On Sun, Jul 19, 2020 at 06:32:22PM -0400, contact@xxxxxxxxxxxxxxxxx wrote: > Hi, > > I'm a bit confused by the interface for interacting with the page rmap. For > context, on a TDP-enabled x86-64 host, I'm logging each time a GFN->PFN > mapping is created/modified/removed for a non-MMIO page (kernel version > 5.4). > > First, my understanding is that the page rmap is a mapping of non-MMIO PFNs > back to the GFNs that use them. The interface for creating an rmap entry > (and thus, a new GFN->PFN mapping) appears to be rmap_add() and is quite > straightforward. However, rmap_remove() does not appear to be the (only) > function for removing an entry from the page rmap. For instance, > kvm_zap_rmapp()---used by the mmu_notifier for invalidations---jumps > straight to pte_list_remove(), while drop_spte() uses rmap_remove(). The rmaps are associated with the memslot, the drop_spte() path allows KVM to clean up SPTEs without having to guarantee the validity of the memslot that was used to create the SPTE. > Would it be fair to say that mmu_spte_clear_track_bits() is found on all > paths for removing an entry from the page rmap? Yes, that should hold true. > Second, for updates to the frame numbers in an existing SPTE, there are both > mmu_set_spte() and mmu_spte_set(). Could someone please clarify the > difference between these functions? mmu_set_spte() is the higher level helper that is used during a page fault or prefetch to convert a host PFN and basic access permissions into a SPTE value, handle large/huge page interactions and accounting, add the rmap, etc..., and of course eventually update the SPTE. mmu_spte_set() is a low level helper that does nothing more than write a SPTE. It's just a wrapper to __set_spte() that also WARNs if the old SPTE is present. > Finally, much of the logic between the page rmap and parent PTE rmaps > (understandably) overlaps. However, with TDP-enabled, I'm not entirely sure > what the role of the parent PTE rmaps is relative to the page rmap. Could > someone possibly clarify? KVM needs the backpointers to remove the SPTE for a shadow page, which exists in the parent shadow page, when the child is zapped, e.g. if a L2 SP is removed, its SPTE in a L3 SP needs to be updated.
