Documentation for HTM (Hardware Trace Macro) debugfs interface and how it can be used to configure/control the HTM operations.
Signed-off-by: Athira Rajeev <atraj...@linux.ibm.com> Tested-by: Venkat Rao Bagalkote <venka...@linux.ibm.com> --- Documentation/arch/powerpc/htm.rst | 104 +++++++++++++++++++++++++++++ 1 file changed, 104 insertions(+) create mode 100644 Documentation/arch/powerpc/htm.rst diff --git a/Documentation/arch/powerpc/htm.rst b/Documentation/arch/powerpc/htm.rst new file mode 100644 index 000000000000..fcb4eb6306b1 --- /dev/null +++ b/Documentation/arch/powerpc/htm.rst @@ -0,0 +1,104 @@ +.. SPDX-License-Identifier: GPL-2.0 +.. _htm: + +=================================== +HTM (Hardware Trace Macro) +=================================== + +Athira Rajeev, 2 Mar 2025 + +.. contents:: + :depth: 3 + + +Basic overview +============== + +H_HTM is used as an interface for executing Hardware Trace Macro (HTM) +functions, including setup, configuration, control and dumping of the HTM data. +For using HTM, it is required to setup HTM buffers and HTM operations can +be controlled using the H_HTM hcall. The hcall can be invoked for any core/chip +of the system from within a partition itself. To use this feature, a debugfs +folder called "htmdump" is present under /sys/kernel/debug/powerpc. + + +HTM debugfs example usage +========================= + +.. code-block:: sh + + # ls /sys/kernel/debug/powerpc/htmdump/ + coreindexonchip htmcaps htmconfigure htmflags htminfo htmsetup + htmstart htmstatus htmtype nodalchipindex nodeindex trace + +Details on each file: + +* nodeindex, nodalchipindex, coreindexonchip specifies which partition to configure the HTM for. +* htmtype: specifies the type of HTM. Supported target is hardwareTarget. +* trace: is to read the HTM data. +* htmconfigure: Configure/Deconfigure the HTM. Writing 1 to the file will configure the trace, writing 0 to the file will do deconfigure. +* htmstart: start/Stop the HTM. Writing 1 to the file will start the tracing, writing 0 to the file will stop the tracing. +* htmstatus: get the status of HTM. This is needed to understand the HTM state after each operation. +* htmsetup: set the HTM buffer size. Size of HTM buffer is in power of 2 +* htminfo: provides the system processor configuration details. This is needed to understand the appropriate values for nodeindex, nodalchipindex, coreindexonchip. +* htmcaps : provides the HTM capabilities like minimum/maximum buffer size, what kind of tracing the HTM supports etc. +* htmflags : allows to pass flags to hcall. Currently supports controlling the wrapping of HTM buffer. + +To see the system processor configuration details: + +.. code-block:: sh + + # cat /sys/kernel/debug/powerpc/htmdump/htminfo > htminfo_file + +The result can be interpreted using hexdump. + +To collect HTM traces for a partition represented by nodeindex as +zero, nodalchipindex as 1 and coreindexonchip as 12 + +.. code-block:: sh + + # cd /sys/kernel/debug/powerpc/htmdump/ + # echo 2 > htmtype + # echo 33 > htmsetup ( sets 8GB memory for HTM buffer, number is size in power of 2 ) + +This requires a CEC reboot to get the HTM buffers allocated. + +.. code-block:: sh + + # cd /sys/kernel/debug/powerpc/htmdump/ + # echo 2 > htmtype + # echo 0 > nodeindex + # echo 1 > nodalchipindex + # echo 12 > coreindexonchip + # echo 1 > htmflags # to set noWrap for HTM buffers + # echo 1 > htmconfigure # Configure the HTM + # echo 1 > htmstart # Start the HTM + # echo 0 > htmstart # Stop the HTM + # echo 0 > htmconfigure # Deconfigure the HTM + # cat htmstatus # Dump the status of HTM entries as data + +Above will set the htmtype and core details, followed by executing respective HTM operation. + +Read the HTM trace data +======================== + +After starting the trace collection, run the workload +of interest. Stop the trace collection after required period +of time, and read the trace file. + +.. code-block:: sh + + # cat /sys/kernel/debug/powerpc/htmdump/trace > trace_file + +This trace file will contain the relevant instruction traces +collected during the workload execution. And can be used as +input file for trace decoders to understand data. + +Benefits of using HTM debugfs interface +======================================= + +It is now possible to collect traces for a particular core/chip +from within any partition of the system and decode it. Through +this enablement, a small partition can be dedicated to collect the +trace data and analyze to provide important information for Performance +analysis, Software tuning, or Hardware debug. -- 2.43.0
