Re: [PATCH v3 2/7] arm64: trap to EL1 accesses to AMU counters from EL0

[Valentin Schneider]

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@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Cc: Steve Capper <steve.capper@xxxxxxx>

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