Date:   Wed,  5 Apr 2023 17:14:16 -0300
From:   Daniel Almeida <daniel.almeida@...labora.com>
To:     wedsonaf@...il.com, ojeda@...nel.org
Cc:     Daniel Almeida <daniel.almeida@...labora.com>,
        rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: [PATCH v2 2/2] rust: virtio: add virtio support

This patch adds virtIO support to the rust crate. This includes the
capability to create a virtIO driver (through the module_virtio_driver
macro and the respective Driver trait) as well as initial virtqueue
support.

A sample virtIO module is included for conveninence.

Signed-off-by: Daniel Almeida <daniel.almeida@...labora.com>
---
 rust/bindings/bindings_helper.h |   3 +
 rust/helpers.c                  |  25 +++
 rust/kernel/lib.rs              |   2 +
 rust/kernel/virtio.rs           | 261 ++++++++++++++++++++++++++++++++
 rust/kernel/virtio/virtqueue.rs | 126 +++++++++++++++
 samples/rust/Kconfig            |  10 ++
 samples/rust/Makefile           |   1 +
 samples/rust/rust_virtio.rs     | 195 ++++++++++++++++++++++++
 8 files changed, 623 insertions(+)
 create mode 100644 rust/kernel/virtio.rs
 create mode 100644 rust/kernel/virtio/virtqueue.rs
 create mode 100644 samples/rust/rust_virtio.rs

diff --git a/rust/bindings/bindings_helper.h b/rust/bindings/bindings_helper.h
index 048bae2197ac..001a492e0554 100644
--- a/rust/bindings/bindings_helper.h
+++ b/rust/bindings/bindings_helper.h
@@ -35,6 +35,9 @@
 #include <linux/sysctl.h>
 #include <linux/uaccess.h>
 #include <linux/uio.h>
+#include <linux/virtio_config.h>
+#include <linux/virtio_ids.h>
+#include <linux/virtio.h>
 
 /* `bindgen` gets confused at certain things. */
 const gfp_t BINDINGS_GFP_KERNEL = GFP_KERNEL;
diff --git a/rust/helpers.c b/rust/helpers.c
index bf790f46c763..a80b0ab8ca64 100644
--- a/rust/helpers.c
+++ b/rust/helpers.c
@@ -39,6 +39,8 @@
 #include <linux/skbuff.h>
 #include <linux/uaccess.h>
 #include <linux/uio.h>
+#include <linux/virtio.h>
+#include <linux/virtio_config.h>
 
 __noreturn void rust_helper_BUG(void)
 {
@@ -655,6 +657,29 @@ int rust_helper_fs_parse(struct fs_context *fc,
 }
 EXPORT_SYMBOL_GPL(rust_helper_fs_parse);
 
+struct virtqueue *rust_helper_virtio_find_single_vq(struct virtio_device *vdev,
+						    vq_callback_t *c,
+						    const char *n)
+{
+	return virtio_find_single_vq(vdev, c, n);
+}
+
+EXPORT_SYMBOL_GPL(rust_helper_virtio_find_single_vq);
+
+void rust_helper_virtio_del_vqs(struct virtio_device *vdev)
+{
+	vdev->config->del_vqs(vdev);
+}
+
+EXPORT_SYMBOL_GPL(rust_helper_virtio_del_vqs);
+
+void rust_helper_virtio_device_ready(struct virtio_device *vdev)
+{
+	virtio_device_ready(vdev);
+}
+
+EXPORT_SYMBOL_GPL(rust_helper_virtio_device_ready);
+
 /*
  * We use `bindgen`'s `--size_t-is-usize` option to bind the C `size_t` type
  * as the Rust `usize` type, so we can use it in contexts where Rust
diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index b23be69919cc..5441b545c5f2 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -74,6 +74,8 @@ pub mod revocable;
 pub mod scatterlist;
 pub mod security;
 pub mod task;
+#[cfg(CONFIG_VIRTIO)]
+pub mod virtio;
 pub mod workqueue;
 
 pub mod linked_list;
diff --git a/rust/kernel/virtio.rs b/rust/kernel/virtio.rs
new file mode 100644
index 000000000000..a95a32605728
--- /dev/null
+++ b/rust/kernel/virtio.rs
@@ -0,0 +1,261 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Virtio abstractions
+//!
