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NAME

     hwpmc — Hardware Performance Monitoring Counter support

SYNOPSIS

     options HWPMC_HOOKS
     device hwpmc

     Additionally, for i386 systems:
     device apic

DESCRIPTION

     The hwpmc driver virtualizes the hardware performance monitoring facilities in modern CPUs
     and provides support for using these facilities from user level processes.

     The driver supports multi-processor systems.

     PMCs are allocated using the PMC_OP_PMCALLOCATE request.  A successful PMC_OP_PMCALLOCATE
     request will return a handle to the requesting process.  Subsequent operations on the
     allocated PMC use this handle to denote the specific PMC.  A process that has successfully
     allocated a PMC is termed an “owner process”.

     PMCs may be allocated with process or system scope.

     Process-scope  The PMC is active only when a thread belonging to a process it is attached to
                    is scheduled on a CPU.

     System-scope   The PMC operates independently of processes and measures hardware events for
                    the system as a whole.

     PMCs may be allocated for counting or for sampling:

     Counting  In counting modes, the PMCs count hardware events.  These counts are retrievable
               using the PMC_OP_PMCREAD system call on all architectures.  Some architectures
               offer faster methods of reading these counts.

     Sampling  In sampling modes, the PMCs are configured to sample the CPU instruction pointer
               (and optionally to capture the call chain leading upto the sampled instruction
               pointer) after a configurable number of hardware events have been observed.
               Instruction pointer samples and call chain records are usually directed to a log
               file for subsequent analysis.

     Scope and operational mode are orthogonal; a PMC may thus be configured to operate in one of
     the following four modes:

     Process-scope, counting
             These PMCs count hardware events whenever a thread in their attached process is
             scheduled on a CPU.  These PMCs normally count from zero, but the initial count may
             be set using the PMC_OP_SETCOUNT operation.  Applications can read the value of the
             PMC anytime using the PMC_OP_PMCRW operation.

     Process-scope, sampling
             These PMCs sample the target processes instruction pointer after they have seen the
             configured number of hardware events.  The PMCs only count events when a thread
             belonging to their attached process is active.  The desired frequency of sampling is
             set using the PMC_OP_SETCOUNT operation prior to starting the PMC.  Log files are
             configured using the PMC_OP_CONFIGURELOG operation.

     System-scope, counting
             These PMCs count hardware events seen by them independent of the processes that are
             executing.  The current count on these PMCs can be read using the PMC_OP_PMCRW
             request.  These PMCs normally count from zero, but the initial count may be set
             using the PMC_OP_SETCOUNT operation.

     System-scope, sampling
             These PMCs will periodically sample the instruction pointer of the CPU they are
             allocated on, and will write the sample to a log for further processing.  The
             desired frequency of sampling is set using the PMC_OP_SETCOUNT operation prior to
             starting the PMC.  Log files are configured using the PMC_OP_CONFIGURELOG operation.

             System-wide statistical sampling can only be enabled by a process with super-user
             privileges.

     Processes are allowed to allocate as many PMCs as the hardware and current operating
     conditions permit.  Processes may mix allocations of system-wide and process-private PMCs.
     Multiple processes may be using PMCs simultaneously.

     Allocated PMCs are started using the PMC_OP_PMCSTART operation, and stopped using the
     PMC_OP_PMCSTOP operation.  Stopping and starting a PMC is permitted at any time the owner
     process has a valid handle to the PMC.

     Process-private PMCs need to be attached to a target process before they can be used.
     Attaching a process to a PMC is done using the PMC_OP_PMCATTACH operation.  An already
     attached PMC may be detached from its target process using the converse PMC_OP_PMCDETACH
     operation.  Issuing a PMC_OP_PMCSTART operation on an as yet unattached PMC will cause it to
     be attached to its owner process.  The following rules determine whether a given process may
     attach a PMC to another target process:
     ·   A non-jailed process with super-user privileges is allowed to attach to any other
         process in the system.
     ·   Other processes are only allowed to attach to targets that they would be able to attach
         to for debugging (as determined by p_candebug(9)).

