For the shortlog, I really want to capture the net effect. It took me a lot of staring and reading (and hopefully not misreading) to figure out that this is a glorified nop. KVM: x86/mmu: Update unsync children metadata via recursion, not bottom-up walk On Sun, Jun 05, 2022, Lai Jiangshan wrote: > From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx> > > mmu_unsync_walk() is designed to be workable in a pagetable which has > unsync child bits set in the shadow pages in the pagetable but without > any unsync shadow pages. > > This can be resulted when the unsync shadow pages of a pagetable > can be walked from other pagetables and have been synced or zapped > when other pagetables are synced or zapped. > > So mmu_pages_clear_parents() is not required even when the callers of > mmu_unsync_walk() zap or sync the pagetable. There's one other critical piece that it took me a quite some time to suss out from the code: the @parent passed to mmu_sync_children() _is_ updated because mmu_sync_children() loops on mmu_unsync_walk(). It's only the parents of @parent that are not updated, but they weren't updated anyways because mmu_pages_clear_parents() doesn't operate on the parents of @parent. > So remove mmu_pages_clear_parents() and the child bits can be cleared in > the next call of mmu_unsync_walk() in one go. Ah, I missed (over and over) that the "next call" is the one right mmu_sync_children() and mmu_unsync_walk(), not a future call. Because I kept losing track of which pagetable was which, how about this for a changelog? When syncing a shadow page with unsync children, do not update the "unsync children" metadata from the bottom up, and instead defer the update to the next "iteration" of mmu_unsync_walk() (all users of mmu_unsync_walk() loop until it returns "no unsync children"). mmu_unsync_walk() is designed to handle the scenario where a shadow page has a false positive on having unsync children, i.e. unsync_children can be elevated without any child shadow pages actually being unsync. Such a scenario already occurs when a child is synced or zapped by a different walk of the page tables, i.e. with a different set of parents, as unmarking parents is done only for the current walk. Note, mmu_pages_clear_parents() doesn't update parents of @parent, so there's no change in functionality from that perspective. Removing mmu_pages_clear_parents() allows for further simplifying mmu_unsync_walk(), including removing the struct mmu_page_path since mmu_pages_clear_parents() was the only the function is the only user of it. With a cleaned up shortlog+changelog, and assuming I didn't misread everything... Reviewed-by: Sean Christopherson <seanjc@xxxxxxxxxx> > > Removing mmu_pages_clear_parents() allows for further simplifying > mmu_unsync_walk() including removing the struct mmu_page_path since > the function is the only user of it. > > Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx> > ---