+//! C header: [`include/linux/virtio.h`](../../../../include/media/virtio.h)
+
+use crate::{
+    device, driver,
+    error::{self, from_kernel_result},
+    prelude::*,
+    str::CStr,
+    to_result, ForeignOwnable, ThisModule,
+};
+
+pub mod virtqueue;
+
+/// A VirtIO Device ID entry. Safe wrapper over `struct virtio_device_id`.
+#[derive(Copy, Clone, Debug, PartialEq, PartialOrd)]
+pub struct DeviceId {
+    /// The device ID in the virtIO protocol.
+    pub device: u32,
+    /// The vendor ID, usually set to VIRTIO_DEV_ANY_ID.
+    pub vendor: u32,
+}
+
+// SAFETY: we ensure that `RawDeviceId::ZERO` is actually a zeroed out version of
+// the raw device id. Offset is not needed, because there is no `data` pointer.
+unsafe impl const crate::driver::RawDeviceId for DeviceId {
+    type RawType = bindings::virtio_device_id;
+
+    const ZERO: Self::RawType = bindings::virtio_device_id {
+        device: 0,
+        vendor: 0,
+    };
+
+    fn to_rawid(&self, _offset: isize) -> Self::RawType {
+        bindings::virtio_device_id {
+            device: self.device,
+            vendor: self.vendor,
+        }
+    }
+}
+
+/// Defines a VirtIO ID table consisting of [`DeviceId`] entries.
+#[macro_export]
+macro_rules! define_virtio_id_table {
+    ($data_type:ty, $($t:tt)*) => {
+        $crate::define_id_table!(ID_TABLE, $crate::virtio::DeviceId, $data_type, $($t)*);
+    };
+}
+
+/// A registration of a virtio driver.
+pub type Registration<T> = driver::Registration<Adapter<T>>;
+
+/// An adapter for the registration of virtio drivers.
+pub struct Adapter<T: Driver>(T);
+
+impl<T: Driver> driver::DriverOps for Adapter<T> {
+    type RegType = bindings::virtio_driver;
+
+    unsafe fn register(
+        reg: *mut bindings::virtio_driver,
+        name: &'static CStr,
+        _module: &'static ThisModule,
+    ) -> Result {
+        // SAFETY: By the safety requirements of this function `reg` is non-null
+        // and valid.
+        let virtio_driver = unsafe { &mut *reg };
+
+        virtio_driver.driver.name = name.as_char_ptr();
+        virtio_driver.probe = Some(Self::probe_callback);
+        virtio_driver.remove = Some(Self::remove_callback);
+        if let Some(t) = T::ID_TABLE {
+            virtio_driver.id_table = t.as_ref();
+        }
+
+        to_result(unsafe { bindings::register_virtio_driver(reg) })
+    }
+
+    unsafe fn unregister(reg: *mut bindings::virtio_driver) {
+        // SAFETY: By the safety requirements of this function `reg` was passed
+        // (and updated) by a previous successful call to
+        // `register_virtio_driver`.
+        unsafe { bindings::unregister_virtio_driver(reg) };
+    }
+}
+
+impl<T: Driver> Adapter<T> {
+    extern "C" fn probe_callback(virtio_device: *mut bindings::virtio_device) -> core::ffi::c_int {
+        from_kernel_result! {
+            // SAFETY: `virtio_device` is valid by the contract with the C code.
+            // `dev` is alive only for the duration of this call, so it is
+            // guaranteed to remain alive for the lifetime of `virtio_device`.
+            let mut dev = unsafe { Device::from_ptr(virtio_device) };
+            if let Some(data) = T::probe(&mut dev)? {
+                // SAFETY: `virtio_device` is passed in by the kernel and
+                // assumed to be valid.