     PMCs are released using PMC_OP_PMCRELEASE.  After a successful PMC_OP_PMCRELEASE operation
     the handle to the PMC will become invalid.

   Modifier Flags
     The PMC_OP_PMCALLOCATE operation supports the following flags that modify the behavior of an
     allocated PMC:

     PMC_F_CALLCHAIN
             This modifier informs sampling PMCs to record a callchain when capturing a sample.
             The maximum depth to which call chains are recorded is specified by the
             kern.hwpmc.callchaindepth kernel tunable.

     PMC_F_DESCENDANTS
             This modifier is valid only for a PMC being allocated in process-private mode.  It
             signifies that the PMC will track hardware events for its target process and the
             target's current and future descendants.

     PMC_F_KGMON
             This modifier is valid only for a PMC being allocated in system-wide sampling mode.
             It signifies that the PMC's sampling interrupt is to be used to drive kernel
             profiling via kgmon(8).  This functionality is currently unimplemented.

     PMC_F_LOG_PROCCSW
             This modifier is valid only for a PMC being allocated in process-private mode.  When
             this modifier is present, at every context switch, hwpmc will log a record
             containing the number of hardware events seen by the target process when it was
             scheduled on the CPU.

     PMC_F_LOG_PROCEXIT
             This modifier is valid only for a PMC being allocated in process-private mode.  With
             this modifier present, hwpmc will maintain per-process counts for each target
             process attached to a PMC.  At process exit time, a record containing the target
             process' PID and the accumulated per-process count for that process will be written
             to the configured log file.

     Modifiers PMC_F_LOG_PROCEXIT and PMC_F_LOG_PROCCSW may be used in combination with modifier
     PMC_F_DESCENDANTS to track the behavior of complex pipelines of processes.  PMCs with
     modifiers PMC_F_LOG_PROCEXIT and PMC_F_LOG_PROCCSW cannot be started until their owner
     process has configured a log file.

   Signals
     The hwpmc driver may deliver signals to processes that have allocated PMCs:

     SIGIO   A PMC_OP_PMCRW operation was attempted on a process-private PMC that does not have
             attached target processes.

     SIGBUS  The hwpmc driver is being unloaded from the kernel.

   PMC ROW DISPOSITIONS
     A PMC row is defined as the set of PMC resources at the same hardware address in the CPUs in
     a system.  Since process scope PMCs need to move between CPUs following their target
     threads, allocation of a process scope PMC reserves all PMCs in a PMC row for use only with
     process scope PMCs.  Accordingly a PMC row will be in one of the following dispositions:
     PMC_DISP_FREE        Hardware counters in this row are free and may be use to satisfy either
                          of system scope or process scope allocation requests.
     PMC_DISP_THREAD      Hardware counters in this row are in use by process scope PMCs and are
                          only available for process scope allocation requests.
     PMC_DISP_STANDALONE  Some hardware counters in this row have been administratively disabled
                          or are in use by system scope PMCs.  Non-disabled hardware counters in
                          such a row may be used for satisfying system scope allocation requests.
                          No process scope PMCs will use hardware counters in this row.

PROGRAMMING API

     The recommended way for application programs to use the facilities of the hwpmc driver is
     using the API provided by the pmc(3) library.

     The hwpmc driver operates using a system call number that is dynamically allotted to it when
     it is loaded into the kernel.

     The hwpmc driver supports the following operations:

     PMC_OP_CONFIGURELOG
             Configure a log file for PMCs that require a log file.  The hwpmc driver will write
             log data to this file asynchronously.  If it encounters an error, logging will be
             stopped and the error code encountered will be saved for subsequent retrieval by a
             PMC_OP_FLUSHLOG request.

