KVM: MMU: flush tlb out of mmu lock when write-protect the sptes

Now we can flush all the TLBs out of the mmu lock without TLB corruption when
write-proect the sptes, it is because:
- we have marked large sptes readonly instead of dropping them that means we
  just change the spte from writable to readonly so that we only need to care
  the case of changing spte from present to present (changing the spte from
  present to nonpresent will flush all the TLBs immediately), in other words,
  the only case we need to care is mmu_spte_update()

- in mmu_spte_update(), we haved checked
  SPTE_HOST_WRITEABLE | PTE_MMU_WRITEABLE instead of PT_WRITABLE_MASK, that
  means it does not depend on PT_WRITABLE_MASK anymore

Acked-by: Marcelo Tosatti <mtosatti@redhat.com>
Signed-off-by: Xiao Guangrong <xiaoguangrong@linux.vnet.ibm.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

arch/x86/kvm/mmu.c

@@ -4309,15 +4309,32 @@ void kvm_mmu_slot_remove_write_access(struct kvm *kvm, int slot)
 			if (*rmapp)
 				__rmap_write_protect(kvm, rmapp, false);
 
-			if (need_resched() || spin_needbreak(&kvm->mmu_lock)) {
-				kvm_flush_remote_tlbs(kvm);
+			if (need_resched() || spin_needbreak(&kvm->mmu_lock))
 				cond_resched_lock(&kvm->mmu_lock);
-			}
 		}
 	}
 
-	kvm_flush_remote_tlbs(kvm);
 	spin_unlock(&kvm->mmu_lock);
+
+	/*
+	 * kvm_mmu_slot_remove_write_access() and kvm_vm_ioctl_get_dirty_log()
+	 * which do tlb flush out of mmu-lock should be serialized by
+	 * kvm->slots_lock otherwise tlb flush would be missed.
+	 */
+	lockdep_assert_held(&kvm->slots_lock);
+
+	/*
+	 * We can flush all the TLBs out of the mmu lock without TLB
+	 * corruption since we just change the spte from writable to
+	 * readonly so that we only need to care the case of changing
+	 * spte from present to present (changing the spte from present
+	 * to nonpresent will flush all the TLBs immediately), in other
+	 * words, the only case we care is mmu_spte_update() where we
+	 * haved checked SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE
+	 * instead of PT_WRITABLE_MASK, that means it does not depend
+	 * on PT_WRITABLE_MASK anymore.
+	 */
+	kvm_flush_remote_tlbs(kvm);
 }
 
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