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Index: share/man/man4/hwt.4
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+.\" Copyright (c) 2025 Ruslan Bukin <br@bsdpad.com>
+.\"
+.\" SPDX-License-Identifier: BSD-2-Clause
+.\"
+.Dd July 7, 2025
+.Dt HWT 4
+.Os
+.Sh NAME
+.Nm hwt
+.Nd Hardware Trace Framework
+.Sh SYNOPSIS
+The following option must be present in the kernel configuration file:
+.Bd -ragged -offset indent
+.Cd "options HWT_HOOKS"
+.Ed
+.Pp
+To load the driver as a module at boot time:
+.Bd -literal -offset indent
+sysrc kld_list+=hwt
+.Ed
+.Pp
+Alternatively, to compile the driver into the kernel:
+.Bd -ragged -offset indent
+.Cd "device hwt"
+.Ed
+.Pp
+Additionally, at least one of these drivers has to be loaded into kernel via
+module or compiled in via kernel configuration file:
+.Pp
+On arm64 systems:
+.Bd -ragged -offset indent
+.Cd "coresight"
+.Cd "spe"
+.Ed
+.Pp
+On amd64 systems:
+.Bd -ragged -offset indent
+.Cd "pt"
+.Ed
+.Sh DESCRIPTION
+The
+.Nm
+framework provides a set of kernel interfaces to manage hardware tracing units
+of a CPU (if provided by an architecture).
+The HW trace units collect information about software execution and store 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 restore the entire program flow of a given
+application without noticeable performance impact.
+.Pp
+The framework supports several tracing technologies found on
+.Cd arm64
+and
+.Cd amd64
+systems:
+.Pp
+.Bl -bullet -compact
+.It
+ARM Coresight
+.It
+ARM Statistical Profiling Extension (SPE)
+.It
+Intel Processor Trace (PT)
+.El
+.Pp
+The
+.Nm
+framework supports two modes of operation:
+.Bl -tag -width "Thread mode"
+.It Em CPU mode
+Capture CPU activity in kernel mode.
+.It Em Thread mode
+Capture activity of each of a process's threads in user mode.
+.El
+.Sh MANAGEMENT
+When loaded into kernel, the
+.Nm
+framework provides
+.Pa /dev/hwt
+character device.
+The only
+.Xr ioctl 2
+request it accepts is
+.Dv HWT_IOC_ALLOC .
+This request allocates kernel tracing context (CTX) based on requested mode of
+operation, set of CPUs and/or pid.
+.Pp
+Upon successfull CTX allocation request, the ioctl returns a CTX identification
+number (ident).
+.Pp
+Each CTX is then managed using its own dedicated character device found at
+.Pa "/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).
+.Sh HOOKS
+During operation (when tracing for a particular process is started), HWT
+collects information about threads creation, as well as calls to exec or
+mmap by the target process.
+This information is stored as "records" to a particular CTX.
+.Pp
+Additionally, HWT could put a thread to sleep if requested by user on exec/mmap
+event, so that user has time to copy the records to userspace and reconfigure
+tracing as needed.
+This is particulary useful if address range filtering feature is configured,
+allowing user to trace particular function of a target executable or dynamic
+library.
+.Sh IOCTLS
+Once a CTX is allocated, it's management character device accept several IOC
+requests:
+.Bl -tag -width "HWT_IOC_RECORD_GET"
+.It Dv HWT_IOC_START
+Start tracing.
+In HWT CPU mode the tracing does actually start with this
+.Xr 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.
+.It Dv HWT_IOC_STOP
+Stop tracing of the particular CTX.
+.It Dv HWT_IOC_RECORD_GET
+Copy all or part of records collected during hook invocation and associated
+with this CTX to userspace.
+.It Dv HWT_IOC_BUFPTR_GET
+Get current pointer in buffer that is filled by tracing units in real-time.
+.It Dv HWT_IOC_SET_CONFIG
+Set achitecture-specific config (optional).
+.It Dv HWT_IOC_WAKEUP
+Wake up a thread that has been put to sleep by HWT framework hooks.
+.It Dv HWT_IOC_SVC_BUF
+For SPE-only, the kernel is waiting for userspace to notify that it's copied
+out a buffer to avoid data loss/overwriting buffers.
+.El
+.Sh SEE ALSO
+.Xr hwt 8
+.Sh HISTORY
+The
+.Nm
+framework first appeared in
+.Fx 15.0 .
+.Sh AUTHORS
+.An Ruslan Bukin Aq Mt br@FreeBSD.org
+.An Bojan Novković Aq Mt bnovkov@freebsd.org
+.An Zachary Leaf Aq Mt zachary.leaf@arm.com

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