     PMC_OP_FLUSHLOG
             Transfer buffered log data inside hwpmc to a configured output file.  This operation
             returns to the caller after the write operation has returned.  The returned error
             code reflects any pending error state inside hwpmc.

     PMC_OP_GETCPUINFO
             Retrieve information about the highest possible CPU number for the system, and the
             number of hardware performance monitoring counters available per CPU.

     PMC_OP_GETDRIVERSTATS
             Retrieve module statistics (for analyzing the behavior of hwpmc itself).

     PMC_OP_GETMODULEVERSION
             Retrieve the version number of API.

     PMC_OP_GETPMCINFO
             Retrieve information about the current state of the PMCs on a given CPU.

     PMC_OP_PMCADMIN
             Set the administrative state (i.e., whether enabled or disabled) for the hardware
             PMCs managed by the hwpmc driver.  The invoking process needs to possess the
             PRIV_PMC_MANAGE privilege.

     PMC_OP_PMCALLOCATE
             Allocate and configure a PMC.  On successful allocation, a handle to the PMC (a 32
             bit value) is returned.

     PMC_OP_PMCATTACH
             Attach a process mode PMC to a target process.  The PMC will be active whenever a
             thread in the target process is scheduled on a CPU.

             If the PMC_F_DESCENDANTS flag had been specified at PMC allocation time, then the
             PMC is attached to all current and future descendants of the target process.

     PMC_OP_PMCDETACH
             Detach a PMC from its target process.

     PMC_OP_PMCRELEASE
             Release a PMC.

     PMC_OP_PMCRW
             Read and write a PMC.  This operation is valid only for PMCs configured in counting
             modes.

     PMC_OP_SETCOUNT
             Set the initial count (for counting mode PMCs) or the desired sampling rate (for
             sampling mode PMCs).

     PMC_OP_PMCSTART
             Start a PMC.

     PMC_OP_PMCSTOP
             Stop a PMC.

     PMC_OP_WRITELOG
             Insert a timestamped user record into the log file.

   i386 Specific API
     Some i386 family CPUs support the RDPMC instruction which allows a user process to read a
     PMC value without needing to invoke a PMC_OP_PMCRW operation.  On such CPUs, the machine
     address associated with an allocated PMC is retrievable using the PMC_OP_PMCX86GETMSR system
     call.

     PMC_OP_PMCX86GETMSR
             Retrieve the MSR (machine specific register) number associated with the given PMC
             handle.

             The PMC needs to be in process-private mode and allocated without the
             PMC_F_DESCENDANTS modifier flag, and should be attached only to its owner process at
             the time of the call.

   amd64 Specific API
     AMD64 CPUs support the RDPMC instruction which allows a user process to read a PMC value
     without needing to invoke a PMC_OP_PMCRW operation.  The machine address associated with an
     allocated PMC is retrievable using the PMC_OP_PMCX86GETMSR system call.

     PMC_OP_PMCX86GETMSR
             Retrieve the MSR (machine specific register) number associated with the given PMC
             handle.

             The PMC needs to be in process-private mode and allocated without the
             PMC_F_DESCENDANTS modifier flag, and should be attached only to its owner process at
             the time of the call.

SYSCTL VARIABLES AND LOADER TUNABLES

     The behavior of hwpmc is influenced by the following sysctl(8) and loader(8) tunables:

     kern.hwpmc.callchaindepth (integer, read-only)
             The maximum number of call chain records to capture per sample.  The default is 8.

     kern.hwpmc.debugflags (string, read-write)
             (Only available if the hwpmc driver was compiled with -DDEBUG.)  Control the
             verbosity of debug messages from the hwpmc driver.

     kern.hwpmc.hashsize (integer, read-only)
             The number of rows in the hash tables used to keep track of owner and target
             processes.  The default is 16.

     kern.hwpmc.logbuffersize (integer, read-only)
             The size in kilobytes of each log buffer used by hwpmc's logging function.  The
             default buffer size is 4KB.

