diff --git a/Cargo.lock b/Cargo.lock
index 3f5d294b2f..3e6649aa34 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -285,6 +285,12 @@ version = "1.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "ae44d1a3d5a19df61dd0c8beb138458ac2a53a7ac09eba97d55592540004306b"
 
+[[package]]
+name = "bytes"
+version = "1.0.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "b700ce4376041dcd0a327fd0097c41095743c4c8af8887265942faf1100bd040"
+
 [[package]]
 name = "cap-fs-ext"
 version = "0.13.7"
@@ -2819,6 +2825,8 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "83f0c8e7c0addab50b663055baf787d0af7f413a46e6e7fb9559a4e4db7137a5"
 dependencies = [
  "autocfg 1.0.1",
+ "bytes",
+ "memchr",
  "num_cpus",
  "pin-project-lite",
  "tokio-macros",
diff --git a/crates/wasi-common/tokio/Cargo.toml b/crates/wasi-common/tokio/Cargo.toml
index 13c5b23281..f480de0635 100644
--- a/crates/wasi-common/tokio/Cargo.toml
+++ b/crates/wasi-common/tokio/Cargo.toml
@@ -15,7 +15,7 @@ include = ["src/**/*", "LICENSE" ]
 wasi-common = { path = "../", version = "0.26.0" }
 wasi-cap-std-sync = { path = "../cap-std-sync", version = "0.26.0" }
 wiggle = { path = "../../wiggle", version = "0.26.0" }
-tokio = { version = "1.5.0", features = [ "rt", "fs", "time" ] }
+tokio = { version = "1.5.0", features = [ "rt", "fs", "time" , "io-util"] }
 cap-std = "0.13.7"
 cap-fs-ext = "0.13.7"
 cap-time-ext = "0.13.7"
diff --git a/crates/wasi-common/tokio/src/file.rs b/crates/wasi-common/tokio/src/file.rs
index 209e1eab15..fa63fe8b09 100644
--- a/crates/wasi-common/tokio/src/file.rs
+++ b/crates/wasi-common/tokio/src/file.rs
@@ -12,56 +12,52 @@ use wasi_common::{
 };
 
 mod internal {
+    use std::sync::{Mutex, MutexGuard};
     #[cfg(not(windows))]
     use unsafe_io::os::posish::{AsRawFd, RawFd};
     #[cfg(windows)]
     use unsafe_io::os::windows::{AsRawHandleOrSocket, RawHandleOrSocket};
     use unsafe_io::OwnsRaw;
-    use wasi_common::Error;
 
     // This internal type wraps tokio's File so that we can impl the
     // `AsUnsafeFile` trait. We impl this on an internal type, rather than on
     // super::File, because we don't want consumers of this library to be able
     // to use our `AsUnsafeFile`.
-    pub(super) struct Internal(pub(super) tokio::fs::File);
+    // Mutex is required because this type requires internal mutation for the
+    // tokio AsyncWriteExt methods to work, and must be Send.
+    pub(super) struct Internal(Mutex<tokio::fs::File>);
+    impl Internal {
+        pub fn new(f: tokio::fs::File) -> Self {
+            Internal(Mutex::new(f))
+        }
+        pub fn inner(&self) -> MutexGuard<tokio::fs::File> {
+            self.0.lock().unwrap()
+        }
+    }
 
     #[cfg(not(windows))]
     impl AsRawFd for Internal {
         fn as_raw_fd(&self) -> RawFd {
-            self.0.as_raw_fd()
+            self.inner().as_raw_fd()
         }
     }
 
     #[cfg(windows)]
     impl AsRawHandleOrSocket for Internal {
         fn as_raw_handle_or_socket(&self) -> RawHandleOrSocket {
-            self.0.as_raw_handle_or_socket()
+            self.inner().as_raw_handle_or_socket()
         }
     }
 
     // Safety: `Internal` owns its handle.
     unsafe impl OwnsRaw for Internal {}
-
-    // Tokio provides implementations of these methods, which are not
-    // available via AsUnsafeFile.
-    impl Internal {
-        pub async fn set_len(&self, size: u64) -> Result<(), Error> {
-            Ok(self.0.set_len(size).await?)
-        }
-        pub async fn sync_data(&self) -> Result<(), Error> {
-            Ok(self.0.sync_data().await?)
-        }
-        pub async fn sync_all(&self) -> Result<(), Error> {
-            Ok(self.0.sync_all().await?)
-        }
-    }
 }
 
 pub struct File(internal::Internal);
 
