/*
 * Copyright (C) 2014 Cloudius Systems, Ltd.
 */

#include "virtio.hh"
#include "core/posix.hh"
#include "core/future-util.hh"
#include "core/vla.hh"
#include "virtio-interface.hh"
#include "core/reactor.hh"
#include "core/stream.hh"
#include "core/circular_buffer.hh"
#include "core/align.hh"
#include "util/function_input_iterator.hh"
#include "util/transform_iterator.hh"
#include <atomic>
#include <vector>
#include <queue>
#include <fcntl.h>
#include <linux/vhost.h>
#include <linux/if_tun.h>
#include "ip.hh"
#include "const.hh"
#include "net/native-stack.hh"

#ifdef HAVE_OSV
#include <osv/virtio-assign.hh>
#endif

using namespace net;

namespace virtio {

using phys = uint64_t;

#ifndef HAVE_OSV

phys virt_to_phys(void* p) {
    return reinterpret_cast<uintptr_t>(p);
}

#else

phys virt_to_phys(void* p) {
    return osv::assigned_virtio::virt_to_phys(p);
}

#endif

class device : public net::device {
private:
    boost::program_options::variables_map _opts;
    net::hw_features _hw_features;
    uint64_t _features;

private:
    uint64_t setup_features() {
        int64_t seastar_supported_features = VIRTIO_RING_F_INDIRECT_DESC | VIRTIO_NET_F_MRG_RXBUF;

        if (!(_opts.count("event-index") && _opts["event-index"].as<std::string>() == "off")) {
            seastar_supported_features |= VIRTIO_RING_F_EVENT_IDX;
        }
        if (!(_opts.count("csum-offload") && _opts["csum-offload"].as<std::string>() == "off")) {
            seastar_supported_features |= VIRTIO_NET_F_CSUM | VIRTIO_NET_F_GUEST_CSUM;
            _hw_features.tx_csum_l4_offload = true;
            _hw_features.rx_csum_offload = true;
        } else {
            _hw_features.tx_csum_l4_offload = false;
            _hw_features.rx_csum_offload = false;
        }
        if (!(_opts.count("tso") && _opts["tso"].as<std::string>() == "off")) {
            seastar_supported_features |= VIRTIO_NET_F_HOST_TSO4;
            seastar_supported_features |= VIRTIO_NET_F_GUEST_TSO4;
            _hw_features.tx_tso = true;
        } else {
            _hw_features.tx_tso = false;
        }
        if (!(_opts.count("ufo") && _opts["ufo"].as<std::string>() == "off")) {
            seastar_supported_features |= VIRTIO_NET_F_HOST_UFO;
            seastar_supported_features |= VIRTIO_NET_F_GUEST_UFO;
            _hw_features.tx_ufo = true;
        } else {
            _hw_features.tx_ufo = false;
        }

        seastar_supported_features |= VIRTIO_NET_F_MAC;
        return seastar_supported_features;
    }

public:
    device(boost::program_options::variables_map opts)
       : _opts(opts), _features(setup_features())
       {}
    ethernet_address hw_address() override {
        return { 0x12, 0x23, 0x34, 0x56, 0x67, 0x78 };
    }

    net::hw_features hw_features() {
        return _hw_features;
    }

    uint64_t features() {
        return _features;
    }

    virtual std::unique_ptr<net::qp> init_local_queue(boost::program_options::variables_map opts, uint16_t qid) override;
};

/* The virtio_notifier class determines how to do host-to-guest and guest-to-
 * host notifications. We have two different implementations - one for vhost
 * (where both notifications occur through eventfds) and one for an assigned
 * virtio device from OSv.
 */
class notifier {
public:
    // Notify the host
    virtual void notify() = 0;
    // Do whatever it takes to wake wait(). A notifier does not need to
    // implement this function if wait() waits for an external even which is
    // generated by an external process (e.g., virtio_notifier_host doesn't
    // need to implement this).
    virtual void wake_wait() {
        abort();
    }
    virtual ~notifier() {
    }
};

class notifier_vhost : public notifier {
private:
    writeable_eventfd _kick;
public:
    virtual void notify() override {
        _kick.signal(1);
    }
    notifier_vhost(writeable_eventfd &&kick)
        : _kick(std::move(kick)) {}
};

