Documentation: x86: convert orc-unwinder.txt to reST

This converts the plain text documentation to reStructuredText format and
add it to Sphinx TOC tree. No essential content change.

Signed-off-by: Changbin Du <changbin.du@gmail.com>
Reviewed-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>

1 files changed, 182 insertions, 0 deletions

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+.. SPDX-License-Identifier: GPL-2.0
+
+============
+ORC unwinder
+============
+
+Overview
+========
+
+The kernel CONFIG_UNWINDER_ORC option enables the ORC unwinder, which is
+similar in concept to a DWARF unwinder.  The difference is that the
+format of the ORC data is much simpler than DWARF, which in turn allows
+the ORC unwinder to be much simpler and faster.
+
+The ORC data consists of unwind tables which are generated by objtool.
+They contain out-of-band data which is used by the in-kernel ORC
+unwinder.  Objtool generates the ORC data by first doing compile-time
+stack metadata validation (CONFIG_STACK_VALIDATION).  After analyzing
+all the code paths of a .o file, it determines information about the
+stack state at each instruction address in the file and outputs that
+information to the .orc_unwind and .orc_unwind_ip sections.
+
+The per-object ORC sections are combined at link time and are sorted and
+post-processed at boot time.  The unwinder uses the resulting data to
+correlate instruction addresses with their stack states at run time.
+
+
+ORC vs frame pointers
+=====================
+
+With frame pointers enabled, GCC adds instrumentation code to every
+function in the kernel.  The kernel's .text size increases by about
+3.2%, resulting in a broad kernel-wide slowdown.  Measurements by Mel
+Gorman [1]_ have shown a slowdown of 5-10% for some workloads.
+
+In contrast, the ORC unwinder has no effect on text size or runtime
+performance, because the debuginfo is out of band.  So if you disable
+frame pointers and enable the ORC unwinder, you get a nice performance
+improvement across the board, and still have reliable stack traces.
+
+Ingo Molnar says:
+
+  "Note that it's not just a performance improvement, but also an
+  instruction cache locality improvement: 3.2% .text savings almost
+  directly transform into a similarly sized reduction in cache
+  footprint. That can transform to even higher speedups for workloads
+  whose cache locality is borderline."
+
+Another benefit of ORC compared to frame pointers is that it can
+reliably unwind across interrupts and exceptions.  Frame pointer based
+unwinds can sometimes skip the caller of the interrupted function, if it
+was a leaf function or if the interrupt hit before the frame pointer was
+saved.
+
+The main disadvantage of the ORC unwinder compared to frame pointers is
+that it needs more memory to store the ORC unwind tables: roughly 2-4MB
+depending on the kernel config.
+
+
+ORC vs DWARF
+============
+
+ORC debuginfo's advantage over DWARF itself is that it's much simpler.
+It gets rid of the complex DWARF CFI state machine and also gets rid of
+the tracking of unnecessary registers.  This allows the unwinder to be
+much simpler, meaning fewer bugs, which is especially important for
+mission critical oops code.
+
+The simpler debuginfo format also enables the unwinder to be much faster
+than DWARF, which is important for perf and lockdep.  In a basic
+performance test by Jiri Slaby [2]_, the ORC unwinder was about 20x
+faster than an out-of-tree DWARF unwinder.  (Note: That measurement was
+taken before some performance tweaks were added, which doubled
+performance, so the speedup over DWARF may be closer to 40x.)
+
+The ORC data format does have a few downsides compared to DWARF.  ORC
+unwind tables take up ~50% more RAM (+1.3MB on an x86 defconfig kernel)
+than DWARF-based eh_frame tables.
+
+Another potential downside is that, as GCC evolves, it's conceivable
+that the ORC data may end up being *too* simple to describe the state of
+the stack for certain optimizations.  But IMO this is unlikely because
+GCC saves the frame pointer for any unusual stack adjustments it does,
+so I suspect we'll really only ever need to keep track of the stack
+pointer and the frame pointer between call frames.  But even if we do
+end up having to track all the registers DWARF tracks, at least we will
+still be able to control the format, e.g. no complex state machines.
+
+