 impl File {
     pub fn from_cap_std(file: cap_std::fs::File) -> Self {
-        File(internal::Internal(tokio::fs::File::from_std(
+        File(internal::Internal::new(tokio::fs::File::from_std(
             file.into_std(),
         )))
     }
@@ -78,10 +74,12 @@ impl WasiFile for File {
         self
     }
     async fn datasync(&self) -> Result<(), Error> {
-        self.0.sync_data().await
+        self.0.inner().sync_data().await?;
+        Ok(())
     }
     async fn sync(&self) -> Result<(), Error> {
-        self.0.sync_all().await
+        self.0.inner().sync_all().await?;
+        Ok(())
     }
     async fn get_filetype(&self) -> Result<FileType, Error> {
         let meta = self.metadata().await?;
@@ -116,8 +114,7 @@ impl WasiFile for File {
         })
     }
     async fn set_filestat_size(&self, size: u64) -> Result<(), Error> {
-        // Tokio asyncified this already:
-        self.0.set_len(size).await?;
+        self.0.inner().set_len(size).await?;
         Ok(())
     }
     async fn advise(&self, offset: u64, len: u64, advice: Advice) -> Result<(), Error> {
@@ -141,9 +138,17 @@ impl WasiFile for File {
         Ok(())
     }
     async fn read_vectored<'a>(&self, bufs: &mut [io::IoSliceMut<'a>]) -> Result<u64, Error> {
-        // XXX use tokio's AsyncReadExt trait instead!
-        let n = self.0.read_vectored(bufs)?;
-        Ok(n.try_into()?)
+        use std::ops::DerefMut;
+        use tokio::io::AsyncReadExt;
+        let mut nbytes: usize = 0;
+        for b in bufs.iter_mut() {
+            let n = self.0.inner().read(b.deref_mut()).await?;
+            nbytes += n;
+            if n < b.len() {
+                break;
+            }
+        }
+        Ok(nbytes.try_into()?)
     }
     async fn read_vectored_at<'a>(
         &self,
@@ -154,8 +159,9 @@ impl WasiFile for File {
         Ok(n.try_into()?)
     }
     async fn write_vectored<'a>(&self, bufs: &[io::IoSlice<'a>]) -> Result<u64, Error> {
-        // XXX use tokio's AsyncWriteExt trait instead!
-        let n = self.0.write_vectored(bufs)?;
+        use tokio::io::AsyncWriteExt;
+        let mut n: usize = 0;
+        n += self.0.inner().write_vectored(bufs).await?;
         Ok(n.try_into()?)
     }
     async fn write_vectored_at<'a>(
@@ -201,23 +207,6 @@ pub fn filetype_from(ft: &cap_std::fs::FileType) -> FileType {
     }
 }
 
-#[cfg(windows)]
-use std::os::windows::io::{AsRawHandle, RawHandle};
-#[cfg(windows)]
-impl AsRawHandle for File {
-    fn as_raw_handle(&self) -> RawHandle {
-        self.0.as_raw_handle()
-    }
-}
-
-#[cfg(unix)]
-use std::os::unix::io::{AsRawFd, RawFd};
-#[cfg(unix)]
-impl AsRawFd for File {
-    fn as_raw_fd(&self) -> RawFd {
-        self.0.as_raw_fd()
-    }
-}
 pub fn convert_systimespec(t: Option<wasi_common::SystemTimeSpec>) -> Option<SystemTimeSpec> {
     match t {
         Some(wasi_common::SystemTimeSpec::Absolute(t)) => {
diff --git a/crates/wasi-common/tokio/src/lib.rs b/crates/wasi-common/tokio/src/lib.rs
index be03c21216..7ae8849259 100644
--- a/crates/wasi-common/tokio/src/lib.rs
+++ b/crates/wasi-common/tokio/src/lib.rs
@@ -85,6 +85,7 @@ impl WasiCtxBuilder {
     pub fn stderr(self, f: Box<dyn wasi_common::WasiFile>) -> Self {
         WasiCtxBuilder(self.0.stderr(f))
     }
+    // XXX our crate needs its own stdios
     pub fn inherit_stdin(self) -> Self {
         self.stdin(Box::new(wasi_cap_std_sync::stdio::stdin()))
     }
@@ -102,7 +103,7 @@ impl WasiCtxBuilder {
         dir: Dir,
         guest_path: impl AsRef<Path>,
     ) -> Result<Self, wasi_common::Error> {
-        let dir = Box::new(wasi_cap_std_sync::dir::Dir::from_cap_std(dir));
+        let dir = Box::new(crate::dir::Dir::from_cap_std(dir));
         Ok(WasiCtxBuilder(self.0.preopened_dir(dir, guest_path)?))
     }
     pub fn build(self) -> Result<WasiCtx, wasi_common::Error> {