     kern.hwpmc.mtxpoolsize (integer, read-only)
             The size of the spin mutex pool used by the PMC driver.  The default is 32.

     kern.hwpmc.nbuffers (integer, read-only)
             The number of log buffers used by hwpmc for logging.  The default is 16.

     kern.hwpmc.nsamples (integer, read-only)
             The number of entries in the per-CPU ring buffer used during sampling.  The default
             is 16.

     security.bsd.unprivileged_syspmcs (boolean, read-write)
             If set to non-zero, allow unprivileged processes to allocate system-wide PMCs.  The
             default value is 0.

     security.bsd.unprivileged_proc_debug (boolean, read-write)
             If set to 0, the hwpmc driver will only allow privileged processes to attach PMCs to
             other processes.

     These variables may be set in the kernel environment using kenv(1) before hwpmc is loaded.

SECURITY CONSIDERATIONS

     PMCs may be used to monitor the actual behavior of the system on hardware.  In situations
     where this constitutes an undesirable information leak, the following options are available:

     1.   Set the sysctl(8) tunable security.bsd.unprivileged_syspmcs to 0.  This ensures that
          unprivileged processes cannot allocate system-wide PMCs and thus cannot observe the
          hardware behavior of the system as a whole.  This tunable may also be set at boot time
          using loader(8), or with kenv(1) prior to loading the hwpmc driver into the kernel.

     2.   Set the sysctl(8) tunable security.bsd.unprivileged_proc_debug to 0.  This will ensure
          that an unprivileged process cannot attach a PMC to any process other than itself and
          thus cannot observe the hardware behavior of other processes with the same credentials.

     System administrators should note that on IA-32 platforms FreeBSD makes the content of the
     IA-32 TSC counter available to all processes via the RDTSC instruction.

IMPLEMENTATION NOTES

   SMP Symmetry
     The kernel driver requires all physical CPUs in an SMP system to have identical performance
     monitoring counter hardware.

   Sparse CPU Numbering
     On platforms that sparsely number CPUs and which support hot-plugging of CPUs, requests that
     specify non-existent or disabled CPUs will fail with an error.  Applications allocating
     system-scope PMCs need to be aware of the possibility of such transient failures.

   x86 TSC Handling
     Historically, on the x86 architecture, FreeBSD has permitted user processes running at a
     processor CPL of 3 to read the TSC using the RDTSC instruction.  The hwpmc driver preserves
     this behavior.

   Intel P4/HTT Handling
     On CPUs with HTT support, Intel P4 PMCs are capable of qualifying only a subset of hardware
     events on a per-logical CPU basis.  Consequently, if HTT is enabled on a system with Intel
     Pentium P4 PMCs, then the hwpmc driver will reject allocation requests for process-private
     PMCs that request counting of hardware events that cannot be counted separately for each
     logical CPU.

   Intel Pentium-Pro Handling
     Writing a value to the PMC MSRs found in Intel Pentium-Pro style PMCs (found in Intel
     Pentium Pro, Pentium II, Pentium III, Pentium M and Celeron processors) will replicate bit
     31 of the value being written into the upper 8 bits of the MSR, bringing down the usable
     width of these PMCs to 31 bits.  For process-virtual PMCs, the hwpmc driver implements a
     workaround in software and makes the corrected 64 bit count available via the PMC_OP_RW
     operation.  Processes that intend to use RDPMC instructions directly or that intend to write
     values larger than 2^31 into these PMCs with PMC_OP_RW need to be aware of this hardware
     limitation.

