Based on kernel version 4.7.2. Page generated on 2016-08-22 22:47 EST.
1 PMU Event Based Branches 2 ======================== 3 4 Event Based Branches (EBBs) are a feature which allows the hardware to 5 branch directly to a specified user space address when certain events occur. 6 7 The full specification is available in Power ISA v2.07: 8 9 https://www.power.org/documentation/power-isa-version-2-07/ 10 11 One type of event for which EBBs can be configured is PMU exceptions. This 12 document describes the API for configuring the Power PMU to generate EBBs, 13 using the Linux perf_events API. 14 15 16 Terminology 17 ----------- 18 19 Throughout this document we will refer to an "EBB event" or "EBB events". This 20 just refers to a struct perf_event which has set the "EBB" flag in its 21 attr.config. All events which can be configured on the hardware PMU are 22 possible "EBB events". 23 24 25 Background 26 ---------- 27 28 When a PMU EBB occurs it is delivered to the currently running process. As such 29 EBBs can only sensibly be used by programs for self-monitoring. 30 31 It is a feature of the perf_events API that events can be created on other 32 processes, subject to standard permission checks. This is also true of EBB 33 events, however unless the target process enables EBBs (via mtspr(BESCR)) no 34 EBBs will ever be delivered. 35 36 This makes it possible for a process to enable EBBs for itself, but not 37 actually configure any events. At a later time another process can come along 38 and attach an EBB event to the process, which will then cause EBBs to be 39 delivered to the first process. It's not clear if this is actually useful. 40 41 42 When the PMU is configured for EBBs, all PMU interrupts are delivered to the 43 user process. This means once an EBB event is scheduled on the PMU, no non-EBB 44 events can be configured. This means that EBB events can not be run 45 concurrently with regular 'perf' commands, or any other perf events. 46 47 It is however safe to run 'perf' commands on a process which is using EBBs. The 48 kernel will in general schedule the EBB event, and perf will be notified that 49 its events could not run. 50 51 The exclusion between EBB events and regular events is implemented using the 52 existing "pinned" and "exclusive" attributes of perf_events. This means EBB 53 events will be given priority over other events, unless they are also pinned. 54 If an EBB event and a regular event are both pinned, then whichever is enabled 55 first will be scheduled and the other will be put in error state. See the 56 section below titled "Enabling an EBB event" for more information. 57 58 59 Creating an EBB event 60 --------------------- 61 62 To request that an event is counted using EBB, the event code should have bit 63 63 set. 64 65 EBB events must be created with a particular, and restrictive, set of 66 attributes - this is so that they interoperate correctly with the rest of the 67 perf_events subsystem. 68 69 An EBB event must be created with the "pinned" and "exclusive" attributes set. 70 Note that if you are creating a group of EBB events, only the leader can have 71 these attributes set. 72 73 An EBB event must NOT set any of the "inherit", "sample_period", "freq" or 74 "enable_on_exec" attributes. 75 76 An EBB event must be attached to a task. This is specified to perf_event_open() 77 by passing a pid value, typically 0 indicating the current task. 78 79 All events in a group must agree on whether they want EBB. That is all events 80 must request EBB, or none may request EBB. 81 82 EBB events must specify the PMC they are to be counted on. This ensures 83 userspace is able to reliably determine which PMC the event is scheduled on. 84 85 86 Enabling an EBB event 87 --------------------- 88 89 Once an EBB event has been successfully opened, it must be enabled with the 90 perf_events API. This can be achieved either via the ioctl() interface, or the 91 prctl() interface. 92 93 However, due to the design of the perf_events API, enabling an event does not 94 guarantee that it has been scheduled on the PMU. To ensure that the EBB event 95 has been scheduled on the PMU, you must perform a read() on the event. If the 96 read() returns EOF, then the event has not been scheduled and EBBs are not 97 enabled. 98 99 This behaviour occurs because the EBB event is pinned and exclusive. When the 100 EBB event is enabled it will force all other non-pinned events off the PMU. In 101 this case the enable will be successful. However if there is already an event 102 pinned on the PMU then the enable will not be successful. 103 104 105 Reading an EBB event 106 -------------------- 107 108 It is possible to read() from an EBB event. However the results are 109 meaningless. Because interrupts are being delivered to the user process the 110 kernel is not able to count the event, and so will return a junk value. 111 112 113 Closing an EBB event 114 -------------------- 115 116 When an EBB event is finished with, you can close it using close() as for any 117 regular event. If this is the last EBB event the PMU will be deconfigured and 118 no further PMU EBBs will be delivered. 119 120 121 EBB Handler 122 ----------- 123 124 The EBB handler is just regular userspace code, however it must be written in 125 the style of an interrupt handler. When the handler is entered all registers 126 are live (possibly) and so must be saved somehow before the handler can invoke 127 other code. 128 129 It's up to the program how to handle this. For C programs a relatively simple 130 option is to create an interrupt frame on the stack and save registers there. 131 132 Fork 133 ---- 134 135 EBB events are not inherited across fork. If the child process wishes to use 136 EBBs it should open a new event for itself. Similarly the EBB state in 137 BESCR/EBBHR/EBBRR is cleared across fork().