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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 up to 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 64.
kern.hwpmc.nsamples (integer, read-only)
The number of entries in the per-CPU ring buffer used during sampling. The default
is 512.
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.
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.
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.