DIAGNOSTICS

     hwpmc: [class/npmc/capabilities]...  Announce the presence of npmc PMCs of class class, with
     capabilities described by bit string capabilities.

     hwpmc: kernel version (0x%x) does not match module version (0x%x).  The module loading
     process failed because a version mismatch was detected between the currently executing
     kernel and the module being loaded.

     hwpmc: this kernel has not been compiled with 'options HWPMC_HOOKS'.  The module loading
     process failed because the currently executing kernel was not configured with the required
     configuration option HWPMC_HOOKS.

     hwpmc: tunable hashsize=%d must be greater than zero.  A negative value was supplied for
     tunable kern.hwpmc.hashsize.

     hwpmc: tunable logbuffersize=%d must be greater than zero.  A negative value was supplied
     for tunable kern.hwpmc.logbuffersize.

     hwpmc: tunable nlogbuffers=%d must be greater than zero.  A negative value was supplied for
     tunable kern.hwpmc.nlogbuffers.

     hwpmc: tunable nsamples=%d out of range.  The value for tunable kern.hwpmc.nsamples was
     negative or greater than 65535.

COMPATIBILITY

     The hwpmc driver is currently under development.  The API and ABI documented in this manual
     page may change in the future.  The recommended method of accessing this driver is using the
     pmc(3) API.

ERRORS

     A command issued to the hwpmc driver may fail with the following errors:

     [EAGAIN]           Helper process creation failed for a PMC_OP_CONFIGURELOG request due to a
                        temporary resource shortage in the kernel.

     [EBUSY]            A PMC_OP_CONFIGURELOG operation was requested while an existing log was
                        active.

     [EBUSY]            A DISABLE operation was requested using the PMC_OP_PMCADMIN request for a
                        set of hardware resources currently in use for process-private PMCs.

     [EBUSY]            A PMC_OP_PMCADMIN operation was requested on an active system mode PMC.

     [EBUSY]            A PMC_OP_PMCATTACH operation was requested for a target process that
                        already had another PMC using the same hardware resources attached to it.

     [EBUSY]            A PMC_OP_PMCRW request writing a new value was issued on a PMC that was
                        active.

     [EBUSY]            A PMC_OP_PMCSETCOUNT request was issued on a PMC that was active.

     [EDOOFUS]          A PMC_OP_PMCSTART operation was requested without a log file being
                        configured for a PMC allocated with PMC_F_LOG_PROCCSW and
                        PMC_F_LOG_PROCEXIT modifiers.

     [EDOOFUS]          A PMC_OP_PMCSTART operation was requested on a system-wide sampling PMC
                        without a log file being configured.

     [EEXIST]           A PMC_OP_PMCATTACH request was reissued for a target process that already
                        is the target of this PMC.

     [EFAULT]           A bad address was passed in to the driver.

     [EINVAL]           An invalid PMC handle was specified.

     [EINVAL]           An invalid CPU number was passed in for a PMC_OP_GETPMCINFO operation.

     [EINVAL]           A PMC_OP_CONFIGURELOG request to de-configure a log file was issued
                        without a log file being configured.

     [EINVAL]           A PMC_OP_FLUSHLOG request was issued without a log file being configured.

     [EINVAL]           An invalid CPU number was passed in for a PMC_OP_PMCADMIN operation.

     [EINVAL]           An invalid operation request was passed in for a PMC_OP_PMCADMIN
                        operation.

     [EINVAL]           An invalid PMC ID was passed in for a PMC_OP_PMCADMIN operation.

     [EINVAL]           A suitable PMC matching the parameters passed in to a PMC_OP_PMCALLOCATE
                        request could not be allocated.

     [EINVAL]           An invalid PMC mode was requested during a PMC_OP_PMCALLOCATE request.

     [EINVAL]           An invalid CPU number was specified during a PMC_OP_PMCALLOCATE request.

     [EINVAL]           A CPU other than PMC_CPU_ANY was specified in a PMC_OP_PMCALLOCATE
                        request for a process-private PMC.

     [EINVAL]           A CPU number of PMC_CPU_ANY was specified in a PMC_OP_PMCALLOCATE request
                        for a system-wide PMC.

     [EINVAL]           The pm_flags argument to an PMC_OP_PMCALLOCATE request contained unknown
                        flags.