+                unsafe { (*virtio_device).priv_ = data.into_foreign() as _ }
+            }
+
+            // Either `probe()` or `device_ready()` must pass Self::Data, as we
+            // assume `virtio_device.priv_` is set to a valid object throughout
+            // the code.
+            // SAFETY: `virtio_device` is passed in by the kernel and
+            // assumed to be valid.
+            if !(unsafe { *virtio_device }).priv_.is_null() {
+                Ok(0)
+            } else {
+                Err(EINVAL)
+            }
+        }
+    }
+
+    // Note, this type is a wrapper over a kernel pointer, meaning that we
+    // cannot be sure that the pointee is actually going away when the wrapper
+    // is dropped. Thus, release resources here instead, since the kernel will
+    // call this when the actual pointee is to be removed.
+    extern "C" fn remove_callback(virtio_device: *mut bindings::virtio_device) {
+        // SAFETY: `virtio_device` is guaranteed to be a valid, non-null
+        // pointer. `priv_` was set on `probe()`.
+        let ptr = unsafe { (*virtio_device).priv_ };
+        // SAFETY:
+        // - We allocated this pointer using `T::Data::into_foreign`, so it is
+        //   safe to turn back into a `T::Data`.
+        // - The allocation happened in probe()`, no-one freed the memory and
+        //   `remove` is the canonical kernel location to free driver data. So,
+        //   it is OK to convert the pointer back to a Rust structure here.
+        let data = unsafe { T::Data::from_foreign(ptr) };
+        T::remove(&data);
+
+        // SAFETY: just a couple of FFI call with a valid pointer.
+        unsafe {
+            bindings::virtio_del_vqs(virtio_device);
+            bindings::virtio_reset_device(virtio_device);
+        }
+        <T::Data as driver::DeviceRemoval>::device_remove(&data);
+    }
+}
+
+/// A virtio driver.
+pub trait Driver {
+    /// Data stored on device by driver in `virtio_device.priv`
+    type Data: ForeignOwnable + Send + Sync + driver::DeviceRemoval = ();
+
+    /// The table of [`DeviceId`]s supported by the driver.
+    const ID_TABLE: Option<driver::IdTable<'static, DeviceId, ()>> = None;
+    /// The virtio features offered. This depends on the specifics of the given
+    /// VirtIO protocol for the device and can be empty.
+    const FEATURES: &'static [i32] = &[];
+
+    /// VirtIO driver probe.
+    ///
+    /// Called when a new virtio device is added or discovered.
+    /// Implementers should attempt to initialize the device here.
+    fn probe(dev: &mut Device) -> Result<Option<Self::Data>>;
+
+    /// VirtIO driver remove.
+    ///
+    /// Called when a virtio device is removed.
+    /// Implementers should prepare the device for complete removal here.
+    ///
+    /// In particular, implementers must ensure no buffers are left over in the
+    /// virtqueues in use by calling [`virtqueue::get_buf()`] until `None` is
+    /// returned.
+    fn remove(_data: &Self::Data) {}
+}
+
+/// A VirtIO device.
+///
+/// # Invariants
+///
+/// The field `ptr` is non-null and valid for the lifetime of the object.
+pub struct Device {
+    ptr: *mut bindings::virtio_device,
+}
+
+impl Device {
+    /// Creates a new device from the given pointer.
+    ///
+    /// # Safety
+    ///
+    /// `ptr` must be non-null and valid. It must remain valid for the lifetime of the returned
+    /// instance.
+    pub unsafe fn from_ptr(ptr: *mut bindings::virtio_device) -> Self {
+        // INVARIANT: The safety requirements of the function ensure the lifetime invariant.
+        Self { ptr }
+    }
+
+    /// Returns id of the VirtIO device.
+    pub fn id(&self) -> i32 {
+        // SAFETY: By the type invariants, we know that `self.ptr` is non-null and valid.
