diff --git a/Tethering/tests/privileged/src/com/android/networkstack/tethering/BpfMapTest.java b/Tethering/tests/privileged/src/com/android/networkstack/tethering/BpfMapTest.java
index cf31a19..6d14f31 100644
--- a/Tethering/tests/privileged/src/com/android/networkstack/tethering/BpfMapTest.java
+++ b/Tethering/tests/privileged/src/com/android/networkstack/tethering/BpfMapTest.java
@@ -25,6 +25,7 @@
 import static org.junit.Assert.assertNotEquals;
 import static org.junit.Assert.assertNotNull;
 import static org.junit.Assert.assertNull;
+import static org.junit.Assert.assertThrows;
 import static org.junit.Assert.assertTrue;
 import static org.junit.Assert.fail;
 
@@ -36,6 +37,7 @@
 import android.util.ArrayMap;
 
 import com.android.net.module.util.BpfMap;
+import com.android.net.module.util.SingleWriterBpfMap;
 import com.android.testutils.DevSdkIgnoreRule.IgnoreUpTo;
 import com.android.testutils.DevSdkIgnoreRunner;
 
@@ -43,11 +45,14 @@
 import org.junit.BeforeClass;
 import org.junit.Test;
 import org.junit.runner.RunWith;
+import org.junit.runners.Parameterized;
 
 import java.io.File;
 import java.net.InetAddress;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Arrays;
+import java.util.Collection;
 import java.util.NoSuchElementException;
 import java.util.concurrent.atomic.AtomicInteger;
 
@@ -64,6 +69,17 @@
 
     private BpfMap<TetherDownstream6Key, Tether6Value> mTestMap;
 
+    private final boolean mShouldTestSingleWriterMap;
+
+    @Parameterized.Parameters
+    public static Collection<Boolean> shouldTestSingleWriterMap() {
+        return Arrays.asList(true, false);
+    }
+
+    public BpfMapTest(boolean shouldTestSingleWriterMap) {
+        mShouldTestSingleWriterMap = shouldTestSingleWriterMap;
+    }
+
     @BeforeClass
     public static void setupOnce() {
         System.loadLibrary(getTetheringJniLibraryName());
@@ -85,11 +101,16 @@
         initTestMap();
     }
 
-    private void initTestMap() throws Exception {
-        mTestMap = new BpfMap<>(
-                TETHER_DOWNSTREAM6_FS_PATH,
-                TetherDownstream6Key.class, Tether6Value.class);
+    private BpfMap<TetherDownstream6Key, Tether6Value> openTestMap() throws Exception {
+        return mShouldTestSingleWriterMap
+                ? new SingleWriterBpfMap<>(TETHER_DOWNSTREAM6_FS_PATH, TetherDownstream6Key.class,
+                Tether6Value.class)
+                : new BpfMap<>(TETHER_DOWNSTREAM6_FS_PATH, TetherDownstream6Key.class,
+                        Tether6Value.class);
+    }
 
+    private void initTestMap() throws Exception {
+        mTestMap = openTestMap();
         mTestMap.forEach((key, value) -> {
             try {
                 assertTrue(mTestMap.deleteEntry(key));
@@ -360,6 +381,25 @@
     }
 
