On Sat, 2018-03-03 at 09:54 +0100, Greg Kroah-Hartman wrote: > On Fri, Mar 02, 2018 at 01:32:08PM -0800, Tim Chen wrote: > > > > Greg, > > > > I will like to propose backporting "x86/speculation: Use Indirect Branch > > Prediction Barrier on context switch" from commit 18bf3c3e in upstream > > to 4.9 and 4.4 stable. The patch has already been ported to 4.14 and > > 4.15 stable. The patch needs mm context id that Andy added in commit > > f39681ed. I have lifted the mm context id change from Andy's upstream > > patch and included it here. > > What does this patch "fix" in those older kernels? Is this a > performance improvement or something else? It's part of the Spectre variant 2 mitigation — a full flush of the branch prediction on context switch to a sensitive¹ process. It was the one I called out as "needs more attention" when I did the rest of the retpoline etc backportingk, and Tim has now fixed it up. (Thanks). ¹ for now, "sensitive" means non-dumpable. This isn't perfect but it's a reasonable approximation for now; it would be too expensive to do it on *every* context switch. And for your purposes, the important part is that it's what's upstream.
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