On Tue, May 03, 2022 at 02:17:25PM +0000, Quentin Perret wrote: > On Friday 22 Apr 2022 at 20:41:47 (+0000), Oliver Upton wrote: > > On Fri, Apr 22, 2022 at 04:00:45PM +0000, Quentin Perret wrote: > > > On Thursday 21 Apr 2022 at 16:40:56 (+0000), Oliver Upton wrote: > > > > The other option would be to not touch the subtree at all until the rcu > > > > callback, as at that point software will not tweak the tables any more. > > > > No need for atomics/spinning and can just do a boring traversal. > > > > > > Right that is sort of what I had in mind. Note that I'm still trying to > > > make my mind about the overall approach -- I can see how RCU protection > > > provides a rather elegant solution to this problem, but this makes the > > > whole thing inaccessible to e.g. pKVM where RCU is a non-starter. > > > > Heh, figuring out how to do this for pKVM seemed hard hence my lazy > > attempt :) > > > > > A > > > possible alternative that comes to mind would be to have all walkers > > > take references on the pages as they walk down, and release them on > > > their way back, but I'm still not sure how to make this race-safe. I'll > > > have a think ... > > > > Does pKVM ever collapse tables into blocks? That is the only reason any > > of this mess ever gets roped in. If not I think it is possible to get > > away with a rwlock with unmap on the write side and everything else on > > the read side, right? > > > > As far as regular KVM goes we get in this business when disabling dirty > > logging on a memslot. Guest faults will lazily collapse the tables back > > into blocks. An equally valid implementation would be just to unmap the > > whole memslot and have the guest build out the tables again, which could > > work with the aforementioned rwlock. > > Apologies for the delay on this one, I was away for a while. > > Yup, that all makes sense. FWIW the pKVM use-case I have in mind is > slightly different. Specifically, in the pKVM world the hypervisor > maintains a stage-2 for the host, that is all identity mapped. So we use > nice big block mappings as much as we can. But when a protected guest > starts, the hypervisor needs to break down the host stage-2 blocks to > unmap the 4K guest pages from the host (which is where the protection > comes from in pKVM). And when the guest is torn down, the host can > reclaim its pages, hence putting us in a position to coallesce its > stage-2 into nice big blocks again. Note that none of this coallescing > is currently implemented even in our pKVM prototype, so it's a bit > unfair to ask you to deal with this stuff now, but clearly it'd be cool > if there was a way we could make these things coexist and even ideally > share some code... Oh, it certainly isn't unfair to make sure we've got good constructs landing for everyone to use :-) I'll need to chew on this a bit more to have a better answer. The reason I hesitate to do the giant unmap for non-pKVM is that I believe we'd be leaving some performance on the table for newer implementations of the architecture. Having said that, avoiding a tlbi vmalls12e1is on every collapsed table is highly desirable. FEAT_BBM=2 semantics in the MMU is also on the todo list. In this case we'd do a direct table->block transformation on the PTE and elide that nasty tlbi. Unless there's objections, I'll probably hobble this series along as-is for the time being. My hope is that other table walkers can join in on the parallel party later down the road. Thanks for getting back to me. -- Best, Oliver
