cloud-hypervisor/vm-virtio/src/vsock/device.rs

// Copyright 2019 Intel Corporation. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0
//
// Portions Copyright 2018 Amazon.com, Inc. or its affiliates. All Rights Reserved.
// SPDX-License-Identifier: Apache-2.0
//
// Portions Copyright 2017 The Chromium OS Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the THIRD-PARTY file.

/// This is the `VirtioDevice` implementation for our vsock device. It handles the virtio-level
/// device logic: feature negociation, device configuration, and device activation.
/// The run-time device logic (i.e. event-driven data handling) is implemented by
/// `super::epoll_handler::EpollHandler`.
///
/// We aim to conform to the VirtIO v1.1 spec:
/// https://docs.oasis-open.org/virtio/virtio/v1.1/virtio-v1.1.html
///
/// The vsock device has two input parameters: a CID to identify the device, and a `VsockBackend`
/// to use for offloading vsock traffic.
///
/// Upon its activation, the vsock device creates its `EpollHandler`, passes it the event-interested
/// file descriptors, and registers these descriptors with the VMM `EpollContext`. Going forward,
/// the `EpollHandler` will get notified whenever an event occurs on the just-registered FDs:
/// - an RX queue FD;
/// - a TX queue FD;
/// - an event queue FD; and
/// - a backend FD.
///
use epoll;
use libc::EFD_NONBLOCK;
use std;
use std::io;
use std::os::unix::io::AsRawFd;
use std::result;
use std::sync::{Arc, RwLock};
use std::thread;

use super::{VsockBackend, VsockPacket};
use crate::Error as DeviceError;
use crate::VirtioInterrupt;
use crate::{
    ActivateError, ActivateResult, DeviceEventT, Queue, VirtioDevice, VirtioDeviceType,
    VirtioInterruptType, VIRTIO_F_IN_ORDER, VIRTIO_F_VERSION_1,
};
use byteorder::{ByteOrder, LittleEndian};
use vm_memory::GuestMemoryMmap;
use vmm_sys_util::eventfd::EventFd;

const QUEUE_SIZE: u16 = 256;
const NUM_QUEUES: usize = 3;
const QUEUE_SIZES: &[u16] = &[QUEUE_SIZE; NUM_QUEUES];

// New descriptors are pending on the rx queue.
pub const RX_QUEUE_EVENT: DeviceEventT = 0;
// New descriptors are pending on the tx queue.
pub const TX_QUEUE_EVENT: DeviceEventT = 1;
// New descriptors are pending on the event queue.
pub const EVT_QUEUE_EVENT: DeviceEventT = 2;
// Notification coming from the backend.
pub const BACKEND_EVENT: DeviceEventT = 3;
// The device has been dropped.
pub const KILL_EVENT: DeviceEventT = 4;
pub const EVENTS_LEN: usize = 5;

/// The `VsockEpollHandler` implements the runtime logic of our vsock device:
/// 1. Respond to TX queue events by wrapping virtio buffers into `VsockPacket`s, then sending those
///    packets to the `VsockBackend`;
/// 2. Forward backend FD event notifications to the `VsockBackend`;
/// 3. Fetch incoming packets from the `VsockBackend` and place them into the virtio RX queue;
/// 4. Whenever we have processed some virtio buffers (either TX or RX), let the driver know by
///    raising our assigned IRQ.
///
/// In a nutshell, the `VsockEpollHandler` logic looks like this:
/// - on TX queue event:
///   - fetch all packets from the TX queue and send them to the backend; then
///   - if the backend has queued up any incoming packets, fetch them into any available RX buffers.
/// - on RX queue event:
///   - fetch any incoming packets, queued up by the backend, into newly available RX buffers.
/// - on backend event:
///   - forward the event to the backend; then
///   - again, attempt to fetch any incoming packets queued by the backend into virtio RX buffers.
///
pub struct VsockEpollHandler<B: VsockBackend> {
    pub mem: Arc<RwLock<GuestMemoryMmap>>,
    pub queues: Vec<Queue>,
    pub queue_evts: Vec<EventFd>,
    pub kill_evt: EventFd,
    pub interrupt_cb: Arc<VirtioInterrupt>,
    pub backend: B,
}

