Messages in this thread | | | Date | Thu, 25 Jan 2018 19:18:52 +0100 | From | Peter Zijlstra <> | Subject | Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process |
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On Thu, Jan 25, 2018 at 09:04:21AM -0800, Andy Lutomirski wrote: > I haven't tried to fully decipher the patch, but I think the idea is > wrong. (I think it's the same wrong idea that Rik and I both had and > that I got into Linus' tree for a while...) The problem is that it's > not actually correct to run indefinitely in kernel mode using stale > cached page table data. The stale PTEs themselves are fine, but the > stale intermediate translations can cause the CPU to speculatively > load complete garbage into the TLB, and that's bad (and causes MCEs on > AMD CPUs).
Urggh.. indeed :/
> I think we only really have two choices: tlb_defer_switch_to_init_mm() > == true and tlb_defer_switch_to_init_mm() == false. The current > heuristic is to not defer if we have PCID, because loading CR3 is > reasonably fast.
I just _really_ _really_ hate idle drivers doing leave_mm(). I don't suppose limiting the !IPI case to just the idle case would be correct either, because between waking from idle and testing our 'should I have invalidated' bit it can (however unlikely) speculate into stale TLB entries too..
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