linux-kernel - Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

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Message-ID: <20170201174105.GK8177@arm.com>
Date:   Wed, 1 Feb 2017 17:41:05 +0000
From:   Will Deacon <will.deacon@....com>
To:     Catalin Marinas <catalin.marinas@....com>
Cc:     Christopher Covington <cov@...eaurora.org>,
        Mark Rutland <mark.rutland@....com>,
        Mark Langsdorf <mlangsdo@...hat.com>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>, timur@...eaurora.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, shankerd@...eaurora.org,
        linux-arm-kernel@...ts.infradead.org,
        Neil Leeder <nleeder@...eaurora.org>,
        Jon Masters <jcm@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        kvmarm@...ts.cs.columbia.edu,
        Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

On Wed, Feb 01, 2017 at 05:36:09PM +0000, Catalin Marinas wrote:
> On Wed, Feb 01, 2017 at 04:33:58PM +0000, Will Deacon wrote:
> > On Wed, Feb 01, 2017 at 11:29:22AM -0500, Christopher Covington wrote:
> > > On 01/31/2017 12:56 PM, Marc Zyngier wrote:
> > > > Given that all ARMv8 CPUs can support SW_PAN, it is more likely to be
> > > > enabled than the ARMv8.1 PAN. I'd vote for supporting the workaround in
> > > > that case too, and hope that people do enable the HW version.
> > > 
> > > Okay, I'll do my best to add support for the SW PAN case. I rebased and
> > > submitted v6 of the E1009 patch [1] so that it no longer depends on this
> > > patch landing first, if you all are inclined to pick it up while work on
> > > this E1003 patch continues.
> > 
> > The alternative is not enabling SW_PAN (at runtime) if this errata is
> > present, along with a warning stating that hardware-PAN should be
> > enabled in kconfig instead. Not sure what distributions will make of that
> > though.
> 
> The problem with this patch is that when ARM64_SW_TTBR0_PAN is enabled
> and in the absence of hardware PAN (or ARM64_PAN disabled),
> cpu_do_switch_mm is no longer called for user process switching, so the
> workaround is pretty much useless.

Oh, I see what you mean now.

> I'm ok with adding the Kconfig dependency below to
> QCOM_FALKOR_ERRATUM_1003:
> 
> 	depends on !ARM64_SW_TTBR0_PAN || ARM64_PAN
> 
> together with a run-time warning if ARM64_SW_TTBR0_PAN is being used.

That makes it look like hardware-PAN is the cause of the erratum. Maybe
just select ARM64_PAN if the erratum workaround is selected, then
runtime warning if we find that the h/w doesn't have PAN but does have
the erratum (which should never fire)?

Will