| PMU Event Based Branches | |
| ======================== | |
| Event Based Branches (EBBs) are a feature which allows the hardware to | |
| branch directly to a specified user space address when certain events occur. | |
| The full specification is available in Power ISA v2.07: | |
| https://www.power.org/documentation/power-isa-version-2-07/ | |
| One type of event for which EBBs can be configured is PMU exceptions. This | |
| document describes the API for configuring the Power PMU to generate EBBs, | |
| using the Linux perf_events API. | |
| Terminology | |
| ----------- | |
| Throughout this document we will refer to an "EBB event" or "EBB events". This | |
| just refers to a struct perf_event which has set the "EBB" flag in its | |
| attr.config. All events which can be configured on the hardware PMU are | |
| possible "EBB events". | |
| Background | |
| ---------- | |
| When a PMU EBB occurs it is delivered to the currently running process. As such | |
| EBBs can only sensibly be used by programs for self-monitoring. | |
| It is a feature of the perf_events API that events can be created on other | |
| processes, subject to standard permission checks. This is also true of EBB | |
| events, however unless the target process enables EBBs (via mtspr(BESCR)) no | |
| EBBs will ever be delivered. | |
| This makes it possible for a process to enable EBBs for itself, but not | |
| actually configure any events. At a later time another process can come along | |
| and attach an EBB event to the process, which will then cause EBBs to be | |
| delivered to the first process. It's not clear if this is actually useful. | |
| When the PMU is configured for EBBs, all PMU interrupts are delivered to the | |
| user process. This means once an EBB event is scheduled on the PMU, no non-EBB | |
| events can be configured. This means that EBB events can not be run | |
| concurrently with regular 'perf' commands, or any other perf events. | |
| It is however safe to run 'perf' commands on a process which is using EBBs. The | |
| kernel will in general schedule the EBB event, and perf will be notified that | |
| its events could not run. | |
| The exclusion between EBB events and regular events is implemented using the | |
| existing "pinned" and "exclusive" attributes of perf_events. This means EBB | |
| events will be given priority over other events, unless they are also pinned. | |
| If an EBB event and a regular event are both pinned, then whichever is enabled | |
| first will be scheduled and the other will be put in error state. See the | |
| section below titled "Enabling an EBB event" for more information. | |
| Creating an EBB event | |
| --------------------- | |
| To request that an event is counted using EBB, the event code should have bit | |
| 63 set. | |
| EBB events must be created with a particular, and restrictive, set of | |
| attributes - this is so that they interoperate correctly with the rest of the | |
| perf_events subsystem. | |
| An EBB event must be created with the "pinned" and "exclusive" attributes set. | |
| Note that if you are creating a group of EBB events, only the leader can have | |
| these attributes set. | |
| An EBB event must NOT set any of the "inherit", "sample_period", "freq" or | |
| "enable_on_exec" attributes. | |
| An EBB event must be attached to a task. This is specified to perf_event_open() | |
| by passing a pid value, typically 0 indicating the current task. | |
| All events in a group must agree on whether they want EBB. That is all events | |
| must request EBB, or none may request EBB. | |
| EBB events must specify the PMC they are to be counted on. This ensures | |
| userspace is able to reliably determine which PMC the event is scheduled on. | |
| Enabling an EBB event | |
| --------------------- | |
| Once an EBB event has been successfully opened, it must be enabled with the | |
| perf_events API. This can be achieved either via the ioctl() interface, or the | |
| prctl() interface. | |
| However, due to the design of the perf_events API, enabling an event does not | |
| guarantee that it has been scheduled on the PMU. To ensure that the EBB event | |
| has been scheduled on the PMU, you must perform a read() on the event. If the | |
| read() returns EOF, then the event has not been scheduled and EBBs are not | |
| enabled. | |
| This behaviour occurs because the EBB event is pinned and exclusive. When the | |
| EBB event is enabled it will force all other non-pinned events off the PMU. In | |
| this case the enable will be successful. However if there is already an event | |
| pinned on the PMU then the enable will not be successful. | |
| Reading an EBB event | |
| -------------------- | |
| It is possible to read() from an EBB event. However the results are | |
| meaningless. Because interrupts are being delivered to the user process the | |
| kernel is not able to count the event, and so will return a junk value. | |
| Closing an EBB event | |
| -------------------- | |
| When an EBB event is finished with, you can close it using close() as for any | |
| regular event. If this is the last EBB event the PMU will be deconfigured and | |
| no further PMU EBBs will be delivered. | |
| EBB Handler | |
| ----------- | |
| The EBB handler is just regular userspace code, however it must be written in | |
| the style of an interrupt handler. When the handler is entered all registers | |
| are live (possibly) and so must be saved somehow before the handler can invoke | |
| other code. | |
| It's up to the program how to handle this. For C programs a relatively simple | |
| option is to create an interrupt frame on the stack and save registers there. | |
| Fork | |
| ---- | |
| EBB events are not inherited across fork. If the child process wishes to use | |
| EBBs it should open a new event for itself. Similarly the EBB state in | |
| BESCR/EBBHR/EBBRR is cleared across fork(). |