#ifdef HAVE_OSV
class notifier_osv : public notifier {
private:
    uint16_t _q_index;
    osv::assigned_virtio &_virtio;
public:
    virtual void notify() override {
        _virtio.kick(_q_index);
    }
    virtual void wake_wait() override {
        _notified->signal();
    }
    notifier_osv(osv::assigned_virtio &virtio, uint16_t q_index)
        , _q_index(q_index)
        , _virtio(virtio)
    {
    }
};
#endif

struct ring_config {
    char* descs;
    char* avail;
    char* used;
    unsigned size;
    bool event_index;
    bool indirect;
    bool mergable_buffers;
};

struct buffer {
    phys addr;
    uint32_t len;
    bool writeable;
};

// The 'buffer_chain' concept, used in vring, is a container of buffers, as in:
//
//   using buffer_chain = std::vector<buffer>;
//
// The 'Completion' concept is a functor with the signature:
//
//     void (buffer_chain&, size_t len);
//
template <typename BufferChain, typename Completion>
class vring {
private:
    class desc {
    public:
        struct flags {
            // This marks a buffer as continuing via the next field.
            uint16_t has_next : 1;
            // This marks a buffer as write-only (otherwise read-only).
            uint16_t writeable : 1;
            // This means the buffer contains a list of buffer descriptors.
            uint16_t indirect : 1;
        };

        phys get_paddr();
        uint32_t get_len() { return _len; }
        uint16_t next_idx() { return _next; }

        phys _paddr;
        uint32_t _len;
        flags _flags;
        uint16_t _next;
    };

    // Guest to host
    struct avail_layout {
        struct flags {
            // Mark that we do not need an interrupt for consuming a descriptor
            // from the ring. Unreliable so it's simply an optimization
            uint16_t no_interrupts : 1;
        };

        std::atomic<uint16_t> _flags;

        // Where we put the next descriptor
        std::atomic<uint16_t> _idx;
        // There may be no more entries than the queue size read from device
        uint16_t _ring[];
        // used event index is an optimization in order to get an interrupt from the host
        // only when the value reaches this number
        // The location of this field is places after the variable length ring array,
        // that's why we cannot fully define it within the struct and use a function accessor
        //std::atomic<uint16_t> used_event;
    };

    struct used_elem {
        // Index of start of used _desc chain. (uint32_t for padding reasons)
        uint32_t _id;
        // Total length of the descriptor chain which was used (written to)
        uint32_t _len;
    };

    // Host to guest
    struct used_layout {
        enum {
            // The Host advise the Guest: don't kick me when
            // you add a buffer.  It's unreliable, so it's simply an
            // optimization. Guest will still kick if it's out of buffers.
            no_notify = 1
        };

        // Using std::atomic since it being changed by the host
        std::atomic<uint16_t> _flags;
        // Using std::atomic in order to have memory barriers for it
        std::atomic<uint16_t> _idx;
        used_elem _used_elements[];
        // avail event index is an optimization kick the host only when the value reaches this number
        // The location of this field is places after the variable length ring array,
        // that's why we cannot fully define it within the struct and use a function accessor
        //std::atomic<uint16_t> avail_event;
    };

    struct avail {
        explicit avail(ring_config conf);
        avail_layout* _shared;
        uint16_t _head = 0;
        uint16_t _avail_added_since_kick = 0;
    };
    struct used {
        explicit used(ring_config conf);
        used_layout* _shared;
        uint16_t _tail = 0;
    };
private:
    ring_config _config;
    Completion _complete;
    std::unique_ptr<notifier> _notifier;
    std::unique_ptr<BufferChain[]> _buffer_chains;
    desc* _descs;
    avail _avail;
    used _used;
    std::atomic<uint16_t>* _avail_event;
    std::atomic<uint16_t>* _used_event;
    semaphore _available_descriptors = { 0 };
    int _free_head = -1;
    int _free_last = -1;
    reactor::poller _poller;
public:

    explicit vring(ring_config conf, Completion complete);
    void set_notifier(std::unique_ptr<notifier> notifier) {
        _notifier = std::move(notifier);
    }
    const ring_config& getconfig() {
        return _config;
    }
    void wake_notifier_wait() {
        _notifier->wake_wait();
    }

