Linux kernel Spectre V2 defense fingered for massively slowing down apps on Intel Hyper-Thread CPUs

guest · Nov 19, 2018

Linux kernel Spectre V2 defense fingered for massively slowing down unlucky apps on Intel Hyper-Thread CPUs
This is on by default? 'Yikes' says Chipzilla techie
November 20, 2018
https://www.theregister.co.uk/2018/11/20/linux_kernel_spectre_v2_patch_slowdown_intel/

Linux supremo Linus Torvalds has voiced support for a kernel patch that limits a previously deployed defense against Spectre Variant 2, a data-leaking vulnerability in modern processors.

Specifically, the proposed patch disables a particular Spectre V2 defense mechanism by default, rather than switching it on automatically. And here's the reason for that suggested change: code runs up to 50 per cent slower on Intel CPUs that use Hyper-Threading with the security defense enabled.

The specific Spectre V2 mitigation in this case was added to Linux 4.20 and backported to Linux 4.19.2. It's called STIBP (Single Thread Indirect Branch Predictors)

"When performance goes down by 50 per cent on some loads, people need to start asking themselves whether it was worth it," Torvalds wrote in a message to the Linux kernel mailing list on Sunday. "It's apparently better to just disable SMT entirely, which is what security-conscious people do anyway."
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guest · Nov 25, 2018

Linux 4.19.4 & 4.14.83 Released With STIBP Code Dropped
November 24, 2018
https://www.phoronix.com/scan.php?page=news_item&px=Linux-4.19.4-Released

[...] 4.19.4 and 4.14.83 are the releases that end up reverting the STIBP behavior that applied Single Thread Indirect Branch Predictors to all processes on supported systems. [...] That is what was introduced in Linux 4.20 and then back-ported to the 4.19/4.14 LTS branches, which in turn hurt the performance a lot. So for now the code is removed.

[...] there is improved STIBP code on the way for Linux 4.20 that by default just apply STIBP to SECCOMP threads and processes requesting it via prctl() but otherwise is off by default (that behavior can also be changed via kernel parameters). Once that code is ready to go for Linux 4.20, we may see it then back-ported to these stable trees.
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Linux kernel Spectre V2 defense fingered for massively slowing down apps on Intel Hyper-Thread CPUs

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