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Subject: Re: [PATCH v3 2/7] arm64: trap to EL1 accesses to AMU counters from
 EL0
To:     Ionela Voinescu <ionela.voinescu@arm.com>, catalin.marinas@arm.com,
        will@kernel.org, mark.rutland@arm.com, maz@kernel.org,
        suzuki.poulose@arm.com, sudeep.holla@arm.com, lukasz.luba@arm.com,
        rjw@rjwysocki.net
Cc:     peterz@infradead.org, mingo@redhat.com, vincent.guittot@linaro.org,
        viresh.kumar@linaro.org, linux-arm-kernel@lists.infradead.org,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-pm@vger.kernel.org, Steve Capper <steve.capper@arm.com>
References: <20200211184542.29585-1-ionela.voinescu@arm.com>
 <20200211184542.29585-3-ionela.voinescu@arm.com>
From:   Valentin Schneider <valentin.schneider@arm.com>
Message-ID: <17a791d0-89ec-9b1e-ff36-502cbf51ab17@arm.com>
Date:   Wed, 12 Feb 2020 15:36:12 +0000
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On 11/02/2020 18:45, Ionela Voinescu wrote:
> The activity monitors extension is an optional extension introduced
> by the ARMv8.4 CPU architecture. In order to access the activity
> monitors counters safely, if desired, the kernel should detect the
> presence of the extension through the feature register, and mediate
> the access.
> 
> Therefore, disable direct accesses to activity monitors counters
> from EL0 (userspace) and trap them to EL1 (kernel).
> 
> To be noted that the ARM64_AMU_EXTN kernel config and the disable_amu
> kernel parameter do not have an effect on this code. Given that the
> amuserenr_el0 resets to an UNKNOWN value, setting the trap of EL0
> accesses to EL1 is always attempted for safety and security
> considerations. Therefore firmware should still ensure accesses to
> AMU registers are not trapped in EL2/EL3 as this code cannot be
> bypassed if the CPU implements the Activity Monitors Unit.
> 
> Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com>
> Cc: Catalin Marinas <catalin.marinas@arm.com>
> Cc: Will Deacon <will@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Steve Capper <steve.capper@arm.com>

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>