1 ======================== 2 PMU Event Based Branches 3 ======================== 4 5 Event Based Branches (EBBs) are a feature which allows the hardware to 6 branch directly to a specified user space address when certain events occur. 7 8 The full specification is available in Power ISA v2.07: 9 10 https://www.power.org/documentation/power-isa-version-2-07/ 11 12 One type of event for which EBBs can be configured is PMU exceptions. This 13 document describes the API for configuring the Power PMU to generate EBBs, 14 using the Linux perf_events API. 15 16 17 Terminology 18 ----------- 19 20 Throughout this document we will refer to an "EBB event" or "EBB events". This 21 just refers to a struct perf_event which has set the "EBB" flag in its 22 attr.config. All events which can be configured on the hardware PMU are 23 possible "EBB events". 24 25 26 Background 27 ---------- 28 29 When a PMU EBB occurs it is delivered to the currently running process. As such 30 EBBs can only sensibly be used by programs for self-monitoring. 31 32 It is a feature of the perf_events API that events can be created on other 33 processes, subject to standard permission checks. This is also true of EBB 34 events, however unless the target process enables EBBs (via mtspr(BESCR)) no 35 EBBs will ever be delivered. 36 37 This makes it possible for a process to enable EBBs for itself, but not 38 actually configure any events. At a later time another process can come along 39 and attach an EBB event to the process, which will then cause EBBs to be 40 delivered to the first process. It's not clear if this is actually useful. 41 42 43 When the PMU is configured for EBBs, all PMU interrupts are delivered to the 44 user process. This means once an EBB event is scheduled on the PMU, no non-EBB 45 events can be configured. This means that EBB events can not be run 46 concurrently with regular 'perf' commands, or any other perf events. 47 48 It is however safe to run 'perf' commands on a process which is using EBBs. The 49 kernel will in general schedule the EBB event, and perf will be notified that 50 its events could not run. 51 52 The exclusion between EBB events and regular events is implemented using the 53 existing "pinned" and "exclusive" attributes of perf_events. This means EBB 54 events will be given priority over other events, unless they are also pinned. 55 If an EBB event and a regular event are both pinned, then whichever is enabled 56 first will be scheduled and the other will be put in error state. See the 57 section below titled "Enabling an EBB event" for more information. 58 59 60 Creating an EBB event 61 --------------------- 62 63 To request that an event is counted using EBB, the event code should have bit 64 63 set. 65 66 EBB events must be created with a particular, and restrictive, set of 67 attributes - this is so that they interoperate correctly with the rest of the 68 perf_events subsystem. 69 70 An EBB event must be created with the "pinned" and "exclusive" attributes set. 71 Note that if you are creating a group of EBB events, only the leader can have 72 these attributes set. 73 74 An EBB event must NOT set any of the "inherit", "sample_period", "freq" or 75 "enable_on_exec" attributes. 76 77 An EBB event must be attached to a task. This is specified to perf_event_open() 78 by passing a pid value, typically 0 indicating the current task. 79 80 All events in a group must agree on whether they want EBB. That is all events 81 must request EBB, or none may request EBB. 82 83 EBB events must specify the PMC they are to be counted on. This ensures 84 userspace is able to reliably determine which PMC the event is scheduled on. 85 86 87 Enabling an EBB event 88 --------------------- 89 90 Once an EBB event has been successfully opened, it must be enabled with the 91 perf_events API. This can be achieved either via the ioctl() interface, or the 92 prctl() interface. 93 94 However, due to the design of the perf_events API, enabling an event does not 95 guarantee that it has been scheduled on the PMU. To ensure that the EBB event 96 has been scheduled on the PMU, you must perform a read() on the event. If the 97 read() returns EOF, then the event has not been scheduled and EBBs are not 98 enabled. 99 100 This behaviour occurs because the EBB event is pinned and exclusive. When the 101 EBB event is enabled it will force all other non-pinned events off the PMU. In 102 this case the enable will be successful. However if there is already an event 103 pinned on the PMU then the enable will not be successful. 104 105 106 Reading an EBB event 107 -------------------- 108 109 It is possible to read() from an EBB event. However the results are 110 meaningless. Because interrupts are being delivered to the user process the 111 kernel is not able to count the event, and so will return a junk value. 112 113 114 Closing an EBB event 115 -------------------- 116 117 When an EBB event is finished with, you can close it using close() as for any 118 regular event. If this is the last EBB event the PMU will be deconfigured and 119 no further PMU EBBs will be delivered. 120 121 122 EBB Handler 123 ----------- 124 125 The EBB handler is just regular userspace code, however it must be written in 126 the style of an interrupt handler. When the handler is entered all registers 127 are live (possibly) and so must be saved somehow before the handler can invoke 128 other code. 129 130 It's up to the program how to handle this. For C programs a relatively simple 131 option is to create an interrupt frame on the stack and save registers there. 132 133 Fork 134 ---- 135 136 EBB events are not inherited across fork. If the child process wishes to use 137 EBBs it should open a new event for itself. Similarly the EBB state in 138 BESCR/EBBHR/EBBRR is cleared across fork().
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