kbuild: rust: keep Rust objects out of Clang LTO with inline helpersgrafted

Under `CONFIG_LTO_CLANG` + `CONFIG_RUST_INLINE_HELPERS`, one may hit
`objtool` errors such as:

    vmlinux.o: error: objtool: _R..._3Gsp4boot+0xd6a:
    can't find jump dest instruction at .text._R..._3Gsp4boot+0x1dfd

The reason is that in such builds, the Clang invocation that compiles
the combined Rust plus helpers bitcode emits LLVM bitcode (again) --
the final code generation happens in the linker's LTO step, which the
`-mllvm` trap options passed to Clang do not reach.

This, in turn, means that unreachable traps are missing, and the
impossible paths do not merely fallthrough to the next symbol, but past
the end of their own section, since LTO builds place each function in
its own section.

Thus filter `CC_FLAGS_LTO` out of the Clang invocation, so that it always
emits machine code directly, with the traps in place.

Assisted-by: LLM
Cc: Gary Guo <gary@garyguo.net>
Cc: Boqun Feng <boqun@kernel.org>
Cc: Alice Ryhl <aliceryhl@google.com>
Cc: Matthew Maurer <mmaurer@google.com>
Cc: Josh Poimboeuf <jpoimboe@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: stable@vger.kernel.org
Fixes: 3a2486cc1da5 ("kbuild: rust: provide an option to inline C helpers into Rust")
Acked-by: Gary Guo <gary@garyguo.net>
Link: https://patch.msgid.link/20260816133233.197500-2-ojeda@kernel.org
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