impl<B> VsockEpollHandler<B>
where
    B: VsockBackend,
{
    /// Signal the guest driver that we've used some virtio buffers that it had previously made
    /// available.
    ///
    fn signal_used_queue(&self, queue: &Queue) -> result::Result<(), DeviceError> {
        debug!("vsock: raising IRQ");

        (self.interrupt_cb)(&VirtioInterruptType::Queue, Some(queue)).map_err(|e| {
            error!("Failed to signal used queue: {:?}", e);
            DeviceError::FailedSignalingUsedQueue(e)
        })
    }

    /// Walk the driver-provided RX queue buffers and attempt to fill them up with any data that we
    /// have pending.
    ///
    fn process_rx(&mut self) -> result::Result<(), DeviceError> {
        debug!("vsock: epoll_handler::process_rx()");

        let mut used_desc_heads = [(0, 0); QUEUE_SIZE as usize];
        let mut used_count = 0;
        let mem = self.mem.read().unwrap();
        for avail_desc in self.queues[0].iter(&mem) {
            let used_len = match VsockPacket::from_rx_virtq_head(&avail_desc) {
                Ok(mut pkt) => {
                    if self.backend.recv_pkt(&mut pkt).is_ok() {
                        pkt.hdr().len() as u32 + pkt.len()
                    } else {
                        // We are using a consuming iterator over the virtio buffers, so, if we can't
                        // fill in this buffer, we'll need to undo the last iterator step.
                        self.queues[0].go_to_previous_position();
                        break;
                    }
                }
                Err(e) => {
                    warn!("vsock: RX queue error: {:?}", e);
                    0
                }
            };

            used_desc_heads[used_count] = (avail_desc.index, used_len);
            used_count += 1;
        }

        for &(desc_index, len) in &used_desc_heads[..used_count] {
            self.queues[0].add_used(&mem, desc_index, len);
        }

        if used_count > 0 {
            self.signal_used_queue(&self.queues[0])
        } else {
            Ok(())
        }
    }

    /// Walk the driver-provided TX queue buffers, package them up as vsock packets, and send them to
    /// the backend for processing.
    ///
    fn process_tx(&mut self) -> result::Result<(), DeviceError> {
        debug!("vsock: epoll_handler::process_tx()");

        let mut used_desc_heads = [(0, 0); QUEUE_SIZE as usize];
        let mut used_count = 0;
        let mem = self.mem.read().unwrap();
        for avail_desc in self.queues[1].iter(&mem) {
            let pkt = match VsockPacket::from_tx_virtq_head(&avail_desc) {
                Ok(pkt) => pkt,
                Err(e) => {
                    error!("vsock: error reading TX packet: {:?}", e);
                    used_desc_heads[used_count] = (avail_desc.index, 0);
                    used_count += 1;
                    continue;
                }
            };

            if self.backend.send_pkt(&pkt).is_err() {
                self.queues[1].go_to_previous_position();
                break;
            }

            used_desc_heads[used_count] = (avail_desc.index, 0);
            used_count += 1;
        }

        for &(desc_index, len) in &used_desc_heads[..used_count] {
            self.queues[1].add_used(&mem, desc_index, len);
        }

        if used_count > 0 {
            self.signal_used_queue(&self.queues[1])
        } else {
            Ok(())
        }
    }

    fn run(&mut self) -> result::Result<(), DeviceError> {
        // Create the epoll file descriptor
        let epoll_fd = epoll::create(true).map_err(DeviceError::EpollCreateFd)?;