    // start the queue
    void run();

    // wait for the used ring to have at least @nr buffers
    future<> on_used(size_t nr);

    // Total number of descriptors in ring
    int size() { return _config.size; }

    template <typename Iterator>
    void post(Iterator begin, Iterator end);

    semaphore& available_descriptors() { return _available_descriptors; }
private:
    bool notifications_disabled() {
        return (_used._shared->_flags.load(std::memory_order_relaxed) & VRING_USED_F_NO_NOTIFY) != 0;
    }

    void kick() {
        bool need_kick = true;
        // Make sure we see the fresh _idx value writen before kick.
        std::atomic_thread_fence(std::memory_order_seq_cst);
        if (_config.event_index) {
            uint16_t avail_idx = _avail._shared->_idx.load(std::memory_order_relaxed);
            uint16_t avail_event = _avail_event->load(std::memory_order_relaxed);
            need_kick = (uint16_t)(avail_idx - avail_event - 1) < _avail._avail_added_since_kick;
        } else {
            if (notifications_disabled())
                return;
        }
        if (need_kick || (_avail._avail_added_since_kick >= (uint16_t)(~0) / 2)) {
            _notifier->notify();
            _avail._avail_added_since_kick = 0;
        }
    }

    void do_complete();
    size_t mask() { return size() - 1; }
    size_t masked(size_t idx) { return idx & mask(); }
    size_t available();
    unsigned allocate_desc();
    void setup();
};

template <typename BufferChain, typename Completion>
vring<BufferChain, Completion>::avail::avail(ring_config conf)
    : _shared(reinterpret_cast<avail_layout*>(conf.avail)) {
}

template <typename BufferChain, typename Completion>
vring<BufferChain, Completion>::used::used(ring_config conf)
    : _shared(reinterpret_cast<used_layout*>(conf.used)) {
}

template <typename BufferChain, typename Completion>
inline
unsigned
vring<BufferChain, Completion>::allocate_desc() {
    assert(_free_head != -1);
    auto desc = _free_head;
    if (desc == _free_last) {
        _free_last = _free_head = -1;
    } else {
        _free_head = _descs[desc]._next;
    }
    return desc;
}

template <typename BufferChain, typename Completion>
vring<BufferChain, Completion>::vring(ring_config conf, Completion complete)
    : _config(conf)
    , _complete(complete)
    , _buffer_chains(new BufferChain[_config.size])
    , _descs(reinterpret_cast<desc*>(conf.descs))
    , _avail(conf)
    , _used(conf)
    , _avail_event(reinterpret_cast<std::atomic<uint16_t>*>(&_used._shared->_used_elements[conf.size]))
    , _used_event(reinterpret_cast<std::atomic<uint16_t>*>(&_avail._shared->_ring[conf.size]))
    , _poller([this] {
        do_complete();
        return true;
    })
{
    setup();
}

template <typename BufferChain, typename Completion>
void vring<BufferChain, Completion>::setup() {
    for (unsigned i = 0; i < _config.size; ++i) {
        _descs[i]._next = i + 1;
    }
    _free_head = 0;
    _free_last = _config.size - 1;
    _available_descriptors.signal(_config.size);
}

// Iterator: points at a buffer_chain
template <typename BufferChain, typename Completion>
template <typename Iterator>
void vring<BufferChain, Completion>::post(Iterator begin, Iterator end) {
    for (auto bci = begin; bci!= end; ++bci) {
        auto&& bc = *bci;
        desc pseudo_head = {};
        desc* prev = &pseudo_head;
        for (auto i = bc.begin(); i != bc.end(); ++i) {
            unsigned desc_idx = allocate_desc();
            prev->_flags.has_next = true;
            prev->_next = desc_idx;
            desc &d = _descs[desc_idx];
            d._flags = {};
            auto&& b = *i;
            d._flags.writeable = b.writeable;
            d._paddr = b.addr;
            d._len = b.len;
            prev = &d;
        }
        auto desc_head = pseudo_head._next;
        _buffer_chains[desc_head] = std::move(bc);
        _avail._shared->_ring[masked(_avail._head++)] = desc_head;
        _avail._avail_added_since_kick++;
    }
    _avail._shared->_idx.store(_avail._head, std::memory_order_release);
    kick();
}

