new file mode 100644
@@ -0,0 +1,114 @@
+=======================================================
+Activity Monitors Unit (AMU) extension in AArch64 Linux
+=======================================================
+
+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, if the capability is not disabled
+at system level (either through CONFIG_ARM64_AMU_EXTN or kernel parameter)
+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 or disable the capability at runtime through the
+corresponding kernel parameter. 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.
@@ -248,6 +248,20 @@ Before jumping into the kernel, the following conditions must be met:
- HCR_EL2.APK (bit 40) must be initialised to 0b1
- HCR_EL2.API (bit 41) must be initialised to 0b1
+ For CPUs with Activity Monitors Unit v1 (AMUv1) extension present:
+ - If EL3 is present:
+ CPTR_EL3.TAM (bit 30) must be initialised to 0b0
+ CPTR_EL2.TAM (bit 30) must be initialised to 0b0
+ AMCNTENSET0_EL0 must be initialised to 0b1111
+ AMCNTENSET1_EL0 must be initialised to a platform specific value
+ having 0b1 set for the corresponding bit for each of the auxiliary
+ counters present.
+ - If the kernel is entered at EL1:
+ AMCNTENSET0_EL0 must be initialised to 0b1111
+ AMCNTENSET1_EL0 must be initialised to a platform specific value
+ having 0b1 set for the corresponding bit for each of the auxiliary
+ counters present.
+
The requirements described above for CPU mode, caches, MMUs, architected
timers, coherency and system registers apply to all CPUs. All CPUs must
enter the kernel in the same exception level.
@@ -6,6 +6,7 @@ ARM64 Architecture
:maxdepth: 1
acpi_object_usage
+ amu
arm-acpi
booting
cpu-feature-registers
The activity monitors extension is an optional extension introduced by the ARMv8.4 CPU architecture. Add initial documentation for the AMUv1 extension: - arm64/amu.txt: AMUv1 documentation - arm64/booting.txt: system registers initialisation Signed-off-by: Ionela Voinescu <ionela.voinescu@arm.com> Cc: Catalin Marinas <catalin.marinas@arm.com> Cc: Will Deacon <will@kernel.org> Cc: Jonathan Corbet <corbet@lwn.net> --- Documentation/arm64/amu.rst | 114 ++++++++++++++++++++++++++++++++ Documentation/arm64/booting.rst | 14 ++++ Documentation/arm64/index.rst | 1 + 3 files changed, 129 insertions(+) create mode 100644 Documentation/arm64/amu.rst