On Thu, Jan 26, 2023, Huang, Kai wrote: > On Thu, 2023-01-26 at 17:28 +0000, Sean Christopherson wrote: > > In other words, once the PTE is zapped/blocked (branch is pruned), it's completely > > removed from the paging tree and no other tasks can access the branch (page table > > and its children). I.e. the only remaining reference to the branch is the pointer > > handed to the RCU callback. That means the RCU callback has exclusive access to the > > branch, i.e. can operate as if it were holding mmu_lock for write. Furthermore, the > > RCU callback also doesn't need to flush TLBs because that was again done when > > pruning the branch. > > > > It's the same idea that KVM already uses for root SPs, the only difference is how > > KVM determines that there is exactly one entity that holds a reference to the SP. > > Right. This works fine for normal non-TDX case. However for TDX unfortunately > the access to the removed branch (or the removed sub-page-table) isn't that > "exclusive" as the SEAMCALL to truly zap that branch still needs to hold the > write lock of the entire Secure EPT tree, so it can still conflict with other > threads handling new faults. I thought TDX was smart enough to read-lock only the part of the tree that it's actually consuming, and write-lock only the part of the tree that it's actually modifying? Hrm, but even if TDX takes a read-lock, there's still the problem of it needing to walk the upper levels, i.e. KVM needs to keep mid-level page tables reachable until they're fully removed. Blech. That should be a non-issue at this time though, as I don't think KVM will ever REMOVE a page table of a live guest. I need to look at the PROMOTE/DEMOTE flows...
