This morning’s “x86/urgent” pull request ahead of Linux 6.17-rc4 being released later today has a fix for old Intel Pentium 4 processors as well as an Attack Vector Controls update for handling the Speculative Store Bypass (SSB) mitigation.
On the Pentium 4 side, there is a fix for some copy-paste fail that was merged earlier this year for Linux 6.15 and only now coming to light and being addressed. This Pentium 4 patch in turn will be back-ported to Linux 6.16 stable but Linux 6.15 is already end-of-life.
The issue at hand is around the Time Stamp Computer (TSC) handling with Prescott cores and later having a constant TSC that runs at a fixed rate even against CPU frequency changes.
The Linux check for setting the constant TSC feature was erroneously updated for between Intel Pentium 4 Prescott and up to Willamette processors. Except that Willamette was for the core for the first Pentium 4 CPUs that came out in 2020 while Prescott cores were only introduced in 2004… With constant TSC being for the Intel Pentium 4 CPUs of Prescott and later, the check is bogus.
The patch being sent to the mainline kernel today adjusts the conditional check for being between Prescott and Cedar Mill as the last of the Pentium 4 cores for constant TSC.
The faulty logic existed in the kernel since early in 2025 when adjusted by an Intel engineer. It’s not clear from the latest patch if this issue was spotted in the real-world by users still compiling latest Linux x86 kernels and running on old Pentium 4 CPUs or rather just spotted when going through the latest Intel kernel code. The fix sums up that this former check was plain wrong and must have been a copy and paste fail.
Another change worth mentioning with today’s x86/urgent pull request is adding Speculative Store Bypass (SSB) to the Attack Vector Controls handling. Not having SSB coverage was an oversight in the original Attack Vector Controls patches.
The full list of patches for this week’s x86/urgent pull can be found via this mailing list post.