template <typename BufferChain, typename Completion>
void vring<BufferChain, Completion>::do_complete() {
    auto used_head = _used._shared->_idx.load(std::memory_order_acquire);
    _complete.bunch(_used._tail - used_head);
    while (used_head != _used._tail) {
        auto ue = _used._shared->_used_elements[masked(_used._tail++)];
        _complete(std::move(_buffer_chains[ue._id]), ue._len);
        auto id = ue._id;
        if (_free_last != -1) {
            _descs[_free_last]._next = id;
        } else {
            _free_head = id;
        }
        while (true) {
            auto& d = _descs[id];
            if (!d._flags.has_next) {
                break;
            }
            id = d._next;
        }
        _free_last = id;
    }
}

class qp : public net::qp {
protected:
    struct net_hdr {
        uint8_t needs_csum : 1;
        uint8_t flags_reserved : 7;
        enum { gso_none = 0, gso_tcpv4 = 1, gso_udp = 3, gso_tcpv6 = 4, gso_ecn = 0x80 };
        uint8_t gso_type;
        uint16_t hdr_len;
        uint16_t gso_size;
        uint16_t csum_start;
        uint16_t csum_offset;
    };
    struct net_hdr_mrg : net_hdr {
        uint16_t num_buffers;
    };
    class txq {
        static buffer fragment_to_buffer(fragment f) {
            buffer b;
            b.addr = virt_to_phys(f.base);
            b.len = f.size;
            b.writeable = false;
            return b;
        };
        struct packet_as_buffer_chain {
            packet p;
            auto begin() {
                return make_transform_iterator(p.fragments().begin(), fragment_to_buffer);
            }
            auto end() {
                return make_transform_iterator(p.fragments().end(), fragment_to_buffer);
            }
        };
        struct complete {
            txq& q;
            void operator()(packet_as_buffer_chain&& bc, size_t len) {
                // move the packet here, to be destroyed on scope exit
                auto p = std::move(bc.p);
                q._ring.available_descriptors().signal(p.nr_frags());
            }
            void bunch(uint64_t c) {}
        };
        qp& _dev;
        vring<packet_as_buffer_chain, complete> _ring;
        std::vector<packet_as_buffer_chain> _packets;
    public:
        txq(qp& dev, ring_config config);
        void set_notifier(std::unique_ptr<notifier> notifier) {
            _ring.set_notifier(std::move(notifier));
        }
        const ring_config& getconfig() {
            return _ring.getconfig();
        }
        void wake_notifier_wait() {
            _ring.wake_notifier_wait();
        }
        uint32_t post(circular_buffer<packet>& p);
    };
    class rxq  {
        struct buffer_and_virt : buffer {
            std::unique_ptr<char[], free_deleter> buf;
        };
        using single_buffer = std::array<buffer_and_virt, 1>;
        struct complete {
            rxq& q;
            void operator()(single_buffer&& bc, size_t len) {
                q.complete_buffer(std::move(bc), len);
            }
            void bunch(uint64_t c) {
                q.update_rx_count(c);
            }
        };
        qp& _dev;
        vring<single_buffer, complete> _ring;
        unsigned _remaining_buffers = 0;
        std::vector<fragment> _fragments;
        std::vector<std::unique_ptr<char[], free_deleter>> _buffers;
    public:
        rxq(qp& _if, ring_config config);
        void set_notifier(std::unique_ptr<notifier> notifier) {
            _ring.set_notifier(std::move(notifier));
        }
        const ring_config& getconfig() {
            return _ring.getconfig();
        }
        void run() {
            keep_doing([this] { return prepare_buffers(); });
        }
        void wake_notifier_wait() {
            _ring.wake_notifier_wait();
        }
        void update_rx_count(uint64_t c) {
            _dev.update_rx_count(c);
        }
    private:
        future<> prepare_buffers();
        void complete_buffer(single_buffer&& b, size_t len);
    };
protected:
    device* _dev;
    size_t _header_len;
    std::unique_ptr<char[], free_deleter> _txq_storage;
    std::unique_ptr<char[], free_deleter> _rxq_storage;
    txq _txq;
    rxq _rxq;
protected:
    ring_config txq_config(size_t txq_ring_size);
    ring_config rxq_config(size_t rxq_ring_size);
    void common_config(ring_config& r);
    size_t vring_storage_size(size_t ring_size);
public:
    explicit qp(device* dev, size_t rx_ring_size, size_t tx_ring_size);
    virtual future<> send(packet p) override {
        abort();
    }
    virtual uint32_t send(circular_buffer<packet>& p) override;
    virtual void rx_start() override;
    friend class rxq;
};

