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ef493f4b12
The BPF subsystem may capture LBR data on a counting event. However, the
current implementation assumes that LBR can/should only be used with
sampling events.
For instance, retsnoop tool ([0]) makes an extensive use of this
functionality and sets up perf event as follows:
struct perf_event_attr attr;
memset(&attr, 0, sizeof(attr));
attr.size = sizeof(attr);
attr.type = PERF_TYPE_HARDWARE;
attr.config = PERF_COUNT_HW_CPU_CYCLES;
attr.sample_type = PERF_SAMPLE_BRANCH_STACK;
attr.branch_sample_type = PERF_SAMPLE_BRANCH_KERNEL;
To limit the LBR for a sampling event is to avoid unnecessary branch
stack setup for a counting event in the sample read. Because LBR is only
read in the sampling event's overflow.
Although in most cases LBR is used in sampling, there is no HW limit to
bind LBR to the sampling mode. Allow an LBR setup for a counting event
unless in the sample read mode.
Fixes:
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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 reStructuredText 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.