HWT(4)

NAME
hwt - Hardware Trace Framework

SYNOPSIS
options HWT_HOOKS
device hwt

At least one of:
device intel_pt (amd64)
device coresight (arm64)
device spe (arm64)

In rc.conf(5):
kld_list="hwt"

DESCRIPTION
The hwt framework provides infrastructure for hardware-assisted tracing. It collects detailed information about software execution and stores it as events in highly compressed format into DRAM. The events cover information about control flow changes of a program, whether branches taken or not, exceptions taken, timing information, cycles elapsed and more. The information collected allows to reconstruct entire program flow of a given application without noticeable performance impact.

HARDWARE
The framework supports several tracing technologies found on arm64 and amd64 systems:

•   ARM Coresight
•   ARM Statistical Profiling Extension (SPE)
•   Intel Processor Trace (PT)

The hwt framework supports two modes of operation:

CPU mode
    Capture CPU activity in kernel mode.

Thread mode
    Capture activity of each of a process's threads in user mode.

MANAGEMENT
When loaded into kernel, the hwt framework provides /dev/hwt character device. The only ioctl(2) request it accepts is HWT_IOC_ALLOC. This request allocates kernel tracing context (CTX) based on requested mode of operation, set of CPUs and/or pid.

Upon successful CTX allocation, the ioctl returns a CTX identification number (ident).

Each CTX is then managed using its own dedicated character device found at /dev/hwt_${ident}_${d}, where ident is a unique identification number of tracing context, d is either cpu_id (in HWT CPU mode) or process pid (in HWT Thread mode).

HOOKS
During tracing of a target process, HWT records runtime events such as threads creation, exec and mmap system calls. These events are logged as "records" within a particular CTX associated with traced process.

Additionally, HWT can suspend the target thread upon exec or mmap system calls if requested by the user. This pause allows user-space tools to retrieve the records and adjust tracing settings before execution continues. This feature is especially useful when address range filtering is enabled, allowing tracing of specific functions within the target executable or a dynamic library.

KERNEL OPTIONS
The following options in the kernel configuration file are mandatory and related to hwt operation:

HWT_HOOKS
    Enable kernel hooks.

IOCTL INTERFACE
Once a CTX is allocated, its management character device accepts several ioctl(2) requests:

HWT_IOC_START
    Start tracing. In HWT CPU mode the tracing does actually start with this ioctl(2) request. In the Thread mode, the tracing "running" flag set, but tracing begins after scheduler switches the target thread onto CPU and return to user mode.

HWT_IOC_STOP
    Stop tracing of the particular CTX.

HWT_IOC_RECORD_GET
    Copy all or part of records collected during hook invocation and associated with this CTX to userspace.

HWT_IOC_BUFPTR_GET
    Get current pointer in buffer that is filled by tracing units in real-time.

HWT_IOC_SET_CONFIG
    Set architecture-specific config (optional).

HWT_IOC_WAKEUP
    Wake up a thread that has been put to sleep by HWT framework hooks.

HWT_IOC_SVC_BUF
    For SPE-only, the kernel is waiting for userspace to notify that it has copied out a buffer to avoid data loss/overwriting buffers.

SEE ALSO
tracing(7), hwt(8)

HISTORY
The hwt framework first appeared in FreeBSD 15.0.

AUTHORS
Ruslan Bukin <br@FreeBSD.org> Bojan Novković <bnovkov@freebsd.org> Zachary Leaf <zachary.leaf@arm.com>