120 lines
4.2 KiB
ReStructuredText
120 lines
4.2 KiB
ReStructuredText
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.. _amu_index:
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=======================================================
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Activity Monitors Unit (AMU) extension in AArch64 Linux
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=======================================================
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Author: Ionela Voinescu <ionela.voinescu@arm.com>
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Date: 2019-09-10
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This document briefly describes the provision of Activity Monitors Unit
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support in AArch64 Linux.
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Architecture overview
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---------------------
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The activity monitors extension is an optional extension introduced by the
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ARMv8.4 CPU architecture.
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The activity monitors unit, implemented in each CPU, provides performance
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counters intended for system management use. The AMU extension provides a
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system register interface to the counter registers and also supports an
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optional external memory-mapped interface.
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Version 1 of the Activity Monitors architecture implements a counter group
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of four fixed and architecturally defined 64-bit event counters.
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- CPU cycle counter: increments at the frequency of the CPU.
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- Constant counter: increments at the fixed frequency of the system
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clock.
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- Instructions retired: increments with every architecturally executed
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instruction.
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- Memory stall cycles: counts instruction dispatch stall cycles caused by
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misses in the last level cache within the clock domain.
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When in WFI or WFE these counters do not increment.
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The Activity Monitors architecture provides space for up to 16 architected
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event counters. Future versions of the architecture may use this space to
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implement additional architected event counters.
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Additionally, version 1 implements a counter group of up to 16 auxiliary
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64-bit event counters.
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On cold reset all counters reset to 0.
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Basic support
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-------------
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The kernel can safely run a mix of CPUs with and without support for the
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activity monitors extension. Therefore, when CONFIG_ARM64_AMU_EXTN is
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selected we unconditionally enable the capability to allow any late CPU
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(secondary or hotplugged) to detect and use the feature.
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When the feature is detected on a CPU, we flag the availability of the
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feature but this does not guarantee the correct functionality of the
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counters, only the presence of the extension.
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Firmware (code running at higher exception levels, e.g. arm-tf) support is
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needed to:
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- Enable access for lower exception levels (EL2 and EL1) to the AMU
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registers.
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- Enable the counters. If not enabled these will read as 0.
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- Save/restore the counters before/after the CPU is being put/brought up
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from the 'off' power state.
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When using kernels that have this feature enabled but boot with broken
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firmware the user may experience panics or lockups when accessing the
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counter registers. Even if these symptoms are not observed, the values
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returned by the register reads might not correctly reflect reality. Most
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commonly, the counters will read as 0, indicating that they are not
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enabled.
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If proper support is not provided in firmware it's best to disable
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CONFIG_ARM64_AMU_EXTN. To be noted that for security reasons, this does not
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bypass the setting of AMUSERENR_EL0 to trap accesses from EL0 (userspace) to
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EL1 (kernel). Therefore, firmware should still ensure accesses to AMU registers
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are not trapped in EL2/EL3.
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The fixed counters of AMUv1 are accessible though the following system
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register definitions:
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- SYS_AMEVCNTR0_CORE_EL0
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- SYS_AMEVCNTR0_CONST_EL0
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- SYS_AMEVCNTR0_INST_RET_EL0
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- SYS_AMEVCNTR0_MEM_STALL_EL0
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Auxiliary platform specific counters can be accessed using
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SYS_AMEVCNTR1_EL0(n), where n is a value between 0 and 15.
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Details can be found in: arch/arm64/include/asm/sysreg.h.
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Userspace access
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----------------
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Currently, access from userspace to the AMU registers is disabled due to:
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- Security reasons: they might expose information about code executed in
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secure mode.
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- Purpose: AMU counters are intended for system management use.
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Also, the presence of the feature is not visible to userspace.
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Virtualization
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--------------
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Currently, access from userspace (EL0) and kernelspace (EL1) on the KVM
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guest side is disabled due to:
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- Security reasons: they might expose information about code executed
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by other guests or the host.
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Any attempt to access the AMU registers will result in an UNDEFINED
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exception being injected into the guest.
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