linux-kernel - Re: [PATCH v5 01/10] arm64: Provide a command line to disable spectre

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Date:   Thu, 28 Feb 2019 18:25:46 +0000
From:   Suzuki K Poulose <suzuki.poulose@....com>
To:     catalin.marinas@....com
Cc:     jeremy.linton@....com, linux-arm-kernel@...ts.infradead.org,
        will.deacon@....com, marc.zyngier@....com, dave.martin@....com,
        shankerd@...eaurora.org, julien.thierry@....com,
        mlangsdo@...hat.com, stefan.wahren@....com, andre.przywara@....com,
        linux-kernel@...r.kernel.org, corbet@....net,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH v5 01/10] arm64: Provide a command line to disable
 spectre_v2 mitigation



On 28/02/2019 18:21, Catalin Marinas wrote:
> On Thu, Feb 28, 2019 at 06:14:34PM +0000, Suzuki K Poulose wrote:
>> 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@....com>
>>> Cc: Jonathan Corbet <corbet@....net>
>>> Cc: linux-doc@...r.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 ?
> 
> There are a few ideas here but I think we settled on always reporting in
> sysfs even if the mitigation is disabled in .config. So I guess we need
> the "cap" around for the reporting part.
> 

Thanks Catalin.

Reviewed-by: Suzuki K Poulose <suzuki.poulose@....com>