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Date:   Thu, 28 Feb 2019 18:21:27 +0000
From:   Catalin Marinas <catalin.marinas@arm.com>
To:     Suzuki K Poulose <suzuki.poulose@arm.com>
Cc:     jeremy.linton@arm.com, linux-arm-kernel@lists.infradead.org,
        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
Subject: Re: [PATCH v5 01/10] arm64: Provide a command line to disable
 spectre_v2 mitigation
Message-ID: <20190228182126.GA252518@arrakis.emea.arm.com>
References: <20190227010544.597579-1-jeremy.linton@arm.com>
 <20190227010544.597579-2-jeremy.linton@arm.com>
 <a0524741-e17a-a4b1-9d7c-3d001b2b581f@arm.com>
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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@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 ?

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.

-- 
Catalin