1 files changed, 150 insertions, 0 deletions

diff --git a/Documentation/trace/intel_th.rst b/Documentation/trace/intel_th.rst
new file mode 100644
index 000000000..b31818d5f
--- /dev/null
+++ b/Documentation/trace/intel_th.rst
@@ -0,0 +1,150 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+=======================
+Intel(R) Trace Hub (TH)
+=======================
+
+Overview
+--------
+
+Intel(R) Trace Hub (TH) is a set of hardware blocks that produce,
+switch and output trace data from multiple hardware and software
+sources over several types of trace output ports encoded in System
+Trace Protocol (MIPI STPv2) and is intended to perform full system
+debugging. For more information on the hardware, see Intel(R) Trace
+Hub developer's manual [1].
+
+It consists of trace sources, trace destinations (outputs) and a
+switch (Global Trace Hub, GTH). These devices are placed on a bus of
+their own ("intel_th"), where they can be discovered and configured
+via sysfs attributes.
+
+Currently, the following Intel TH subdevices (blocks) are supported:
+  - Software Trace Hub (STH), trace source, which is a System Trace
+    Module (STM) device,
+  - Memory Storage Unit (MSU), trace output, which allows storing
+    trace hub output in system memory,
+  - Parallel Trace Interface output (PTI), trace output to an external
+    debug host via a PTI port,
+  - Global Trace Hub (GTH), which is a switch and a central component
+    of Intel(R) Trace Hub architecture.
+
+Common attributes for output devices are described in
+Documentation/ABI/testing/sysfs-bus-intel_th-output-devices, the most
+notable of them is "active", which enables or disables trace output
+into that particular output device.
+
+GTH allows directing different STP masters into different output ports
+via its "masters" attribute group. More detailed GTH interface
+description is at Documentation/ABI/testing/sysfs-bus-intel_th-devices-gth.
+
+STH registers an stm class device, through which it provides interface
+to userspace and kernelspace software trace sources. See
+Documentation/trace/stm.rst for more information on that.
+
+MSU can be configured to collect trace data into a system memory
+buffer, which can later on be read from its device nodes via read() or
+mmap() interface and directed to a "software sink" driver that will
+consume the data and/or relay it further.
+
+On the whole, Intel(R) Trace Hub does not require any special
+userspace software to function; everything can be configured, started
+and collected via sysfs attributes, and device nodes.
+
+[1] https://software.intel.com/sites/default/files/managed/d3/3c/intel-th-developer-manual.pdf
+
+Bus and Subdevices
+------------------
+
+For each Intel TH device in the system a bus of its own is
+created and assigned an id number that reflects the order in which TH
+devices were enumerated. All TH subdevices (devices on intel_th bus)
+begin with this id: 0-gth, 0-msc0, 0-msc1, 0-pti, 0-sth, which is
+followed by device's name and an optional index.
+
+Output devices also get a device node in /dev/intel_thN, where N is
+the Intel TH device id. For example, MSU's memory buffers, when
+allocated, are accessible via /dev/intel_th0/msc{0,1}.
+
+Quick example
+-------------
+
+# figure out which GTH port is the first memory controller::
+
+	$ cat /sys/bus/intel_th/devices/0-msc0/port
+	0
+
+# looks like it's port 0, configure master 33 to send data to port 0::
+
+	$ echo 0 > /sys/bus/intel_th/devices/0-gth/masters/33
+
+# allocate a 2-windowed multiblock buffer on the first memory
+# controller, each with 64 pages::
+
+	$ echo multi > /sys/bus/intel_th/devices/0-msc0/mode
+	$ echo 64,64 > /sys/bus/intel_th/devices/0-msc0/nr_pages
+
+# enable wrapping for this controller, too::
+
+	$ echo 1 > /sys/bus/intel_th/devices/0-msc0/wrap
+
+# and enable tracing into this port::
+
+	$ echo 1 > /sys/bus/intel_th/devices/0-msc0/active
+
+# .. send data to master 33, see stm.txt for more details ..
+# .. wait for traces to pile up ..
+# .. and stop the trace::
+
+	$ echo 0 > /sys/bus/intel_th/devices/0-msc0/active
+
+# and now you can collect the trace from the device node::
+
+	$ cat /dev/intel_th0/msc0 > my_stp_trace
+
+Host Debugger Mode
+------------------
+
+It is possible to configure the Trace Hub and control its trace
+capture from a remote debug host, which should be connected via one of
+the hardware debugging interfaces, which will then be used to both
+control Intel Trace Hub and transfer its trace data to the debug host.
+
+The driver needs to be told that such an arrangement is taking place
+so that it does not touch any capture/port configuration and avoids
+conflicting with the debug host's configuration accesses. The only
+activity that the driver will perform in this mode is collecting
+software traces to the Software Trace Hub (an stm class device). The
+user is still responsible for setting up adequate master/channel
+mappings that the decoder on the receiving end would recognize.
+
+In order to enable the host mode, set the 'host_mode' parameter of the
+'intel_th' kernel module to 'y'. None of the virtual output devices
+will show up on the intel_th bus. Also, trace configuration and
+capture controlling attribute groups of the 'gth' device will not be
+exposed. The 'sth' device will operate as usual.
+
+Software Sinks
+--------------
+
+The Memory Storage Unit (MSU) driver provides an in-kernel API for
+drivers to register themselves as software sinks for the trace data.
+Such drivers can further export the data via other devices, such as
+USB device controllers or network cards.
+
+The API has two main parts::
+ - notifying the software sink that a particular window is full, and
+   "locking" that window, that is, making it unavailable for the trace
+   collection; when this happens, the MSU driver will automatically
+   switch to the next window in the buffer if it is unlocked, or stop
+   the trace capture if it's not;
+ - tracking the "locked" state of windows and providing a way for the
+   software sink driver to notify the MSU driver when a window is
+   unlocked and can be used again to collect trace data.
+
+An example sink driver, msu-sink illustrates the implementation of a
+software sink. Functionally, it simply unlocks windows as soon as they
+are full, keeping the MSU running in a circular buffer mode. Unlike the
+"multi" mode, it will fill out all the windows in the buffer as opposed
+to just the first one. It can be enabled by writing "sink" to the "mode"
+file (assuming msu-sink.ko is loaded).