qp::txq::txq(qp& dev, ring_config config)
    : _dev(dev), _ring(config, complete{*this}) {
}

uint32_t
qp::txq::post(circular_buffer<packet>& pb) {
    net_hdr_mrg vhdr = {};
    _packets.clear();

    while (!pb.empty() && pb.front().nr_frags() + 1 <= _ring.available_descriptors().current()) {
        auto p = std::move(pb.front());
        pb.pop_front();
        // Handle TCP checksum offload
        auto oi = p.offload_info();
        if (_dev._dev->hw_features().tx_csum_l4_offload) {
            auto eth_hdr_len = sizeof(eth_hdr);
            auto ip_hdr_len = oi.ip_hdr_len;
            auto mtu = _dev._dev->hw_features().mtu;
            if (oi.protocol == ip_protocol_num::tcp) {
                auto tcp_hdr_len = oi.tcp_hdr_len;
                if (oi.needs_csum) {
                    vhdr.needs_csum = 1;
                    vhdr.csum_start = eth_hdr_len + ip_hdr_len;
                    // TCP checksum filed's offset within the TCP header is 16 bytes
                    vhdr.csum_offset = 16;
                }
                if (_dev._dev->hw_features().tx_tso && p.len() > mtu + eth_hdr_len) {
                    // IPv4 TCP TSO
                    vhdr.gso_type = net_hdr::gso_tcpv4;
                    // Sum of Ethernet, IP and TCP header size
                    vhdr.hdr_len = eth_hdr_len + ip_hdr_len + tcp_hdr_len;
                    // Maximum segment size of packet after the offload
                    vhdr.gso_size = mtu - ip_hdr_len - tcp_hdr_len;
                }
            } else if (oi.protocol == ip_protocol_num::udp) {
                auto udp_hdr_len = oi.udp_hdr_len;
                if (oi.needs_csum) {
                    vhdr.needs_csum = 1;
                    vhdr.csum_start = eth_hdr_len + ip_hdr_len;
                    // UDP checksum filed's offset within the UDP header is 6 bytes
                    vhdr.csum_offset = 6;
                }
                if (_dev._dev->hw_features().tx_ufo && p.len() > mtu + eth_hdr_len) {
                    vhdr.gso_type = net_hdr::gso_udp;
                    vhdr.hdr_len = eth_hdr_len + ip_hdr_len + udp_hdr_len;
                    vhdr.gso_size = mtu - ip_hdr_len - udp_hdr_len;
                }
            }
        }
        // prepend virtio-net header
        packet q = packet(fragment{reinterpret_cast<char*>(&vhdr), _dev._header_len},
                std::move(p));
        auto fut = _ring.available_descriptors().wait(q.nr_frags());
        assert(fut.available()); // how it cannot?
        _packets.emplace_back(packet_as_buffer_chain{ std::move(q) });
    }
    _ring.post(_packets.begin(), _packets.end());
    return _packets.size();
}

qp::rxq::rxq(qp& dev, ring_config config)
    : _dev(dev), _ring(config, complete{*this}) {
}

future<>
qp::rxq::prepare_buffers() {
    auto& available = _ring.available_descriptors();
    return available.wait(1).then([this, &available] {
        unsigned count = 1;
        auto opportunistic = available.current();
        if (available.try_wait(opportunistic)) {
            count += opportunistic;
        }
        auto make_buffer_chain = [this] {
            single_buffer bc;
            std::unique_ptr<char[], free_deleter> buf(reinterpret_cast<char*>(malloc(4096)));
            buffer_and_virt& b = bc[0];
            b.addr = virt_to_phys(buf.get());
            b.len = 4096;
            b.writeable = true;
            b.buf = std::move(buf);
            return bc;
        };
        auto start = make_function_input_iterator(make_buffer_chain, 0U);
        auto finish = make_function_input_iterator(make_buffer_chain, count);
        _ring.post(start, finish);
    });
}

