KVM: x86/mmu: Skip !MMU-present SPTEs when removing SP in exclusive mode

If mmu_lock is held for write, don't bother setting !PRESENT SPTEs to
REMOVED_SPTE when recursively zapping SPTEs as part of shadow page
removal.  The concurrent write protections provided by REMOVED_SPTE are
not needed, there are no backing page side effects to record, and MMIO
SPTEs can be left as is since they are protected by the memslot
generation, not by ensuring that the MMIO SPTE is unreachable (which
is racy with respect to lockless walks regardless of zapping behavior).

Skipping !PRESENT drastically reduces the number of updates needed to
tear down sparsely populated MMUs, e.g. when tearing down a 6gb VM that
didn't touch much memory, 6929/7168 (~96.6%) of SPTEs were '0' and could
be skipped.

Avoiding the write itself is likely close to a wash, but avoiding
__handle_changed_spte() is a clear-cut win as that involves saving and
restoring all non-volatile GPRs (it's a subtly big function), as well as
several conditional branches before bailing out.

Cc: Ben Gardon <[email protected]>
Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

Author: sean-jc

arch/x86/kvm/mmu/tdp_mmu.c

@@ -337,7 +337,18 @@ static void handle_removed_tdp_mmu_page(struct kvm *kvm, u64 *pt,
 				cpu_relax();
 			}
 		} else {
+			/*
+			 * If the SPTE is not MMU-present, there is no backing
+			 * page associated with the SPTE and so no side effects
+			 * that need to be recorded, and exclusive ownership of
+			 * mmu_lock ensures the SPTE can't be made present.
+			 * Note, zapping MMIO SPTEs is also unnecessary as they
+			 * are guarded by the memslots generation, not by being
+			 * unreachable.
+			 */
 			old_child_spte = READ_ONCE(*sptep);
+			if (!is_shadow_present_pte(old_child_spte))
+				continue;
 
 			/*
 			 * Marking the SPTE as a removed SPTE is not