linux-kernel - Re: [PATCH 16/16] arm64: Add work around for Arm Cortex-A55 Erratum 1024718

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Message-ID: <20180126153314.GW5862@e103592.cambridge.arm.com>
Date:   Fri, 26 Jan 2018 15:33:15 +0000
From:   Dave Martin <Dave.Martin@....com>
To:     Suzuki K Poulose <suzuki.poulose@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        ckadabi@...eaurora.org, ard.biesheuvel@...aro.org,
        marc.zyngier@....com, catalin.marinas@....com, will.deacon@....com,
        linux-kernel@...r.kernel.org, jnair@...iumnetworks.com
Subject: Re: [PATCH 16/16] arm64: Add work around for Arm Cortex-A55 Erratum
 1024718

On Tue, Jan 23, 2018 at 12:28:09PM +0000, Suzuki K Poulose wrote:
> Some variants of the Arm Cortex-55 cores (r0p0, r0p1, r1p0) suffer
> from an erratum 1024718, which causes incorrect updates when DBM/AP
> bits in a page table entry is modified without a break-before-make
> sequence. The work around is to skip enabling the hardware DBM feature
> on the affected cores. The hardware Access Flag management features
> is not affected.
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@....com>
> ---
>  Documentation/arm64/silicon-errata.txt |  1 +
>  arch/arm64/Kconfig                     | 14 ++++++++++++++
>  arch/arm64/kernel/cpufeature.c         | 14 +++++++++++++-
>  3 files changed, 28 insertions(+), 1 deletion(-)

[...]

> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 8af755b8219d..64f1e911c6af 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -914,9 +914,21 @@ static inline void __cpu_enable_hw_dbm(void)
>  	isb();
>  }
>  
> +static bool cpu_has_erratum_1024718(void)
> +{
> +	static const struct midr_range __maybe_unused cpus[] = {

Do you need __maybe_unused?  If #ifdef were used here then
__maybe_unused would be needed, but I thought that if code is optimised
out instead of conditionally copiled, this didn't apply.

> +		MIDR_RANGE(MIDR_CORTEX_A55, 0, 0, 1, 0),	// A55 r0p0 -r1p0
> +		{},
> +	};
> +
> +	return IS_ENABLED(CONFIG_ARM64_ERRATUM_1024718) &&
> +	       is_midr_in_range_list(read_cpuid_id(), cpus);

Why have a list with just one entry?  Do you expect more entries over
time?

[...]

Cheers
---Dave