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⇱ Linux 6.18 Lands Retpoline Optimization To Help With Intel E Cores - Phoronix

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Linux 6.18 Lands Retpoline Optimization To Help With Intel E Cores

Written by Michael Larabel in Intel on 11 October 2025 at 03:46 PM EDT. 7 Comments
The Linux 6.18 merge window is winding down this weekend ahead of Linux 6.18-rc1 expected on Sunday. Merged today were some remaining x86 core updates, which includes a Retpoline optimization patch intended to help out Intel E core CPUs.

Return trampolines "Retpolines" are needed for Spectre Variant Two mitigations. Intel engineer Peter Zijlstra landed a patch for optimizing the x86 patch_retpoline() code within the kernel. He explains with the patch:
Currently the very common retpoline: "CS CALL __x86_indirect_thunk_r11" is transformed into "CALL *R11; NOP3" for eIBRS/BHI_NO parts.

Similarly, paranoid fineibt has: "CALL *R11; NOP".

Recognise that CS stuffing can avoid the extra NOP. However, due to prefix decode penalties, make sure to not emit too many CS prefixes. Notably: "CS CALL __x86_indirect_thunk_rax" must not become "CS CS CS CS CALL *RAX". Prefix decode penalties are typically many more cycles than decoding an extra NOP.

Additionally, if the retpoline is a tail-call, the "JMP *%\reg" should be followed by INT3 for straight-line-speculation mitigation, since emit_indirect() now has a length argument, move this into emit_indirect() such that other users (paranoid-fineibt) also do this.

The original mailing list post for the patch adds more context:
"Finding the exact prefix decode penalties for uarchs that have eIBRS/BHI_NO is not a fun time. I've stuck to the general wisdom that 3 prefixes is mostly good (notably, the instruction at hand has no 0x0f escape which is sometimes counted towards the prefix budget -- it can have a REX prefix, but those are generally not counted towards the prefix budget).

In general Intel P-cores do not have prefix decode penalties, but the E-cores (or rather the Atom line) generally does. And since this all runs on hybrid cores, the code must accommodate them.

I hate all this."

That patch was merged to Linux Git today via the x86/core pull ahead of Linux 6.18-rc1 tomorrow.

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.