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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(). |