The existing pmu documentation describes the limitation of perf infrastructure
in RISC-V ISA and limited feature set of perf in RISC-V.
However, SBI PMU extension and sscofpmf extension(ISA extension) allows to
implement most of the required features of perf. Remove the old documentation
which is not accurate anymore.
Signed-off-by: Atish Patra <[email protected]>
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Documentation/riscv/pmu.rst | 255 ------------------------------------
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-===================================
-Supporting PMUs on RISC-V platforms
-===================================
-
-Alan Kao <[email protected]>, Mar 2018
-
-Introduction
-------------
-
-As of this writing, perf_event-related features mentioned in The RISC-V ISA
-Privileged Version 1.10 are as follows:
-(please check the manual for more details)
-
-* [m|s]counteren
-* mcycle[h], cycle[h]
-* minstret[h], instret[h]
-* mhpeventx, mhpcounterx[h]
-
-With such function set only, porting perf would require a lot of work, due to
-the lack of the following general architectural performance monitoring features:
-
-* Enabling/Disabling counters
- Counters are just free-running all the time in our case.
-* Interrupt caused by counter overflow
- No such feature in the spec.
-* Interrupt indicator
- It is not possible to have many interrupt ports for all counters, so an
- interrupt indicator is required for software to tell which counter has
- just overflowed.
-* Writing to counters
- There will be an SBI to support this since the kernel cannot modify the
- counters [1]. Alternatively, some vendor considers to implement
- hardware-extension for M-S-U model machines to write counters directly.
-
-This document aims to provide developers a quick guide on supporting their
-PMUs in the kernel. The following sections briefly explain perf' mechanism
-and todos.
-
-You may check previous discussions here [1][2]. Also, it might be helpful
-to check the appendix for related kernel structures.
-
-
-1. Initialization
------------------
-
-*riscv_pmu* is a global pointer of type *struct riscv_pmu*, which contains
-various methods according to perf's internal convention and PMU-specific
-parameters. One should declare such instance to represent the PMU. By default,
-*riscv_pmu* points to a constant structure *riscv_base_pmu*, which has very
-basic support to a baseline QEMU model.
-
-Then he/she can either assign the instance's pointer to *riscv_pmu* so that
-the minimal and already-implemented logic can be leveraged, or invent his/her
-own *riscv_init_platform_pmu* implementation.
-
-In other words, existing sources of *riscv_base_pmu* merely provide a
-reference implementation. Developers can flexibly decide how many parts they
-can leverage, and in the most extreme case, they can customize every function
-according to their needs.
-
-
-2. Event Initialization
------------------------
-
-When a user launches a perf command to monitor some events, it is first
-interpreted by the userspace perf tool into multiple *perf_event_open*
-system calls, and then each of them calls to the body of *event_init*
-member function that was assigned in the previous step. In *riscv_base_pmu*'s
-case, it is *riscv_event_init*.
-
-The main purpose of this function is to translate the event provided by user
-into bitmap, so that HW-related control registers or counters can directly be
-manipulated. The translation is based on the mappings and methods provided in
-*riscv_pmu*.
-
-Note that some features can be done in this stage as well:
-
-(1) interrupt setting, which is stated in the next section;
-(2) privilege level setting (user space only, kernel space only, both);
-(3) destructor setting. Normally it is sufficient to apply *riscv_destroy_event*;
-(4) tweaks for non-sampling events, which will be utilized by functions such as
- *perf_adjust_period*, usually something like the follows::
-
- if (!is_sampling_event(event)) {
- hwc->sample_period = x86_pmu.max_period;
- hwc->last_period = hwc->sample_period;
- local64_set(&hwc->period_left, hwc->sample_period);
- }
-
-In the case of *riscv_base_pmu*, only (3) is provided for now.
-
-
-3. Interrupt
-------------
-
-3.1. Interrupt Initialization
-
-This often occurs at the beginning of the *event_init* method. In common
-practice, this should be a code segment like::
-
- int x86_reserve_hardware(void)
- {
- int err = 0;
-
- if (!atomic_inc_not_zero(&pmc_refcount)) {
- mutex_lock(&pmc_reserve_mutex);
- if (atomic_read(&pmc_refcount) == 0) {
- if (!reserve_pmc_hardware())
- err = -EBUSY;
- else
- reserve_ds_buffers();
- }
- if (!err)
- atomic_inc(&pmc_refcount);
- mutex_unlock(&pmc_reserve_mutex);
- }
-
- return err;
- }
-
-And the magic is in *reserve_pmc_hardware*, which usually does atomic
-operations to make implemented IRQ accessible from some global function pointer.
-*release_pmc_hardware* serves the opposite purpose, and it is used in event
-destructors mentioned in previous section.
