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

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Date:   Wed, 1 Feb 2017 17:49:34 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Will Deacon <will.deacon@....com>
Cc:     Mark Rutland <mark.rutland@....com>,
        Mark Langsdorf <mlangsdo@...hat.com>,
        Christoffer Dall <christoffer.dall@...aro.org>,
        kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <marc.zyngier@....com>,
        Jon Masters <jcm@...hat.com>, timur@...eaurora.org,
        Jonathan Corbet <corbet@....net>, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, kvmarm@...ts.cs.columbia.edu,
        Christopher Covington <cov@...eaurora.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Neil Leeder <nleeder@...eaurora.org>, shankerd@...eaurora.org,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v4 2/4] arm64: Work around Falkor erratum 1003

On Wed, Feb 01, 2017 at 05:41:05PM +0000, Will Deacon wrote:
> 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.

With the right Kconfig comment we could make this clearer.

> 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)?

You still need this workaround even if you don't want any PAN (both sw
and hw PAN disabled). I wouldn't want to select ARM64_PAN since it's not
a dependency. It's more like if you do need a PAN, make sure you only
use the hw one.

-- 
Catalin