Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process

[Arjan van de Ven]

> The idea is simple, do what we do for virt. Don't send IPI's to CPUs
> that don't need them (in virt's case because the vCPU isn't running, in
> our case because we're not in fact running a user process), but mark the
> CPU as having needed a TLB flush.

I am really uncomfortable with that idea.
You really can't run code safely on a cpu where the TLBs in the CPU are invalid
or where a CPU that does (partial) page walks would install invalid PTEs either
through actual or through speculative execution.

(in the virt case there's a cheat, since the code is not actually running
there isn't a cpu with TLBs live. You can't do that same cheat for this case)