-
-(Note: From the implementations in all the architectures, the *reserve/release*
-pair are always IRQ settings, so the *pmc_hardware* seems somehow misleading.
-It does NOT deal with the binding between an event and a physical counter,
-which will be introduced in the next section.)
-
-3.2. IRQ Structure
-
-Basically, a IRQ runs the following pseudo code::
-
- for each hardware counter that triggered this overflow
-
- get the event of this counter
-
- // following two steps are defined as *read()*,
- // check the section Reading/Writing Counters for details.
- count the delta value since previous interrupt
- update the event->count (# event occurs) by adding delta, and
- event->hw.period_left by subtracting delta
-
- if the event overflows
- sample data
- set the counter appropriately for the next overflow
-
- if the event overflows again
- too frequently, throttle this event
- fi
- fi
-
- end for
-
-However as of this writing, none of the RISC-V implementations have designed an
-interrupt for perf, so the details are to be completed in the future.
-
-4. Reading/Writing Counters
----------------------------
-
-They seem symmetric but perf treats them quite differently. For reading, there
-is a *read* interface in *struct pmu*, but it serves more than just reading.
-According to the context, the *read* function not only reads the content of the
-counter (event->count), but also updates the left period to the next interrupt
-(event->hw.period_left).
-
-But the core of perf does not need direct write to counters. Writing counters
-is hidden behind the abstraction of 1) *pmu->start*, literally start counting so one
-has to set the counter to a good value for the next interrupt; 2) inside the IRQ
-it should set the counter to the same resonable value.
-
-Reading is not a problem in RISC-V but writing would need some effort, since
-counters are not allowed to be written by S-mode.
-
-
-5. add()/del()/start()/stop()
------------------------------
-
-Basic idea: add()/del() adds/deletes events to/from a PMU, and start()/stop()
-starts/stop the counter of some event in the PMU. All of them take the same
-arguments: *struct perf_event *event* and *int flag*.
-
-Consider perf as a state machine, then you will find that these functions serve
-as the state transition process between those states.
-Three states (event->hw.state) are defined:
-
-* PERF_HES_STOPPED: the counter is stopped
-* PERF_HES_UPTODATE: the event->count is up-to-date
-* PERF_HES_ARCH: arch-dependent usage ... we don't need this for now
-
-A normal flow of these state transitions are as follows:
-
-* A user launches a perf event, resulting in calling to *event_init*.
-* When being context-switched in, *add* is called by the perf core, with a flag
- PERF_EF_START, which means that the event should be started after it is added.
- At this stage, a general event is bound to a physical counter, if any.
- The state changes to PERF_HES_STOPPED and PERF_HES_UPTODATE, because it is now
- stopped, and the (software) event count does not need updating.
-
- - *start* is then called, and the counter is enabled.
- With flag PERF_EF_RELOAD, it writes an appropriate value to the counter (check
- previous section for detail).
- Nothing is written if the flag does not contain PERF_EF_RELOAD.
- The state now is reset to none, because it is neither stopped nor updated
- (the counting already started)
-
-* When being context-switched out, *del* is called. It then checks out all the
- events in the PMU and calls *stop* to update their counts.
-
- - *stop* is called by *del*
- and the perf core with flag PERF_EF_UPDATE, and it often shares the same
- subroutine as *read* with the same logic.
- The state changes to PERF_HES_STOPPED and PERF_HES_UPTODATE, again.
-
- - Life cycle of these two pairs: *add* and *del* are called repeatedly as
- tasks switch in-and-out; *start* and *stop* is also called when the perf core
- needs a quick stop-and-start, for instance, when the interrupt period is being
- adjusted.
-
-Current implementation is sufficient for now and can be easily extended to
-features in the future.
-
-A. Related Structures
----------------------
-
-* struct pmu: include/linux/perf_event.h
-* struct riscv_pmu: arch/riscv/include/asm/perf_event.h
-
- Both structures are designed to be read-only.
-
- *struct pmu* defines some function pointer interfaces, and most of them take
- *struct perf_event* as a main argument, dealing with perf events according to
- perf's internal state machine (check kernel/events/core.c for details).
-
- *struct riscv_pmu* defines PMU-specific parameters. The naming follows the
- convention of all other architectures.
-
-* struct perf_event: include/linux/perf_event.h
-* struct hw_perf_event
-
- The generic structure that represents perf events, and the hardware-related
- details.
-
-* struct riscv_hw_events: arch/riscv/include/asm/perf_event.h
-
- The structure that holds the status of events, has two fixed members:
- the number of events and the array of the events.
-
-References
-----------
-
-[1] https://github.com/riscv/riscv-linux/pull/124
-
-[2] https://groups.google.com/a/groups.riscv.org/forum/#!topic/sw-dev/f19TmCNP6yA
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