+        unsafe { (*self.ptr).id.device as _ }
+    }
+
+    /// Returns the `Virtqueue` associated with `name`, if any.
+    pub fn find_virtqueue<T: virtqueue::Operations>(
+        &self,
+        name: &CStr,
+    ) -> Result<virtqueue::Virtqueue<T>> {
+        let callback = if T::HAS_CALLBACK {
+            Some(virtqueue::Virtqueue::<T>::vq_callback as _)
+        } else {
+            None
+        };
+
+        // SAFETY: ptr is valid due to the type invariant, callback points to a
+        // valid address and the pointer passed in is passed in by the kernel,
+        // so it is assumed to be valid.
+        let vq = unsafe { bindings::virtio_find_single_vq(self.ptr, callback, name.as_char_ptr()) };
+        let vq = error::from_kernel_err_ptr(vq)?;
+
+        // SAFETY: vq is checked with `from_kernel_err_ptr`
+        Ok(unsafe { virtqueue::Virtqueue::from_ptr(vq) })
+    }
+
+    /// Returns the Virtio Device's private data.
+    pub fn data<T: Driver>(&self) -> <T::Data as ForeignOwnable>::Borrowed<'_> {
+        unsafe { <T::Data as ForeignOwnable>::borrow((*self.ptr).priv_) }
+    }
+
+    /// Returns a pointer to the [`bindings::virtio_device`] associated with
+    /// this [`Device`].
+    pub fn raw_device(&self) -> *mut bindings::virtio_device {
+        self.ptr
+    }
+
+    /// Wrapper over the C-side `virtio_device_ready`. Enable virtqueue use in
+    /// the probe function. Drivers must call this to use virtqueues in the
+    /// probe function.
+    ///
+    /// `data` can be optionally be set through `device_ready()` instead of
+    /// `probe()`.
+    pub fn device_ready<T: Driver>(&self, data: Option<T::Data>) {
+        if let Some(data) = data {
+            // SAFETY: `virtio_device` is passed in by the kernel and
+            // assumed to be valid.
+            unsafe { (*self.ptr).priv_ = data.into_foreign() as _ }
+        }
+
+        // SAFETY: just a FFI call on a valid pointer.
+        unsafe {
+            bindings::virtio_device_ready(self.ptr);
+        }
+    }
+}
+
+// SAFETY: The device returned by `raw_device` is the raw virtio device.
+unsafe impl device::RawDevice for Device {
+    fn raw_device(&self) -> *mut bindings::device {
+        // SAFETY: By the type invariants, we know that `self.ptr` is non-null and valid.
+        unsafe { &mut (*self.ptr).dev }
+    }
+}
+
+/// Declares a kernel module that exposes a single virtio driver.
+#[macro_export]
+macro_rules! module_virtio_driver {
+    ($($f:tt)*) => {
+        $crate::module_driver!(<T>, $crate::virtio::Adapter<T>, { $($f)* });
+    };
+}
diff --git a/rust/kernel/virtio/virtqueue.rs b/rust/kernel/virtio/virtqueue.rs
new file mode 100644
index 000000000000..a735f3d997a9
--- /dev/null
+++ b/rust/kernel/virtio/virtqueue.rs
@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Virtqueue abstractions.
+//!
+//! C header: [`include/linux/virtio.h`](../../../../include/linux/virtio.h)
+
+use core::marker::PhantomData;
+
+use crate::error;
+use crate::prelude::*;
+use crate::scatterlist::Scatterlist;
+use crate::virtio;
+use crate::ForeignOwnable;
+
+/// The Virtqueue's operations.
+#[vtable]
+pub trait Operations: Sized {
+    /// Data passed into the callback.
+    type PrivateData: ForeignOwnable;
+
+    /// The virtqueue callback.
+    fn callback(virtqueue: Virtqueue<Self>);
+}
+
+/// A Virtqueue abstraction.
+pub struct Virtqueue<T: Operations> {
+    /// The underlying pointer.
+    ptr: *mut bindings::virtqueue,
+    /// A Phantom so T gets used.
