Messages in this thread | | | Date | Wed, 2 Aug 2017 11:18:43 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC][PATCH 1/5] mm: Rework {set,clear,mm}_tlb_flush_pending() |
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On Wed, Aug 02, 2017 at 10:02:28AM +0100, Will Deacon wrote: > On Wed, Aug 02, 2017 at 10:45:51AM +0200, Peter Zijlstra wrote: > > On Wed, Aug 02, 2017 at 09:15:23AM +0100, Will Deacon wrote: > > > On Wed, Aug 02, 2017 at 10:11:06AM +0200, Peter Zijlstra wrote: > > > > > > arm64 looks good too, although it plays silly games with the first > > > > barrier, but I trust that to be sufficient. > > > > > > The first barrier only orders prior stores for us, because page table > > > updates are made using stores. A prior load could be reordered past the > > > invalidation, but can't make it past the second barrier. > > > > So then you rely on the program not having any loads pending to the > > address you're about to invalidate, right? Otherwise we can do the TLBI > > and then the load to insta-repopulate the TLB entry you just wanted > > dead. > > > > That later DSB ISH is too late for that. > > > > Isn't that somewhat fragile? > > We only initiate the TLB invalidation after the page table update is > observable to the page table walker, so any repopulation will cause a fill > using the new page table entry.
Ah, indeed. Might be worth a comment tho.
> > > I really think we should avoid defining TLB invalidation in terms of > > > smp_mb() because it's a lot more subtle than that. > > > > I'm tempted to say stronger, smp_mb() only provides order, we want full > > serialization. Everything before stays before and _completes_ before. > > Everything after happens after (if the primitives actually do something > > at all of course, sparc64 for instance has no-op flush_tlb*). > > > > While such semantics might be slightly too strong for what we currently > > need, it is what powerpc, x86 and arm currently implement and are fairly > > easy to reason about. If we weaken it, stuff gets confusing again. > > My problem with this is that we're strengthening the semantics for no actual > use-case, but at the same time this will have a real performance impact.
Well, you could put in a dmb(ish) in the local case, that's loads cheaper than the dsb(ish) you need for the !local case. But OK..
Back to staring at dodgy arch code..
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