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I may be wrong but I don’t think there is any mechanism in place to do that. And even then by the time the context switch is actually executed, you may already have speculatively executed further instructions
There is no *user-accessible* architecturally defined mechanism for that that I'm aware of, but it's entirely possible they have a way of doing it in microcode. "Flush/disable/whatever the BTB" sounds like the kind of chicken bit they'd have. Hence microcode fix.
I doubt flushing the branch prediction on syscalls will have nearly as much impact as flushing TLBs/changing pagetables as KPTI is doing. Branch state is a lot more transient and relevant to tight loops than page tables.
I may be missing something but wouldn’t flushing the BTB not only not fix the issue, but makes it easier to exploit by making the behavior of the BP its default?
The Spectre exploit is "priming" the branch predictor with a bogus (exploit) target address. There is no default.
I can’t think of a μcode fix besides “flush any cache line touched by an incorrect prediction IF it wasn’t there before” and I highly doubt the μcode has enough memory to keep track of all that in something in the hot path
I'm not talking about Meltdown, I'm talking about Spectre. Meltdown is mitigated by KPTI (with perf impact) on Intel. I speculate Spectre may be mitigatable in microcode.
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