        // Add events
        epoll::ctl(
            epoll_fd,
            epoll::ControlOptions::EPOLL_CTL_ADD,
            self.queue_evts[0].as_raw_fd(),
            epoll::Event::new(epoll::Events::EPOLLIN, u64::from(RX_QUEUE_EVENT)),
        )
        .map_err(DeviceError::EpollCtl)?;
        epoll::ctl(
            epoll_fd,
            epoll::ControlOptions::EPOLL_CTL_ADD,
            self.queue_evts[1].as_raw_fd(),
            epoll::Event::new(epoll::Events::EPOLLIN, u64::from(TX_QUEUE_EVENT)),
        )
        .map_err(DeviceError::EpollCtl)?;
        epoll::ctl(
            epoll_fd,
            epoll::ControlOptions::EPOLL_CTL_ADD,
            self.queue_evts[2].as_raw_fd(),
            epoll::Event::new(epoll::Events::EPOLLIN, u64::from(EVT_QUEUE_EVENT)),
        )
        .map_err(DeviceError::EpollCtl)?;
        epoll::ctl(
            epoll_fd,
            epoll::ControlOptions::EPOLL_CTL_ADD,
            self.backend.get_polled_fd(),
            epoll::Event::new(self.backend.get_polled_evset(), u64::from(BACKEND_EVENT)),
        )
        .map_err(DeviceError::EpollCtl)?;
        epoll::ctl(
            epoll_fd,
            epoll::ControlOptions::EPOLL_CTL_ADD,
            self.kill_evt.as_raw_fd(),
            epoll::Event::new(epoll::Events::EPOLLIN, u64::from(KILL_EVENT)),
        )
        .map_err(DeviceError::EpollCtl)?;

        let mut events = vec![epoll::Event::new(epoll::Events::empty(), 0); EVENTS_LEN];

        'epoll: loop {
            let num_events = match epoll::wait(epoll_fd, -1, &mut events[..]) {
                Ok(res) => res,
                Err(e) => {
                    if e.kind() == io::ErrorKind::Interrupted {
                        // It's well defined from the epoll_wait() syscall
                        // documentation that the epoll loop can be interrupted
                        // before any of the requested events occurred or the
                        // timeout expired. In both those cases, epoll_wait()
                        // returns an error of type EINTR, but this should not
                        // be considered as a regular error. Instead it is more
                        // appropriate to retry, by calling into epoll_wait().
                        continue;
                    }
                    return Err(DeviceError::EpollWait(e));
                }
            };

            for event in events.iter().take(num_events) {
                let evset = match epoll::Events::from_bits(event.events) {
                    Some(evset) => evset,
                    None => {
                        let evbits = event.events;
                        warn!("epoll: ignoring unknown event set: 0x{:x}", evbits);
                        continue;
                    }
                };

                let ev_type = event.data as DeviceEventT;

                if self.handle_event(ev_type, evset)? {
                    break 'epoll;
                }
            }
        }

