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diff --git a/Documentation/arm64/amu.rst b/Documentation/arm64/amu.rst deleted file mode 100644 index 452ec8b115c2..000000000000 --- a/Documentation/arm64/amu.rst +++ /dev/null @@ -1,117 +0,0 @@ -======================================================= -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, 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. |