     [EINVAL]           (On Intel Pentium 4 CPUs with HTT support) A PMC_OP_PMCALLOCATE request
                        for a process-private PMC was issued for an event that does not support
                        counting on a per-logical CPU basis.

     [EINVAL]           A PMC allocated for system-wide operation was specified with a
                        PMC_OP_PMCATTACH or PMC_OP_PMCDETACH request.

     [EINVAL]           The pm_pid argument to a PMC_OP_PMCATTACH or PMC_OP_PMCDETACH request
                        specified an illegal process ID.

     [EINVAL]           A PMC_OP_PMCDETACH request was issued for a PMC not attached to the
                        target process.

     [EINVAL]           Argument pm_flags to a PMC_OP_PMCRW request contained illegal flags.

     [EINVAL]           A PMC_OP_PMCX86GETMSR operation was requested for a PMC not in process-
                        virtual mode, or for a PMC that is not solely attached to its owner
                        process, or for a PMC that was allocated with flag PMC_F_DESCENDANTS.

     [EINVAL]           A PMC_OP_WRITELOG request was issued for an owner process without a log
                        file configured.

     [ENOMEM]           The system was not able to allocate kernel memory.

     [ENOSYS]           (On i386 and amd64 architectures) A PMC_OP_PMCX86GETMSR operation was
                        requested for hardware that does not support reading PMCs directly with
                        the RDPMC instruction.

     [ENXIO]            A PMC_OP_GETPMCINFO operation was requested for an absent or disabled
                        CPU.

     [ENXIO]            A PMC_OP_PMCALLOCATE operation specified allocation of a system-wide PMC
                        on an absent or disabled CPU.

     [ENXIO]            A PMC_OP_PMCSTART or PMC_OP_PMCSTOP request was issued for a system-wide
                        PMC that was allocated on a CPU that is currently absent or disabled.

     [EOPNOTSUPP]       A PMC_OP_PMCALLOCATE request was issued for PMC capabilities not
                        supported by the specified PMC class.

     [EOPNOTSUPP]       (i386 architectures) A sampling mode PMC was requested on a CPU lacking
                        an APIC.

     [EPERM]            A PMC_OP_PMCADMIN request was issued by a process without super-user
                        privilege or by a jailed super-user process.

     [EPERM]            A PMC_OP_PMCATTACH operation was issued for a target process that the
                        current process does not have permission to attach to.

     [EPERM]            (i386 and amd64 architectures) A PMC_OP_PMCATTACH operation was issued on
                        a PMC whose MSR has been retrieved using PMC_OP_PMCX86GETMSR.

     [ESRCH]            A process issued a PMC operation request without having allocated any
                        PMCs.

     [ESRCH]            A process issued a PMC operation request after the PMC was detached from
                        all of its target processes.

     [ESRCH]            A PMC_OP_PMCATTACH or PMC_OP_PMCDETACH request specified a non-existent
                        process ID.

     [ESRCH]            The target process for a PMC_OP_PMCDETACH operation is not being
                        monitored by hwpmc.

SEE ALSO

     kenv(1), pmc(3), pmclog(3), kgmon(8), kldload(8), pmccontrol(8), pmcstat(8), sysctl(8),
     kproc_create(9), p_candebug(9)

HISTORY

     The hwpmc driver first appeared in FreeBSD 6.0.

AUTHORS

     The hwpmc driver was written by Joseph Koshy ⟨jkoshy@FreeBSD.org⟩.

BUGS

     The driver samples the state of the kernel's logical processor support at the time of
     initialization (i.e., at module load time).  On CPUs supporting logical processors, the
     driver could misbehave if logical processors are subsequently enabled or disabled while the
     driver is active.

     On the i386 architecture, the driver requires that the local APIC on the CPU be enabled for
     sampling mode to be supported.  Many single-processor motherboards keep the APIC disabled in
     BIOS; on such systems hwpmc will not support sampling PMCs.