNKNOWN;

Doesn't this static variable in a header file mean each file that includes this has its
own copy? Is that intentional?


Thanks,

James