linux-kernel - Re: [PATCH v3 2/2] arm64: Add software workaround for Falkor erratum 1041

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Date:   Tue, 14 Nov 2017 17:05:05 -0800
From:   Stephen Boyd <sboyd@...eaurora.org>
To:     Shanker Donthineni <shankerd@...eaurora.org>
Cc:     Will Deacon <will.deacon@....com>,
        Marc Zyngier <marc.zyngier@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-efi@...r.kernel.org,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Matt Fleming <matt@...eblueprint.co.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        linux-kernel@...r.kernel.org, James Morse <james.morse@....com>,
        Robin Murphy <robin.murphy@....com>,
        kvmarm@...ts.cs.columbia.edu,
        Christoffer Dall <christoffer.dall@...aro.org>
Subject: Re: [PATCH v3 2/2] arm64: Add software workaround for Falkor erratum
 1041

On 11/12, Shanker Donthineni wrote:
> The ARM architecture defines the memory locations that are permitted
> to be accessed as the result of a speculative instruction fetch from
> an exception level for which all stages of translation are disabled.
> Specifically, the core is permitted to speculatively fetch from the
> 4KB region containing the current program counter 4K and next 4K.
> 
> When translation is changed from enabled to disabled for the running
> exception level (SCTLR_ELn[M] changed from a value of 1 to 0), the
> Falkor core may errantly speculatively access memory locations outside
> of the 4KB region permitted by the architecture. The errant memory
> access may lead to one of the following unexpected behaviors.
> 
> 1) A System Error Interrupt (SEI) being raised by the Falkor core due
>    to the errant memory access attempting to access a region of memory
>    that is protected by a slave-side memory protection unit.
> 2) Unpredictable device behavior due to a speculative read from device
>    memory. This behavior may only occur if the instruction cache is
>    disabled prior to or coincident with translation being changed from
>    enabled to disabled.
> 
> The conditions leading to this erratum will not occur when either of the
> following occur:
>  1) A higher exception level disables translation of a lower exception level
>    (e.g. EL2 changing SCTLR_EL1[M] from a value of 1 to 0).
>  2) An exception level disabling its stage-1 translation if its stage-2
>     translation is enabled (e.g. EL1 changing SCTLR_EL1[M] from a value of 1
>     to 0 when HCR_EL2[VM] has a value of 1).
> 
> To avoid the errant behavior, software must execute an ISB immediately
> prior to executing the MSR that will change SCTLR_ELn[M] from 1 to 0.
> 

This also applies to Kryo CPUs. I have a patch[1] for the 1003
Falkor errata that adds the Kryo MIDR check which can also be
used for this errata.

[1] https://patchwork.kernel.org/patch/10048987/

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