KVM: x86/mmu: Avoid unnecessary page table allocation in kvm_tdp_mmu_map()

In kvm_tdp_mmu_map(), while iterating TDP MMU page table entries, it is
possible SPTE has already been frozen by another thread but the frozen
is not done yet, for instance, when another thread is still in middle of
zapping large page.  In this case, the !is_shadow_present_pte() check
for old SPTE in tdp_mmu_for_each_pte() may hit true, and in this case
allocating new page table is unnecessary since tdp_mmu_set_spte_atomic()
later will return false and page table will need to be freed.  Add
is_removed_spte() check before allocating new page table to avoid this.
Signed-off-by: Kai Huang <kai.huang@intel.com>
Message-Id: <20210429041226.50279-1-kai.huang@intel.com>
Reviewed-by: Ben Gardon <bgardon@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/mmu/tdp_mmu.c

@@ -1009,6 +1009,14 @@ int kvm_tdp_mmu_map(struct kvm_vcpu *vcpu, gpa_t gpa, u32 error_code,
 		}
 
 		if (!is_shadow_present_pte(iter.old_spte)) {
+			/*
+			 * If SPTE has been forzen by another thread, just
+			 * give up and retry, avoiding unnecessary page table
+			 * allocation and free.
+			 */
+			if (is_removed_spte(iter.old_spte))
+				break;
+
 			sp = alloc_tdp_mmu_page(vcpu, iter.gfn, iter.level);
 			child_pt = sp->spt;