+    phantom: PhantomData<T>,
+}
+
+impl<T: Operations> Virtqueue<T> {
+    /// # Safety
+    /// The caller must ensure that `ptr` is valid and remains valid for the lifetime of the
+    /// returned [`Virtqueue`] instance.
+    ///
+    /// Caller must ensure that get_buf() returns None before the Virtqueue is
+    /// dropped, so that ownership of the passed in T::PrivateData can be
+    /// retrieved.
+    pub unsafe fn from_ptr(ptr: *mut bindings::virtqueue) -> Self {
+        Self {
+            ptr,
+            phantom: PhantomData,
+        }
+    }
+
+    /// A wrapper over the C side `virtqueue_add_sgs()`. Adds scatterlists to
+    /// the [`Virtqueue`] for processing.
+    pub fn add_sgs(
+        &mut self,
+        out_sgs: &mut [Scatterlist],
+        in_sgs: &mut [Scatterlist],
+        data: T::PrivateData,
+        gfp: u32,
+    ) -> Result {
+        let mut sgs = Vec::new();
+
+        let num_out = out_sgs.len();
+        let num_in = in_sgs.len();
+
+        for s in out_sgs {
+            sgs.try_push(s.inner().get())?;
+        }
+        for s in in_sgs {
+            sgs.try_push(s.inner().get())?;
+        }
+
+        // SAFETY: `self.ptr` is valid as per the type invariant.
+        let res = unsafe {
+            bindings::virtqueue_add_sgs(
+                self.ptr,
+                sgs.as_mut_ptr(),
+                num_out as u32,
+                num_in as u32,
+                data.into_foreign() as _,
+                gfp,
+            )
+        };
+
+        error::to_result(res)
+    }
+
+    /// A wrapper over the C side `virtqueue_kick()`. Notifies the VirtIO device
+    /// of activity in this [`Virtqueue`].
+    pub fn kick(&mut self) -> Result {
+        let res = unsafe { bindings::virtqueue_kick(self.ptr) };
+        error::to_result(res as i32)
+    }
+
+    /// Returns the underlying [`virtio::Device`].
+    pub fn virtio_device(&self) -> virtio::Device {
+        // SAFETY: The virtio framework places a pointer to the VirtIO device
+        // when creating the virtqueue
+        unsafe { virtio::Device::from_ptr((*self.ptr).vdev) }
+    }
+
+    /// A wrapper over the C side `virtqueue_get_buf()`. Returns the next used
+    /// buffer, if any, alongside with the amount of data written.
+    pub fn get_buf(&mut self) -> Option<(T::PrivateData, u32)> {
+        let mut len = 0;
+
+        let data = unsafe {
+            let buf = bindings::virtqueue_get_buf(self.ptr, &mut len as _);
+
+            if buf.is_null() {
+                return None;
+            }
+
+            // SAFETY: if there is a buffer token, it came from
+            // `into_foreign()` as called in `add_sgs()`.
+            <T::PrivateData as ForeignOwnable>::from_foreign(buf)
+        };
+
+        Some((data, len))
+    }
+
+    pub(crate) unsafe extern "C" fn vq_callback(vq: *mut bindings::virtqueue) {
+        // SAFETY: the caller should guarantee that `vq` is valid for the lifetime
+        // of `Self`.
+        let virtqueue = unsafe { Self::from_ptr(vq) };
+        T::callback(virtqueue);
+    }
+}
+
+unsafe impl<T: Operations> Send for Virtqueue<T> {}
diff --git a/samples/rust/Kconfig b/samples/rust/Kconfig
index 189c10ced6d4..0507c57170ec 100644
--- a/samples/rust/Kconfig
+++ b/samples/rust/Kconfig
@@ -163,4 +163,14 @@ config SAMPLE_RUST_SELFTESTS
 
 	  If unsure, say N.
 
+config SAMPLE_RUST_VIRTIO
+	tristate "Virtio"
+	help
+	  This option builds the Rust virtio module sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_minimal.
