A Last Minute Linux 4.17 Pull To Help Non-PCID Systems With KPTI Meltdown Performance
Written by Michael Larabel in Linux Kernel on 15 April 2018 at 03:42 PM EDT. 4 Comments
LINUX KERNEL --
While the Linux 4.17 kernel merge window is closing today and is already carrying a lot of interesting changes as covered by our Linux 4.17 feature overview, Thomas Gleixner today sent in a final round of x86 (K)PTI updates for Meltdown mitigation with this upcoming kernel release.

This latest round of page-table isolation updates should help out systems lacking PCID, Process Context Identifiers. The KPTI code makes use of PCID for reducing the performance overhead of this Meltdown mitigation technique. PCID has been around since the Intel Westmere days, but now the latest kernel patches will help offset the KPTI performance impact for systems lacking PCID.

The patches support using global pages with page-table isolation. Allowing the user/kernel shared areas to be marked "GLOBAL" should reduce the TLB overhead on systems lacking PCID. Back when this Meltdown mitigation work was known as KAISER they carried similar functionality, but got lost along the way.

Now thanks to work by Dave Hansen and other kernel developers, this using of global pages with PTI has been restored and ready for mainlining with Linux 4.17. But as a reminder it is expected to just be benefiting pre-Westmere era hardware, which was introduced in 2010.

This work and other changes are found in the latest x86/pti pull request.

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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 10,000 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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