        Ok(())
    }

    pub fn handle_event(
        &mut self,
        device_event: DeviceEventT,
        evset: epoll::Events,
    ) -> Result<bool, DeviceError> {
        match device_event {
            RX_QUEUE_EVENT => {
                debug!("vsock: RX queue event");
                if let Err(e) = self.queue_evts[0].read() {
                    error!("Failed to get RX queue event: {:?}", e);
                    return Err(DeviceError::FailedReadingQueue {
                        event_type: "rx queue event",
                        underlying: e,
                    });
                } else if self.backend.has_pending_rx() {
                    self.process_rx()?;
                }
            }
            TX_QUEUE_EVENT => {
                debug!("vsock: TX queue event");
                if let Err(e) = self.queue_evts[1].read() {
                    error!("Failed to get TX queue event: {:?}", e);
                    return Err(DeviceError::FailedReadingQueue {
                        event_type: "tx queue event",
                        underlying: e,
                    });
                } else {
                    self.process_tx()?;
                    // The backend may have queued up responses to the packets we sent during TX queue
                    // processing. If that happened, we need to fetch those responses and place them
                    // into RX buffers.
                    if self.backend.has_pending_rx() {
                        self.process_rx()?;
                    }
                }
            }
            EVT_QUEUE_EVENT => {
                debug!("vsock: EVT queue event");
                if let Err(e) = self.queue_evts[2].read() {
                    error!("Failed to get EVT queue event: {:?}", e);
                    return Err(DeviceError::FailedReadingQueue {
                        event_type: "evt queue event",
                        underlying: e,
                    });
                }
            }
            BACKEND_EVENT => {
                debug!("vsock: backend event");
                self.backend.notify(evset);
                // After the backend has been kicked, it might've freed up some resources, so we
                // can attempt to send it more data to process.
                // In particular, if `self.backend.send_pkt()` halted the TX queue processing (by
                // reurning an error) at some point in the past, now is the time to try walking the
                // TX queue again.
                self.process_tx()?;
                if self.backend.has_pending_rx() {
                    self.process_rx()?;
                }
            }
            KILL_EVENT => {
                debug!("KILL_EVENT received, stopping epoll loop");
                return Ok(true);
            }
            other => {
                error!("Unknown event for virtio-vsock");
                return Err(DeviceError::UnknownEvent {
                    device: "vsock",
                    event: other,
                });
            }
        }

        Ok(false)
    }
}

/// Virtio device exposing virtual socket to the guest.
pub struct Vsock<B: VsockBackend> {
    cid: u64,
    backend: Option<B>,
    kill_evt: Option<EventFd>,
    avail_features: u64,
    acked_features: u64,
}

impl<B> Vsock<B>
where
    B: VsockBackend,
{
    /// Create a new virtio-vsock device with the given VM CID and vsock
    /// backend.
    pub fn new(cid: u64, backend: B) -> io::Result<Vsock<B>> {
        let avail_features = 1u64 << VIRTIO_F_VERSION_1 | 1u64 << VIRTIO_F_IN_ORDER;

        Ok(Vsock {
            cid,
            backend: Some(backend),
            kill_evt: None,
            avail_features,
            acked_features: 0u64,
        })
    }
}

impl<B> Drop for Vsock<B>
where
    B: VsockBackend,
{
    fn drop(&mut self) {
        if let Some(kill_evt) = self.kill_evt.take() {
            // Ignore the result because there is nothing we can do about it.
            let _ = kill_evt.write(1);
        }
    }
}

impl<B> VirtioDevice for Vsock<B>
where
    B: VsockBackend + 'static,
{
    fn device_type(&self) -> u32 {
        VirtioDeviceType::TYPE_VSOCK as u32
    }

    fn queue_max_sizes(&self) -> &[u16] {
        QUEUE_SIZES
    }

    fn features(&self, page: u32) -> u32 {
        match page {
            // Get the lower 32-bits of the features bitfield.
            0 => self.avail_features as u32,
            // Get the upper 32-bits of the features bitfield.
            1 => (self.avail_features >> 32) as u32,
            _ => {
                warn!("Received request for unknown features page.");
                0u32
            }
        }
    }

    fn ack_features(&mut self, page: u32, value: u32) {
        let mut v = match page {
            0 => u64::from(value),
            1 => u64::from(value) << 32,
            _ => {
                warn!("Cannot acknowledge unknown features page.");
                0u64
            }
        };

        // Check if the guest is ACK'ing a feature that we didn't claim to have.
        let unrequested_features = v & !self.avail_features;
        if unrequested_features != 0 {
            warn!("Received acknowledge request for unknown feature.");

