diff options
author | Costa Shulyupin <costa.shul@redhat.com> | 2023-08-26 19:56:08 +0300 |
---|---|---|
committer | Jonathan Corbet <corbet@lwn.net> | 2023-10-10 22:35:55 +0300 |
commit | 17e02586ed18501c11115b8dea9055a5973f45a4 (patch) | |
tree | aabd2f7273060d14ea90bdcb0fa1bf28972f90ef /Documentation/powerpc/pmu-ebb.rst | |
parent | 2087f270bebb78adc5059fd040e2691cd7f9bb5c (diff) | |
download | linux-17e02586ed18501c11115b8dea9055a5973f45a4.tar.xz |
docs: move powerpc under arch
and fix all in-tree references.
Architecture-specific documentation is being moved into Documentation/arch/
as a way of cleaning up the top-level documentation directory and making
the docs hierarchy more closely match the source hierarchy.
Signed-off-by: Costa Shulyupin <costa.shul@redhat.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230826165737.2101199-1-costa.shul@redhat.com
Diffstat (limited to 'Documentation/powerpc/pmu-ebb.rst')
-rw-r--r-- | Documentation/powerpc/pmu-ebb.rst | 138 |
1 files changed, 0 insertions, 138 deletions
diff --git a/Documentation/powerpc/pmu-ebb.rst b/Documentation/powerpc/pmu-ebb.rst deleted file mode 100644 index 4f474758eb55..000000000000 --- a/Documentation/powerpc/pmu-ebb.rst +++ /dev/null @@ -1,138 +0,0 @@ -======================== -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(). |