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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.
