linux-kernel - Re: [PATCH v3] random: remove early archrandom abstraction

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Date:   Tue, 1 Nov 2022 11:39:11 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Jason A. Donenfeld" <Jason@...c4.com>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
        Will Deacon <will@...nel.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>
Subject: Re: [PATCH v3] random: remove early archrandom abstraction

On Mon, Oct 31, 2022 at 11:28:40AM +0100, Jason A. Donenfeld wrote:
> Catalin - Though this touches arm64's archrandom.h, I intend to take
> this through the random.git tree, if that's okay. I have other patches
> that will build off of this one. -Jason

I'm fine with the patch going through your tree but I have a comment
below.

> diff --git a/arch/arm64/include/asm/archrandom.h b/arch/arm64/include/asm/archrandom.h
> index 109e2a4454be..4b0f28730ab2 100644
> --- a/arch/arm64/include/asm/archrandom.h
> +++ b/arch/arm64/include/asm/archrandom.h
> @@ -58,6 +58,20 @@ static inline bool __arm64_rndrrs(unsigned long *v)
>  	return ok;
>  }
>  
> +static inline bool __early_cpu_has_rndr(void)
> +{
> +	/* Open code as we run prior to the first call to cpufeature. */
> +	unsigned long ftr = read_sysreg_s(SYS_ID_AA64ISAR0_EL1);
> +	return (ftr >> ID_AA64ISAR0_EL1_RNDR_SHIFT) & 0xf;
> +}
> +
> +static __always_inline bool __cpu_has_rng(void)
> +{
> +	if (!system_capabilities_finalized() && early_boot_irqs_disabled)
> +		return __early_cpu_has_rndr();
> +	return cpus_have_const_cap(ARM64_HAS_RNG);
> +}

I'm not sure about using early_boot_irqs_disabled, it is described as a
debug helper. It's also set to 'false' before
system_capabilities_finalized() (once the full SMP is enabled).

Would something like this work:

	if (system_capabilities_finalized())
		return cpus_have_final_cap(ARM64_HAS_RNG);
	if (!preemptible())
		return __early_cpu_has_rndr();
	return false;

We also have a this_cpu_has_cap() function, though it's likely more
expensive than the hand-coded __early_cpu_has_rndr() (if we care about
performance here).

-- 
Catalin