+
+	  If unsure, say N.
+
 endif # SAMPLES_RUST
diff --git a/samples/rust/Makefile b/samples/rust/Makefile
index 420bcefeb082..24059795480f 100644
--- a/samples/rust/Makefile
+++ b/samples/rust/Makefile
@@ -15,5 +15,6 @@ obj-$(CONFIG_SAMPLE_RUST_NETFILTER)		+= rust_netfilter.o
 obj-$(CONFIG_SAMPLE_RUST_ECHO_SERVER)		+= rust_echo_server.o
 obj-$(CONFIG_SAMPLE_RUST_FS)			+= rust_fs.o
 obj-$(CONFIG_SAMPLE_RUST_SELFTESTS)		+= rust_selftests.o
+obj-$(CONFIG_SAMPLE_RUST_VIRTIO)		+= rust_virtio.o
 
 subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS)		+= hostprogs
diff --git a/samples/rust/rust_virtio.rs b/samples/rust/rust_virtio.rs
new file mode 100644
index 000000000000..fddab8674909
--- /dev/null
+++ b/samples/rust/rust_virtio.rs
@@ -0,0 +1,195 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust virtio device driver sample. Retrieves a random number from the guest
+//! using VirtIO Entropy.
+
+// impl_self_work_adapter!() does not compile otherwise.
+#![allow(unreachable_code)]
+
+use kernel::bindings;
+use kernel::module_virtio_driver;
+use kernel::prelude::*;
+use kernel::scatterlist;
+use kernel::sync::smutex::Mutex;
+use kernel::sync::Arc;
+use kernel::sync::UniqueArc;
+use kernel::virtio::virtqueue;
+use kernel::virtio::virtqueue::Virtqueue;
+use kernel::workqueue;
+use kernel::Opaque;
+
+module_virtio_driver! {
+    type: Driver,
+    name: "rust_virtio",
+    license: "GPL",
+}
+
+struct AckWork {
+    work: workqueue::Work,
+    raw_dev: *mut kernel::bindings::virtio_device,
+}
+
+impl AckWork {
+    pub(crate) fn new(raw_dev: *mut kernel::bindings::virtio_device) -> Result<Arc<Self>> {
+        let work = UniqueArc::try_new(Self {
+            // SAFETY: init_work_item!() is called below
+            work: unsafe { workqueue::Work::new() },
+            raw_dev,
+        })?;
+
+        kernel::init_work_item!(&work);
+
+        Ok(work.into())
+    }
+}
+
+kernel::impl_self_work_adapter!(AckWork, work, |w| {
+    // SAFETY:
+    //
+    // If the work still exists, the driver data still exists (as it is its
+    // sole owner).
+    //
+    // If the driver data still exisits, then the underlying kernel::Device
+    // is alive (as it holds a reference obtained from get_device())
+    //
+    // If the contained struct device* is alive, the outermost struct
+    // virtio_device* is presumed alive and valid.
+    //
+    // Other virtio drivers (in C) just use container_of here as they embed
+    // the work struct in their driver struct itself.
+    let virtio_dev = unsafe { kernel::virtio::Device::from_ptr(w.raw_dev) };
+    let virtio_device_data = virtio_dev.data::<Driver>();
+
+    let mut vq = virtio_device_data.vq.lock();
+
+    if let Some(resp) = vq.get_buf() {
+        pr_crit!("Retrieving response, host wrote {} bytes", resp.1);
+        // SAFETY: Just a FFI call on a completion that was previously
+        // initialized with `init_completion`.
+        unsafe { bindings::complete(resp.0.completion.get()) }
+    }
+});
+
+// SAFETY: the fields are not exposed. Nothing can be done.
