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Set the enable bits for general purpose counters in IA32_PERF_GLOBAL_CTRL when refreshing the PMU to emulate the MSR's architecturally defined post-RESET behavior. Per Intel's SDM: IA32_PERF_GLOBAL_CTRL: Sets bits n-1:0 and clears the upper bits. and Where "n" is the number of general-purpose counters available in the processor. AMD also documents this behavior for PerfMonV2 CPUs in one of AMD's many PPRs. Do not set any PERF_GLOBAL_CTRL bits if there are no general purpose counters, although a literal reading of the SDM would require the CPU to set either bits 63:0 or 31:0. The intent of the behavior is to globally enable all GP counters; honor the intent, if not the letter of the law. Leaving PERF_GLOBAL_CTRL '0' effectively breaks PMU usage in guests that haven't been updated to work with PMUs that support PERF_GLOBAL_CTRL. This bug was recently exposed when KVM added supported for AMD's PerfMonV2, i.e. when KVM started exposing a vPMU with PERF_GLOBAL_CTRL to guest software that only knew how to program v1 PMUs (that don't support PERF_GLOBAL_CTRL). Failure to emulate the post-RESET behavior results in such guests unknowingly leaving all general purpose counters globally disabled (the entire reason the post-RESET value sets the GP counter enable bits is to maintain backwards compatibility). The bug has likely gone unnoticed because PERF_GLOBAL_CTRL has been supported on Intel CPUs for as long as KVM has existed, i.e. hardly anyone is running guest software that isn't aware of PERF_GLOBAL_CTRL on Intel PMUs. And because up until v6.0, KVM _did_ emulate the behavior for Intel CPUs, although the old behavior was likely dumb luck. Because (a) that old code was also broken in its own way (the history of this code is a comedy of errors), and (b) PERF_GLOBAL_CTRL was documented as having a value of '0' post-RESET in all SDMs before March 2023. Initial vPMU support in commit |
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| README | ||
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.