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8910075d61
The AMD Branch Sampling features does not provide hardware filtering by privilege level. The associated PMU counter does but not the branch sampling by itself. Given how BRS operates there is a possibility that BRS captures kernel level branches even though the event is programmed to count only at the user level. Implement a workaround in software by removing the branches which belong to the wrong privilege level. The privilege level is evaluated on the target of the branch and not the source so as to be compatible with other architectures. As a consequence of this patch, the number of entries in the PERF_RECORD_BRANCH_STACK buffer may be less than the maximum (16). It could even be zero. Another consequence is that consecutive entries in the branch stack may not reflect actual code path and may have discontinuities, in case kernel branches were suppressed. But this is no different than what happens on other architectures. Signed-off-by: Stephane Eranian <eranian@google.com> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org> Link: https://lore.kernel.org/r/20220322221517.2510440-6-eranian@google.com
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.