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Implement architecture-agnostic parts of USDT parsing logic. The code is the documentation in this case, it's futile to try to succinctly describe how USDT parsing is done in any sort of concreteness. But still, USDTs are recorded in special ELF notes section (.note.stapsdt), where each USDT call site is described separately. Along with USDT provider and USDT name, each such note contains USDT argument specification, which uses assembly-like syntax to describe how to fetch value of USDT argument. USDT arg spec could be just a constant, or a register, or a register dereference (most common cases in x86_64), but it technically can be much more complicated cases, like offset relative to global symbol and stuff like that. One of the later patches will implement most common subset of this for x86 and x86-64 architectures, which seems to handle a lot of real-world production application. USDT arg spec contains a compact encoding allowing usdt.bpf.h from previous patch to handle the above 3 cases. Instead of recording which register might be needed, we encode register's offset within struct pt_regs to simplify BPF-side implementation. USDT argument can be of different byte sizes (1, 2, 4, and 8) and signed or unsigned. To handle this, libbpf pre-calculates necessary bit shifts to do proper casting and sign-extension in a short sequences of left and right shifts. The rest is in the code with sometimes extensive comments and references to external "documentation" for USDTs. Signed-off-by: Andrii Nakryiko <andrii@kernel.org> Signed-off-by: Alexei Starovoitov <ast@kernel.org> Reviewed-by: Alan Maguire <alan.maguire@oracle.com> Reviewed-by: Dave Marchevsky <davemarchevsky@fb.com> Link: https://lore.kernel.org/bpf/20220404234202.331384-4-andrii@kernel.org
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.