KVM: x86/mmu: Clear dirtied pages mask bit before early break

In clear_dirty_pt_masked, the loop is intended to exit early after
processing each of the GFNs with corresponding bits set in mask. This
does not work as intended if another thread has already cleared the
dirty bit or writable bit on the SPTE. In that case, the loop would
proceed to the next iteration early and the bit in mask would not be
cleared. As a result the loop could not exit early and would proceed
uselessly. Move the unsetting of the mask bit before the check for a
no-op SPTE change.

Fixes: a6a0b05d ("kvm: x86/mmu: Support dirty logging for the TDP
MMU")
Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Ben Gardon <bgardon@google.com>
Message-Id: <20210202185734.1680553-17-bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1007,6 +1007,8 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
 		    !(mask & (1UL << (iter.gfn - gfn))))
 			continue;
 
+		mask &= ~(1UL << (iter.gfn - gfn));
+
 		if (wrprot || spte_ad_need_write_protect(iter.old_spte)) {
 			if (is_writable_pte(iter.old_spte))
 				new_spte = iter.old_spte & ~PT_WRITABLE_MASK;
@@ -1020,8 +1022,6 @@ static void clear_dirty_pt_masked(struct kvm *kvm, struct kvm_mmu_page *root,
 		}
 
 		tdp_mmu_set_spte_no_dirty_log(kvm, &iter, new_spte);
-
-		mask &= ~(1UL << (iter.gfn - gfn));
 	}
 }