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>
