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HWPMC(4)		 BSD Kernel Interfaces Manual		      HWPMC(4)

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 facili‐
     ties 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 config‐
	       urable number of hardware events have been observed.  Instruc‐
	       tion 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 cur‐
     rent operating conditions permit.	Processes may mix allocations of sys‐
     tem-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.	Issu‐
     ing 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 inter‐
	     rupt 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 combi‐
     nation 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 config‐
     ured 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 disposi‐
     tions:
     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 admin‐
			  istratively 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 oper‐
	     ation 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 alloca‐
	     tion 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 con‐
	     figured 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 allo‐
     cated 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 sam‐
	     pling.  The default is 16.

     security.bsd.unprivileged_syspmcs (boolean, read-write)
	     If set to non-zero, allow unprivileged processes to allocate sys‐
	     tem-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 pro‐
	     cesses 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 hard‐
     ware.  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 pro‐
     cesses 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 docu‐
     mented 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 tar‐
			get 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 speci‐
			fied 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 hard‐
			ware 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 capa‐
			bilities 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 permis‐
			sion 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 hav‐
			ing 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 speci‐
			fied 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 support‐
     ing 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.

BSD			      September 22, 2008			   BSD
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