Intel Architectural LBR Support Going Into Linux 5.9
Written by Michael Larabel in Intel on 8 July 2020 at 07:47 AM EDT. Add A Comment
INTEL --
Intel CPUs have long supported LBR for last branch records as a means of recording the branches to which software has taken along with exposing other control flow information. This has relied upon model-specific registers while with future Intel CPUs this is being folded into a more universal CPU architectural feature. Support for Intel "Arch LBR" is set to come later this year with the Linux 5.9 kernel.

The Linux kernel has been working with the existing MSR-based LBR functionality where with the likes of the perf subsystem can be used for recording the branches taken and other control flow information exposed for analysis and profiling. Intel recently has been working on added this Architectural LBR support to the Linux kernel.

These Linux patches come after Intel detailed the "architectural" LBR back in March with their programming reference guide. Chapter 7 there details the new LBR implementation.

With the new Linux kernel patches and their Architectural LBR support, there is faster context switching thanks to XSAVES support and new MSR features, faster LBR reading, better support for LBR features without needing to know CPU model specific information, the ability to expose LBRs to guests without model-specific features, and lower overhead. Existing LBR capabilities are still maintained.

The Intel Architectural Last Branch Records functionality is wired up and as of this morning has been queued into the perf/core branch. This is where the patches will reside until this perf code is sent in for the Linux 5.9 kernel merge window in August. Linux 5.9 in turn will debut as stable in the October timeframe.
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Michael Larabel is the principal author of Phoronix.com and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 20,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and OpenBenchmarking.org automated benchmarking software. He can be followed via Twitter or contacted via MichaelLarabel.com.

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