void
qp::rxq::complete_buffer(single_buffer&& bc, size_t len) {
    auto&& sb = bc[0];
    auto&& buf = sb.buf;
    auto frag_buf = buf.get();
    auto frag_len = len;
    // First buffer
    if (_remaining_buffers == 0) {
        auto hdr = reinterpret_cast<net_hdr_mrg*>(frag_buf);
        assert(hdr->num_buffers >= 1);
        _remaining_buffers = hdr->num_buffers;
        frag_buf += _dev._header_len;
        frag_len -= _dev._header_len;
        _fragments.clear();
        _buffers.clear();
    };

    // Append current buffer
    _fragments.emplace_back(fragment{frag_buf, frag_len});
    _buffers.push_back(std::move(buf));
    _remaining_buffers--;

    // Last buffer
    if (_remaining_buffers == 0) {
        deleter del;
        if (_buffers.size() == 1) {
            del = make_free_deleter(_buffers[0].release());
            _buffers.clear();
        } else {
            del = make_object_deleter(std::move(_buffers));
        }
        packet p(_fragments.begin(), _fragments.end(), std::move(del));
        _dev._dev->l2receive(std::move(p));
        _ring.available_descriptors().signal(_fragments.size());
    }
}

// Allocate and zero-initialize a buffer which is page-aligned and can be
// used for virt_to_phys (i.e., physically contiguous).
static std::unique_ptr<char[], free_deleter> virtio_buffer(size_t size) {
    void* ret;
    auto r = posix_memalign(&ret, 4096, size);
    assert(r == 0);
    bzero(ret, size);
    return std::unique_ptr<char[], free_deleter>(reinterpret_cast<char*>(ret));
}

qp::qp(device* dev, size_t rx_ring_size, size_t tx_ring_size)
    : _dev(dev)
    , _txq_storage(virtio_buffer(vring_storage_size(tx_ring_size)))
    , _rxq_storage(virtio_buffer(vring_storage_size(rx_ring_size)))
    , _txq(*this, txq_config(tx_ring_size))
    , _rxq(*this, rxq_config(rx_ring_size)) {
}

size_t qp::vring_storage_size(size_t ring_size) {
    // overestimate, but not by much.
    return 3 * 4096 + ring_size * (16 + 2 + 8);
}

void qp::common_config(ring_config& r) {
    r.avail = r.descs + 16 * r.size;
    r.used = align_up(r.avail + 2 * r.size + 6, 4096);
    r.event_index = (_dev->features() & VIRTIO_RING_F_EVENT_IDX) != 0;
    r.indirect = false;
}

ring_config qp::txq_config(size_t tx_ring_size) {
    ring_config r;
    r.size = tx_ring_size;
    r.descs = _txq_storage.get();
    r.mergable_buffers = false;
    common_config(r);
    return r;
}

ring_config qp::rxq_config(size_t rx_ring_size) {
    ring_config r;
    r.size = rx_ring_size;
    r.descs = _rxq_storage.get();
    r.mergable_buffers = true;
    common_config(r);
    return r;
}

void
qp::rx_start() {
    _rxq.run();
}

uint32_t
qp::send(circular_buffer<packet>& p) {
    return _txq.post(p);
}

class qp_vhost : public qp {
private:
    // The vhost file descriptor needs to remain open throughout the life of
    // this driver, as as soon as we close it, vhost stops servicing us.
    file_desc _vhost_fd;
public:
    qp_vhost(device* dev, boost::program_options::variables_map opts);
};

static size_t config_ring_size(boost::program_options::variables_map &opts) {
    if (opts.count("event-index")) {
        return opts["virtio-ring-size"].as<unsigned>();
    } else {
        return 256;
    }
}

qp_vhost::qp_vhost(device *dev, boost::program_options::variables_map opts)
    : qp(dev, config_ring_size(opts), config_ring_size(opts))
    , _vhost_fd(file_desc::open("/dev/vhost-net", O_RDWR))
{
    auto tap_device = opts["tap-device"].as<std::string>();
    int64_t vhost_supported_features;
    _vhost_fd.ioctl(VHOST_GET_FEATURES, vhost_supported_features);
    vhost_supported_features &= _dev->features();
    _vhost_fd.ioctl(VHOST_SET_FEATURES, vhost_supported_features);
    if (vhost_supported_features & VIRTIO_NET_F_MRG_RXBUF) {
        _header_len = sizeof(net_hdr_mrg);
    } else {
        _header_len = sizeof(net_hdr);
    }

