Do modern CPUs flush branch predictor state on context switch? https://lkml.org/lkml/2018/1/4/635 … seems to indicate that they don’t. If not, why are we treating process separation as “the thing that fixes Spectre”?
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Replying to @pcwalton
Even if the cpu doesn't, in theory the kernel can, and should if necessary. The variants of Spectre that are cross-process are always fixable with heavy machinery by kernel. Only intraprocess ones are unfixable without hardware fixes or pure emulated execution.
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Replying to @RichFelker
Why is the kernel able to do it but userspace can’t? There’s no “flush BTB” instruction that I can see, privileged or not…
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Replying to @pcwalton
Flushing the whole cache hierarchy probably does it..?
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I was corrected: there actually is a flush BTB instruction now.https://software.intel.com/security-software-guidance/insights/deep-dive-indirect-branch-predictor-barrier …
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