+ORC unwind table generation
+===========================
+
+The ORC data is generated by objtool.  With the existing compile-time
+stack metadata validation feature, objtool already follows all code
+paths, and so it already has all the information it needs to be able to
+generate ORC data from scratch.  So it's an easy step to go from stack
+validation to ORC data generation.
+
+It should be possible to instead generate the ORC data with a simple
+tool which converts DWARF to ORC data.  However, such a solution would
+be incomplete due to the kernel's extensive use of asm, inline asm, and
+special sections like exception tables.
+
+That could be rectified by manually annotating those special code paths
+using GNU assembler .cfi annotations in .S files, and homegrown
+annotations for inline asm in .c files.  But asm annotations were tried
+in the past and were found to be unmaintainable.  They were often
+incorrect/incomplete and made the code harder to read and keep updated.
+And based on looking at glibc code, annotating inline asm in .c files
+might be even worse.
+
+Objtool still needs a few annotations, but only in code which does
+unusual things to the stack like entry code.  And even then, far fewer
+annotations are needed than what DWARF would need, so they're much more
+maintainable than DWARF CFI annotations.
+
+So the advantages of using objtool to generate ORC data are that it
+gives more accurate debuginfo, with very few annotations.  It also
+insulates the kernel from toolchain bugs which can be very painful to
+deal with in the kernel since we often have to workaround issues in
+older versions of the toolchain for years.
+
+The downside is that the unwinder now becomes dependent on objtool's
+ability to reverse engineer GCC code flow.  If GCC optimizations become
+too complicated for objtool to follow, the ORC data generation might
+stop working or become incomplete.  (It's worth noting that livepatch
+already has such a dependency on objtool's ability to follow GCC code
+flow.)
+
+If newer versions of GCC come up with some optimizations which break
+objtool, we may need to revisit the current implementation.  Some
+possible solutions would be asking GCC to make the optimizations more
+palatable, or having objtool use DWARF as an additional input, or
+creating a GCC plugin to assist objtool with its analysis.  But for now,
+objtool follows GCC code quite well.
+
+
+Unwinder implementation details
+===============================
+
+Objtool generates the ORC data by integrating with the compile-time
+stack metadata validation feature, which is described in detail in
+tools/objtool/Documentation/stack-validation.txt.  After analyzing all
+the code paths of a .o file, it creates an array of orc_entry structs,
+and a parallel array of instruction addresses associated with those
+structs, and writes them to the .orc_unwind and .orc_unwind_ip sections
+respectively.
+
+The ORC data is split into the two arrays for performance reasons, to
+make the searchable part of the data (.orc_unwind_ip) more compact.  The
+arrays are sorted in parallel at boot time.
+
+Performance is further improved by the use of a fast lookup table which
+is created at runtime.  The fast lookup table associates a given address
+with a range of indices for the .orc_unwind table, so that only a small
+subset of the table needs to be searched.
+
+
+Etymology
+=========
+
+Orcs, fearsome creatures of medieval folklore, are the Dwarves' natural
+enemies.  Similarly, the ORC unwinder was created in opposition to the
+complexity and slowness of DWARF.
+
+"Although Orcs rarely consider multiple solutions to a problem, they do
+excel at getting things done because they are creatures of action, not
+thought." [3]_  Similarly, unlike the esoteric DWARF unwinder, the
+veracious ORC unwinder wastes no time or siloconic effort decoding
+variable-length zero-extended unsigned-integer byte-coded
+state-machine-based debug information entries.
+
+Similar to how Orcs frequently unravel the well-intentioned plans of
+their adversaries, the ORC unwinder frequently unravels stacks with
+brutal, unyielding efficiency.
+
+ORC stands for Oops Rewind Capability.
+
+
+.. [1] https://lkml.kernel.org/r/20170602104048.jkkzssljsompjdwy@suse.de
+.. [2] https://lkml.kernel.org/r/d2ca5435-6386-29b8-db87-7f227c2b713a@suse.cz
+.. [3] http://dustin.wikidot.com/half-orcs-and-orcs