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Activity Monitors Unit (AMU) extension in AArch64 Linux
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Author: Ionela Voinescu <ionela.voinescu@arm.com>

Date: 2019-09-10

This document briefly describes the provision of Activity Monitors Unit
support in AArch64 Linux.


Architecture overview
---------------------

The activity monitors extension is an optional extension introduced by the
ARMv8.4 CPU architecture.

The activity monitors unit, implemented in each CPU, provides performance
counters intended for system management use. The AMU extension provides a
system register interface to the counter registers and also supports an
optional external memory-mapped interface.

Version 1 of the Activity Monitors architecture implements a counter group
of four fixed and architecturally defined 64-bit event counters.

  - CPU cycle counter: increments at the frequency of the CPU.
  - Constant counter: increments at the fixed frequency of the system
    clock.
  - Instructions retired: increments with every architecturally executed
    instruction.
  - Memory stall cycles: counts instruction dispatch stall cycles caused by
    misses in the last level cache within the clock domain.

When in WFI or WFE these counters do not increment.

The Activity Monitors architecture provides space for up to 16 architected
event counters. Future versions of the architecture may use this space to
implement additional architected event counters.

Additionally, version 1 implements a counter group of up to 16 auxiliary
64-bit event counters.

On cold reset all counters reset to 0.


Basic support
-------------

The kernel can safely run a mix of CPUs with and without support for the
activity monitors extension. Therefore, when CONFIG_ARM64_AMU_EXTN is
selected we unconditionally enable the capability to allow any late CPU
(secondary or hotplugged) to detect and use the feature.

When the feature is detected on a CPU, we flag the availability of the
feature but this does not guarantee the correct functionality of the
counters, only the presence of the extension.

Firmware (code running at higher exception levels, e.g. arm-tf) support is
needed to:

 - Enable access for lower exception levels (EL2 and EL1) to the AMU
   registers.
 - Enable the counters. If not enabled these will read as 0.
 - Save/restore the counters before/after the CPU is being put/brought up
   from the 'off' power state.

When using kernels that have this feature enabled but boot with broken
firmware the user may experience panics or lockups when accessing the
counter registers. Even if these symptoms are not observed, the values
returned by the register reads might not correctly reflect reality. Most
commonly, the counters will read as 0, indicating that they are not
enabled.

If proper support is not provided in firmware it's best to disable
CONFIG_ARM64_AMU_EXTN. To be noted that for security reasons, this does not
bypass the setting of AMUSERENR_EL0 to trap accesses from EL0 (userspace) to
EL1 (kernel). Therefore, firmware should still ensure accesses to AMU registers
are not trapped in EL2/EL3.

The fixed counters of AMUv1 are accessible though the following system
register definitions:

 - SYS_AMEVCNTR0_CORE_EL0
 - SYS_AMEVCNTR0_CONST_EL0
 - SYS_AMEVCNTR0_INST_RET_EL0
 - SYS_AMEVCNTR0_MEM_STALL_EL0

Auxiliary platform specific counters can be accessed using
SYS_AMEVCNTR1_EL0(n), where n is a value between 0 and 15.

Details can be found in: arch/arm64/include/asm/sysreg.h.


Userspace access
----------------

Currently, access from userspace to the AMU registers is disabled due to:

 - Security reasons: they might expose information about code executed in
   secure mode.
 - Purpose: AMU counters are intended for system management use.

Also, the presence of the feature is not visible to userspace.


Virtualization
--------------

Currently, access from userspace (EL0) and kernelspace (EL1) on the KVM
guest side is disabled due to:

 - Security reasons: they might expose information about code executed
   by other guests or the host.

Any attempt to access the AMU registers will result in an UNDEFINED
exception being injected into the guest.