            // Don't count these features as acked.
            v &= !unrequested_features;
        }
        self.acked_features |= v;
    }

    fn read_config(&self, offset: u64, data: &mut [u8]) {
        match offset {
            0 if data.len() == 8 => LittleEndian::write_u64(data, self.cid),
            0 if data.len() == 4 => LittleEndian::write_u32(data, (self.cid & 0xffff_ffff) as u32),
            4 if data.len() == 4 => {
                LittleEndian::write_u32(data, ((self.cid >> 32) & 0xffff_ffff) as u32)
            }
            _ => warn!(
                "vsock: virtio-vsock received invalid read request of {} bytes at offset {}",
                data.len(),
                offset
            ),
        }
    }

    fn write_config(&mut self, offset: u64, data: &[u8]) {
        warn!(
            "vsock: guest driver attempted to write device config (offset={:x}, len={:x})",
            offset,
            data.len()
        );
    }

    fn activate(
        &mut self,
        mem: Arc<RwLock<GuestMemoryMmap>>,
        interrupt_cb: Arc<VirtioInterrupt>,
        queues: Vec<Queue>,
        queue_evts: Vec<EventFd>,
    ) -> ActivateResult {
        if queues.len() != NUM_QUEUES || queue_evts.len() != NUM_QUEUES {
            error!(
                "Cannot perform activate. Expected {} queue(s), got {}",
                NUM_QUEUES,
                queues.len()
            );
            return Err(ActivateError::BadActivate);
        }

        let (self_kill_evt, kill_evt) =
            match EventFd::new(EFD_NONBLOCK).and_then(|e| Ok((e.try_clone()?, e))) {
                Ok(v) => v,
                Err(e) => {
                    error!("failed creating kill EventFd pair: {}", e);
                    return Err(ActivateError::BadActivate);
                }
            };
        self.kill_evt = Some(self_kill_evt);

        let mut handler = VsockEpollHandler {
            mem,
            queues,
            queue_evts,
            kill_evt,
            interrupt_cb,
            backend: self.backend.take().unwrap(),
        };

        let worker_result = thread::Builder::new()
            .name("virtio_vsock".to_string())
            .spawn(move || handler.run());

        if let Err(e) = worker_result {
            error!("failed to spawn virtio_vsock worker: {}", e);
            return Err(ActivateError::BadActivate);;
        }

        Ok(())
    }
}

#[cfg(test)]
mod tests {
    use super::super::tests::TestContext;
    use super::super::*;
    use super::*;
    use crate::vsock::device::{BACKEND_EVENT, EVT_QUEUE_EVENT, RX_QUEUE_EVENT, TX_QUEUE_EVENT};

    #[test]
    fn test_virtio_device() {
        let mut ctx = TestContext::new();
        let avail_features = 1u64 << VIRTIO_F_VERSION_1 | 1u64 << VIRTIO_F_IN_ORDER;
        let device_features = avail_features;
        let driver_features: u64 = avail_features | 1 | (1 << 32);
        let device_pages = [
            (device_features & 0xffff_ffff) as u32,
            (device_features >> 32) as u32,
        ];
        let driver_pages = [
            (driver_features & 0xffff_ffff) as u32,
            (driver_features >> 32) as u32,
        ];
        assert_eq!(
            ctx.device.device_type(),
            VirtioDeviceType::TYPE_VSOCK as u32
        );
        assert_eq!(ctx.device.queue_max_sizes(), QUEUE_SIZES);
        assert_eq!(ctx.device.features(0), device_pages[0]);
        assert_eq!(ctx.device.features(1), device_pages[1]);
        assert_eq!(ctx.device.features(2), 0);

        // Ack device features, page 0.
        ctx.device.ack_features(0, driver_pages[0]);
        // Ack device features, page 1.
        ctx.device.ack_features(1, driver_pages[1]);
        // Ack some bogus page (i.e. 2). This should have no side effect.
        ctx.device.ack_features(2, 0);
        // Attempt to un-ack the first feature page. This should have no side effect.
        ctx.device.ack_features(0, !driver_pages[0]);
        // Check that no side effect are present, and that the acked features are exactly the same
        // as the device features.
        assert_eq!(ctx.device.acked_features, device_features & driver_features);