     @Test
+    public void testMapContentsCorrectOnOpen() throws Exception {
+        final BpfMap<TetherDownstream6Key, Tether6Value> map1, map2;
+
+        map1 = openTestMap();
+        map1.clear();
+        for (int i = 0; i < mTestData.size(); i++) {
+            map1.insertEntry(mTestData.keyAt(i), mTestData.valueAt(i));
+        }
+
+        // We can't close and reopen map1, because close does nothing. Open another map instead.
+        map2 = openTestMap();
+        for (int i = 0; i < mTestData.size(); i++) {
+            assertEquals(mTestData.valueAt(i), map2.getValue(mTestData.keyAt(i)));
+        }
+
+        map1.clear();
+    }
+
+    @Test
     public void testInsertOverflow() throws Exception {
         final ArrayMap<TetherDownstream6Key, Tether6Value> testData =
                 new ArrayMap<>();
@@ -432,4 +472,10 @@
         // Check that the number of open fds is the same as before.
         assertEquals("Fd leak after " + iterations + " iterations: ", before, after);
     }
+
+    @Test
+    public void testNullKey() {
+        assertThrows(NullPointerException.class, () ->
+                mTestMap.insertOrReplaceEntry(null, mTestData.valueAt(0)));
+    }
 }
diff --git a/staticlibs/Android.bp b/staticlibs/Android.bp
index ede6d3f..e2834b0 100644
--- a/staticlibs/Android.bp
+++ b/staticlibs/Android.bp
@@ -135,6 +135,7 @@
         "device/com/android/net/module/util/BpfUtils.java",
         "device/com/android/net/module/util/IBpfMap.java",
         "device/com/android/net/module/util/JniUtil.java",
+        "device/com/android/net/module/util/SingleWriterBpfMap.java",
         "device/com/android/net/module/util/TcUtils.java",
     ],
     sdk_version: "module_current",
diff --git a/staticlibs/device/com/android/net/module/util/SingleWriterBpfMap.java b/staticlibs/device/com/android/net/module/util/SingleWriterBpfMap.java
new file mode 100644
index 0000000..3eb59d8
--- /dev/null
+++ b/staticlibs/device/com/android/net/module/util/SingleWriterBpfMap.java
@@ -0,0 +1,129 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.net.module.util;
+
+import android.os.Build;
+import android.system.ErrnoException;
+
+import androidx.annotation.NonNull;
+import androidx.annotation.RequiresApi;
+
+import java.util.HashMap;
+import java.util.NoSuchElementException;
+
+/**
+ * Subclass of BpfMap for maps that are only ever written by one userspace writer.
+ *
+ * This class stores all map data in a userspace HashMap in addition to in the BPF map. This makes
+ * reads (but not iterations) much faster because they do not require a system call or converting
+ * the raw map read to the Value struct. See, e.g., b/343166906 .
+ *
+ * Users of this class must ensure that no BPF program ever writes to the map, and that all
+ * userspace writes to the map occur through this object. Other userspace code may still read from
+ * the map; only writes are required to go through this object.
+ *
+ * Reads and writes to this object are thread-safe and internally synchronized. The read and write
+ * methods are synchronized to ensure that current writers always result in a consistent internal
+ * state (without synchronization, two concurrent writes might update the underlying map and the
+ * cache in the opposite order, resulting in the cache being out of sync with the map).
+ *
+ * getNextKey and iteration over the map are not synchronized or cached and always access the
+ * isunderlying map. The values returned by these calls may be temporarily out of sync with the
+ * values read and written through this object.
+ *
+ * TODO: consider caching reads on iterations as well. This is not trivial because the semantics for
+ * iterating BPF maps require passing in the previously-returned key, and Java iterators only
+ * support iterating from the beginning. It could be done by implementing forEach and possibly by
+ * making getFirstKey and getNextKey private (if no callers are using them). Because HashMap is not
+ * thread-safe, implementing forEach would require either making that method synchronized (and
+ * block reads and updates from other threads until iteration is complete) or switching the
+ * internal HashMap to ConcurrentHashMap.
+ *
+ * @param <K> the key of the map.
+ * @param <V> the value of the map.
+ */
+@RequiresApi(Build.VERSION_CODES.S)
+public class SingleWriterBpfMap<K extends Struct, V extends Struct> extends BpfMap<K, V> {
+    // HashMap instead of ArrayMap because it performs better on larger maps, and many maps used in
+    // our code can contain hundreds of items.
+    private final HashMap<K, V> mCache = new HashMap<>();
+
+    protected SingleWriterBpfMap(@NonNull final String path, final int flag, final Class<K> key,
+            final Class<V> value) throws ErrnoException, NullPointerException {
+        super(path, flag, key, value);
+
+        if (flag != BPF_F_RDWR) {
+            throw new IllegalArgumentException(
+                    "Using " + getClass().getName() + " for read-only maps does not make sense");
+        }
+
+        // Populate cache with the current map contents.
+        K currentKey = super.getFirstKey();
+        while (currentKey != null) {
+            mCache.put(currentKey, super.getValue(currentKey));
+            currentKey = super.getNextKey(currentKey);
+        }
+    }
+
+    public SingleWriterBpfMap(@NonNull final String path, final Class<K> key,
+            final Class<V> value) throws ErrnoException, NullPointerException {
+        this(path, BPF_F_RDWR, key, value);
+    }
+
+    @Override
+    public synchronized void updateEntry(K key, V value) throws ErrnoException {
+        super.updateEntry(key, value);
+        mCache.put(key, value);
+    }
+
+    @Override
+    public synchronized void insertEntry(K key, V value)
+            throws ErrnoException, IllegalStateException {
+        super.insertEntry(key, value);
+        mCache.put(key, value);
+    }
+
+    @Override
+    public synchronized void replaceEntry(K key, V value)
+            throws ErrnoException, NoSuchElementException {
+        super.replaceEntry(key, value);
+        mCache.put(key, value);
+    }
+
+    @Override
+    public synchronized boolean insertOrReplaceEntry(K key, V value) throws ErrnoException {
+        final boolean ret = super.insertOrReplaceEntry(key, value);
+        mCache.put(key, value);
+        return ret;
+    }
+
+    @Override
+    public synchronized boolean deleteEntry(K key) throws ErrnoException {
+        final boolean ret = super.deleteEntry(key);
+        mCache.remove(key);
+        return ret;
+    }
+
+    @Override
+    public synchronized boolean containsKey(@NonNull K key) throws ErrnoException {
+        return mCache.containsKey(key);
+    }
+
+    @Override
+    public synchronized V getValue(@NonNull K key) throws ErrnoException {
+        return mCache.get(key);
+    }
+}