linux-kernel - Re: [PATCH v2 1/1] arm64: Add workaround for Fujitsu A64FX erratum 010001

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Message-ID: <20190125180813.GM25901@arrakis.emea.arm.com>
Date:   Fri, 25 Jan 2019 18:08:14 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     "Zhang, Lei" <zhang.lei@...fujitsu.com>
Cc:     'Mark Rutland' <mark.rutland@....com>,
        "'linux-arm-kernel@...ts.infradead.org'" 
        <linux-arm-kernel@...ts.infradead.org>,
        "'will.deacon@....com'" <will.deacon@....com>,
        "'linux-kernel@...r.kernel.org'" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/1] arm64: Add workaround for Fujitsu A64FX erratum
 010001

On Tue, Jan 22, 2019 at 08:54:33AM +0000, Zhang, Lei wrote:
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index efb7b2c..37e4f18 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -666,6 +666,28 @@ static int do_sea(unsigned long addr, unsigned int esr, struct pt_regs *regs)
>  	return 0;
>  }
>  
> +static int do_bad_unknown_63(unsigned long addr, unsigned int esr, struct pt_regs *regs)
> +{
> +	/*
> +	 * On some variants of the Fujitsu-A64FX cores ver(1.0, 1.1),
> +	 * memory accesses may spuriously trigger data aborts with
> +	 * DFSC=0b111111.
> +	 */
> +	if (IS_ENABLED(CONFIG_FUJITSU_ERRATUM_010001)) {
> +		if (cpus_have_cap(ARM64_WORKAROUND_FUJITSU_A64FX_0100001)) {
> +			return 0;
> +		} else { /* cpu capabilities maybe not ready*/
> +			unsigned int current_cpu_midr = read_cpuid_id();
> +			const struct midr_range fujitsu_a64fx_midr_range = {
> +				MIDR_FUJITSU_A64FX, MIDR_CPU_VAR_REV(0, 0), MIDR_CPU_VAR_REV(1, 0)
> +			};
> +			if (is_midr_in_range(current_cpu_midr, &fujitsu_a64fx_midr_range) == TRUE)
> +				return 0;
> +		}
> +	}
> +	return do_bad(addr, esr, regs);
> +}

IIUC, this can happen very early when the errata framework isn't yet
ready. Given that this is not on a fast path (you already took a fault),
I don't think it's worth optimising for cpus_have_cap() (and
ARM64_WORKAROUND_FUJITSU_A64FX_0100001). I've seen Mark's comments on
why checking MIDR in a preemptible context is not a good idea but I
suspect your platform is homogeneous (i.e. not big.LITTLE).

-- 
Catalin