        // Test reading 32-bit chunks.
        let mut data = [0u8; 8];
        ctx.device.read_config(0, &mut data[..4]);
        assert_eq!(
            u64::from(LittleEndian::read_u32(&data)),
            ctx.cid & 0xffff_ffff
        );
        ctx.device.read_config(4, &mut data[4..]);
        assert_eq!(
            u64::from(LittleEndian::read_u32(&data[4..])),
            (ctx.cid >> 32) & 0xffff_ffff
        );

        // Test reading 64-bit.
        let mut data = [0u8; 8];
        ctx.device.read_config(0, &mut data);
        assert_eq!(LittleEndian::read_u64(&data), ctx.cid);

        // Check that out-of-bounds reading doesn't mutate the destination buffer.
        let mut data = [0u8, 1, 2, 3, 4, 5, 6, 7];
        ctx.device.read_config(2, &mut data);
        assert_eq!(data, [0u8, 1, 2, 3, 4, 5, 6, 7]);

        // Just covering lines here, since the vsock device has no writable config.
        // A warning is, however, logged, if the guest driver attempts to write any config data.
        ctx.device.write_config(0, &data[..4]);

        // Test a bad activation.
        let bad_activate = ctx.device.activate(
            Arc::new(RwLock::new(ctx.mem.clone())),
            Arc::new(
                Box::new(move |_: &VirtioInterruptType, _: Option<&Queue>| Ok(()))
                    as VirtioInterrupt,
            ),
            Vec::new(),
            Vec::new(),
        );
        match bad_activate {
            Err(ActivateError::BadActivate) => (),
            other => panic!("{:?}", other),
        }

        // Test a correct activation.
        ctx.device
            .activate(
                Arc::new(RwLock::new(ctx.mem.clone())),
                Arc::new(
                    Box::new(move |_: &VirtioInterruptType, _: Option<&Queue>| Ok(()))
                        as VirtioInterrupt,
                ),
                vec![Queue::new(256), Queue::new(256), Queue::new(256)],
                vec![
                    EventFd::new(EFD_NONBLOCK).unwrap(),
                    EventFd::new(EFD_NONBLOCK).unwrap(),
                    EventFd::new(EFD_NONBLOCK).unwrap(),
                ],
            )
            .unwrap();
    }

    #[test]
    fn test_irq() {
        // Test case: successful IRQ signaling.
        {
            let test_ctx = TestContext::new();
            let ctx = test_ctx.create_epoll_handler_context();

            let queue = Queue::new(256);
            assert!(ctx.handler.signal_used_queue(&queue).is_ok());
        }
    }

    #[test]
    fn test_txq_event() {
        // Test case:
        // - the driver has something to send (there's data in the TX queue); and
        // - the backend has no pending RX data.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(false);
            ctx.signal_txq_event();

            // The available TX descriptor should have been used.
            assert_eq!(ctx.guest_txvq.used.idx.get(), 1);
            // The available RX descriptor should be untouched.
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 0);
        }

        // Test case:
        // - the driver has something to send (there's data in the TX queue); and
        // - the backend also has some pending RX data.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(true);
            ctx.signal_txq_event();

            // Both available RX and TX descriptors should have been used.
            assert_eq!(ctx.guest_txvq.used.idx.get(), 1);
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 1);
        }

        // Test case:
        // - the driver has something to send (there's data in the TX queue); and
        // - the backend errors out and cannot process the TX queue.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(false);
            ctx.handler.backend.set_tx_err(Some(VsockError::NoData));
            ctx.signal_txq_event();

            // Both RX and TX queues should be untouched.
            assert_eq!(ctx.guest_txvq.used.idx.get(), 0);
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 0);
        }

        // Test case:
        // - the driver supplied a malformed TX buffer.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            // Invalidate the packet header descriptor, by setting its length to 0.
            ctx.guest_txvq.dtable[0].len.set(0);
            ctx.signal_txq_event();

            // The available descriptor should have been consumed, but no packet should have
            // reached the backend.
            assert_eq!(ctx.guest_txvq.used.idx.get(), 1);
            assert_eq!(ctx.handler.backend.tx_ok_cnt, 0);
        }

