AMD Zen 4 LbrExtV2 Feature Queued Ahead Of Linux 6.1

Written by Michael Larabel in AMD on 27 August 2022 at 06:05 AM EDT. 1 Comment
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Earlier this month AMD posted Linux kernel patches preparing LbrExtV2 as updated Last Branch Record functionality being introduced with upcoming AMD Zen 4 processors. That LbrExtV2 support for the Linux kernel's "perf" subsystem has now been queued up in its respective branch ahead of the Linux 6.1 feature merge window beginning in early October.

As explained in the earlier Phoronix article on the LbrExtV2 enablement work, LbrExtV2 is a new version of the Last Branch Record (LBR) functionality that is coming with Zen 4 CPUs. Last Branch Record is used for determining/analyzing the control flow of software by logging branch information to registers in real-time. The LBR information can be used for determining of hot code paths and useful for areas like profile-guided compiler optimizations and other tuning based on being able to analyze the branches frequently taken in software.

Last Branch Record Extension Version Two adds LBR-Freeze-on-PMI for better correlation with PMC overflow events, new speculation information, and new hardware-based filtering support if desiring data just on specific branch types. Among the branch filtering capabilities are for kernel-only, user-space only, conditional branches, near relative calls, near indirect calls, near returns, near indirect jumps, near relative jumps, and far branches. The AMD patches get the LbrExtV2 functionality interfacing with the Linux perf subsystem for performance event monitoring.


The news ending out this week is that AMD LbrExtV2 work has been queued into TIP's perf/core branch. Now that it has made it there, it will be submitted to Linus Torvalds come the Linux 6.1 merge window kicking off in early October. So barring any last minute issues from coming about, the AMD LbrExtV2 functionality with Zen 4 CPUs will be enabled under Linux on the stable kernel around year's end.
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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, LinkedIn, or contacted via MichaelLarabel.com.

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