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Subject: Re: [PATCH v5 01/10] arm64: Provide a command line to disable
 spectre_v2 mitigation
To:     jeremy.linton@arm.com, linux-arm-kernel@lists.infradead.org
Cc:     catalin.marinas@arm.com, will.deacon@arm.com, marc.zyngier@arm.com,
        dave.martin@arm.com, shankerd@codeaurora.org,
        julien.thierry@arm.com, mlangsdo@redhat.com, stefan.wahren@i2e.com,
        andre.przywara@arm.com, linux-kernel@vger.kernel.org,
        corbet@lwn.net, linux-doc@vger.kernel.org
References: <20190227010544.597579-1-jeremy.linton@arm.com>
 <20190227010544.597579-2-jeremy.linton@arm.com>
From:   Suzuki K Poulose <suzuki.poulose@arm.com>
Message-ID: <a0524741-e17a-a4b1-9d7c-3d001b2b581f@arm.com>
Date:   Thu, 28 Feb 2019 18:14:34 +0000
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Hi Jeremy

On 27/02/2019 01:05, Jeremy Linton wrote:
> There are various reasons, including bencmarking, to disable spectrev2
> mitigation on a machine. Provide a command-line to do so.
> 
> Signed-off-by: Jeremy Linton <jeremy.linton@arm.com>
> Cc: Jonathan Corbet <corbet@lwn.net>
> Cc: linux-doc@vger.kernel.org


> diff --git a/arch/arm64/kernel/cpu_errata.c b/arch/arm64/kernel/cpu_errata.c
> index 9950bb0cbd52..d2b2c69d31bb 100644
> --- a/arch/arm64/kernel/cpu_errata.c
> +++ b/arch/arm64/kernel/cpu_errata.c
> @@ -220,6 +220,14 @@ static void qcom_link_stack_sanitization(void)
>   		     : "=&r" (tmp));
>   }
>   
> +static bool __nospectre_v2;
> +static int __init parse_nospectre_v2(char *str)
> +{
> +	__nospectre_v2 = true;
> +	return 0;
> +}
> +early_param("nospectre_v2", parse_nospectre_v2);
> +
>   static void
>   enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
>   {
> @@ -231,6 +239,11 @@ enable_smccc_arch_workaround_1(const struct arm64_cpu_capabilities *entry)
>   	if (!entry->matches(entry, SCOPE_LOCAL_CPU))
>   		return;
>   
> +	if (__nospectre_v2) {
> +		pr_info_once("spectrev2 mitigation disabled by command line option\n");
> +		return;
> +	}
> +

Could we not disable the "cap" altogether instead, rather than disabling the
work around ? Or do we need that information ?

Cheers
Suzuki