        // Test case: spurious TXQ_EVENT.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            match ctx
                .handler
                .handle_event(TX_QUEUE_EVENT, epoll::Events::EPOLLIN)
            {
                Err(DeviceError::FailedReadingQueue { .. }) => (),
                other => panic!("{:?}", other),
            }
        }
    }

    #[test]
    fn test_rxq_event() {
        // Test case:
        // - there is pending RX data in the backend; and
        // - the driver makes RX buffers available; and
        // - the backend successfully places its RX data into the queue.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(true);
            ctx.handler.backend.set_rx_err(Some(VsockError::NoData));
            ctx.signal_rxq_event();

            // The available RX buffer should've been left untouched.
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 0);
        }

        // Test case:
        // - there is pending RX data in the backend; and
        // - the driver makes RX buffers available; and
        // - the backend errors out, when attempting to receive data.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(true);
            ctx.signal_rxq_event();

            // The available RX buffer should have been used.
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 1);
        }

        // Test case: the driver provided a malformed RX descriptor chain.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            // Invalidate the packet header descriptor, by setting its length to 0.
            ctx.guest_rxvq.dtable[0].len.set(0);

            // The chain should've been processed, without employing the backend.
            assert!(ctx.handler.process_rx().is_ok());
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 1);
            assert_eq!(ctx.handler.backend.rx_ok_cnt, 0);
        }

        // Test case: spurious RXQ_EVENT.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();
            ctx.handler.backend.set_pending_rx(false);
            match ctx
                .handler
                .handle_event(RX_QUEUE_EVENT, epoll::Events::EPOLLIN)
            {
                Err(DeviceError::FailedReadingQueue { .. }) => (),
                other => panic!("{:?}", other),
            }
        }
    }

    #[test]
    fn test_evq_event() {
        // Test case: spurious EVQ_EVENT.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();
            ctx.handler.backend.set_pending_rx(false);
            match ctx
                .handler
                .handle_event(EVT_QUEUE_EVENT, epoll::Events::EPOLLIN)
            {
                Err(DeviceError::FailedReadingQueue { .. }) => (),
                other => panic!("{:?}", other),
            }
        }
    }

    #[test]
    fn test_backend_event() {
        // Test case:
        // - a backend event is received; and
        // - the backend has pending RX data.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(true);
            ctx.handler
                .handle_event(BACKEND_EVENT, epoll::Events::EPOLLIN)
                .unwrap();

            // The backend should've received this event.
            assert_eq!(ctx.handler.backend.evset, Some(epoll::Events::EPOLLIN));
            // TX queue processing should've been triggered.
            assert_eq!(ctx.guest_txvq.used.idx.get(), 1);
            // RX queue processing should've been triggered.
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 1);
        }

        // Test case:
        // - a backend event is received; and
        // - the backend doesn't have any pending RX data.
        {
            let test_ctx = TestContext::new();
            let mut ctx = test_ctx.create_epoll_handler_context();

            ctx.handler.backend.set_pending_rx(false);
            ctx.handler
                .handle_event(BACKEND_EVENT, epoll::Events::EPOLLIN)
                .unwrap();

            // The backend should've received this event.
            assert_eq!(ctx.handler.backend.evset, Some(epoll::Events::EPOLLIN));
            // TX queue processing should've been triggered.
            assert_eq!(ctx.guest_txvq.used.idx.get(), 1);
            // The RX queue should've been left untouched.
            assert_eq!(ctx.guest_rxvq.used.idx.get(), 0);
        }
    }

    #[test]
    fn test_unknown_event() {
        let test_ctx = TestContext::new();
        let mut ctx = test_ctx.create_epoll_handler_context();

        match ctx.handler.handle_event(0xff, epoll::Events::EPOLLIN) {
            Err(DeviceError::UnknownEvent { .. }) => (),
            other => panic!("{:?}", other),
        }
    }
}