+unsafe impl Send for AckWork {}
+unsafe impl Sync for AckWork {}
+
+struct Request {
+    inner: Mutex<RequestInner>,
+    completion: Opaque<bindings::completion>,
+}
+
+struct RequestInner {
+    data: [u8; 64],
+}
+
+fn send_request(
+    vq: &Mutex<Virtqueue<VirtqueueCallback>>,
+    request: <VirtqueueCallback as virtqueue::Operations>::PrivateData,
+) -> Result {
+    let mut inner_guard = request.inner.lock();
+    // SAFETY: inner_guard.data points to an owned array, buflen is the size of
+    // the array.
+    let mut in_sgs = unsafe {
+        [scatterlist::Scatterlist::init_one(
+            inner_guard.data.as_mut_ptr() as *mut core::ffi::c_void,
+            core::mem::size_of_val(&inner_guard.data) as u32,
+        )]
+    };
+
+    // SAFETY: Just a FFI call on a valid bindgen type.
+    unsafe { bindings::init_completion(request.completion.get()) }
+
+    {
+        let mut vq = vq.lock();
+        let mut out_sgs = [];
+        vq.add_sgs(
+            &mut out_sgs[..],
+            &mut in_sgs[..],
+            Arc::clone(&request),
+            bindings::GFP_KERNEL,
+        )?;
+
+        vq.kick()?;
+    }
+
+    pr_crit!("Request sent, waiting");
+
+    // SAFETY: Just a FFI call on a valid bindgen type that was previously
+    // passed to `init_completion`.
+    unsafe { bindings::wait_for_completion(request.completion.get()) }
+
+    pr_crit!("Request completed, response was {:?}", inner_guard.data);
+    Ok(())
+}
+
+struct Driver {
+    /// The virtqueue.
+    vq: Mutex<Virtqueue<VirtqueueCallback>>,
+    /// The ack work that is awaken by the ack callback.
+    work: Arc<AckWork>,
+}
+
+impl kernel::driver::DeviceRemoval for Driver {
+    fn device_remove(&self) {
+        let mut vq = self.vq.lock();
+        while let Some(_buf) = vq.get_buf() {
+            // Actual drivers should take care to retrieve any left over buffers
+            // so `T::PrivateData` can drop.
+        }
+    }
+}
+
+impl kernel::virtio::Driver for Driver {
+    type Data = Arc<Self>;
+
+    kernel::define_virtio_id_table! {(), [
+         (kernel::virtio::DeviceId {
+            device: bindings::VIRTIO_ID_RNG,
+            vendor: bindings::VIRTIO_DEV_ANY_ID,
+         }, None),
+    ]}
+
+    fn probe(virtio_dev: &mut kernel::virtio::Device) -> Result<Option<Self::Data>> {
+        pr_info!("rust_virtio: probing");
+
+        let vq = virtio_dev.find_virtqueue::<VirtqueueCallback>(kernel::c_str!("input"))?;
+        let vq = Mutex::new(vq);
+
+        let work = AckWork::new(virtio_dev.raw_device())?;
+        let data = Arc::try_new(Self { vq, work })?;
+
+        // We want to use the virtqueues here in this example.
+        virtio_dev.device_ready::<Self>(Some(Arc::clone(&data)));
+
+        let request = Arc::try_new(Request {
+            inner: Mutex::new(RequestInner { data: [0; 64] }),
+            completion: Opaque::new(Default::default()),
+        })?;
+
+        send_request(&data.vq, Arc::clone(&request))?;
+
+        pr_info!("rust_virtio probed succesfully!");
+
+        Ok(None)
+    }
+}
+
+pub(crate) struct VirtqueueCallback;
+
+#[vtable]
+impl virtqueue::Operations for VirtqueueCallback {
+    type PrivateData = Arc<Request>;
+
+    fn callback(vq: Virtqueue<Self>) {
+        let vdev = vq.virtio_device();
+        let data = vdev.data::<Driver>();
+        let data = &*data;
+
+        pr_info!("virtqueue callback called successfully!");
+
+        // Enqueue work in order to retrieve the data and complete the
+        // completion.
+        kernel::workqueue::system().enqueue(Arc::clone(&data.work));
+    }
+}
-- 
2.40.0

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