    // Open and set up the tap device, which we'll tell vhost to use.
    // Note that the tap_fd we open here will be closed at the end of
    // this function. It appears that this is fine - i.e., after we pass
    // this fd to VHOST_NET_SET_BACKEND, the Linux kernel keeps the reference
    // to it and it's fine to close the file descriptor.
    file_desc tap_fd(file_desc::open("/dev/net/tun", O_RDWR | O_NONBLOCK));
    assert(tap_device.size() + 1 <= IFNAMSIZ);
    ifreq ifr = {};
    ifr.ifr_flags = IFF_TAP | IFF_NO_PI | IFF_ONE_QUEUE | IFF_VNET_HDR;
    strcpy(ifr.ifr_ifrn.ifrn_name, tap_device.c_str());
    tap_fd.ioctl(TUNSETIFF, ifr);
    unsigned int offload = 0;
    auto hw_features = _dev->hw_features();
    if (hw_features.tx_csum_l4_offload && hw_features.rx_csum_offload) {
        offload = TUN_F_CSUM;
        if (hw_features.tx_tso) {
            offload |= TUN_F_TSO4;
        }
        if (hw_features.tx_ufo) {
            offload |= TUN_F_UFO;
        }
    }
    tap_fd.ioctl(TUNSETOFFLOAD, offload);
    tap_fd.ioctl(TUNSETVNETHDRSZ, _header_len);

    // Additional vhost setup:
    _vhost_fd.ioctl(VHOST_SET_OWNER);
    auto mem_table = make_struct_with_vla(&vhost_memory::regions, 1);
    mem_table->nregions = 1;
    auto& region = mem_table->regions[0];
    region.guest_phys_addr = 0;
    region.memory_size = (size_t(1) << 47) - 4096;
    region.userspace_addr = 0;
    region.flags_padding = 0;
    _vhost_fd.ioctl(VHOST_SET_MEM_TABLE, *mem_table);
    vhost_vring_state vvs0 = { 0, _rxq.getconfig().size };
    _vhost_fd.ioctl(VHOST_SET_VRING_NUM, vvs0);
    vhost_vring_state vvs1 = { 1, _txq.getconfig().size };
    _vhost_fd.ioctl(VHOST_SET_VRING_NUM, vvs1);
    auto tov = [](char* x) { return reinterpret_cast<uintptr_t>(x); };

    _vhost_fd.ioctl(VHOST_SET_VRING_ADDR, vhost_vring_addr{
        0, 0, tov(_rxq.getconfig().descs), tov(_rxq.getconfig().used),
        tov(_rxq.getconfig().avail), 0
    });
    _vhost_fd.ioctl(VHOST_SET_VRING_ADDR, vhost_vring_addr{
        1, 0, tov(_txq.getconfig().descs), tov(_txq.getconfig().used),
        tov(_txq.getconfig().avail), 0
    });

    readable_eventfd _txq_notify;
    writeable_eventfd _txq_kick;
    readable_eventfd _rxq_notify;
    writeable_eventfd _rxq_kick;
    _vhost_fd.ioctl(VHOST_SET_VRING_KICK, vhost_vring_file{0, _rxq_kick.get_read_fd()});
    _vhost_fd.ioctl(VHOST_SET_VRING_CALL, vhost_vring_file{0, _rxq_notify.get_write_fd()});
    _vhost_fd.ioctl(VHOST_SET_VRING_KICK, vhost_vring_file{1, _txq_kick.get_read_fd()});
    _vhost_fd.ioctl(VHOST_SET_VRING_CALL, vhost_vring_file{1, _txq_notify.get_write_fd()});
    _rxq.set_notifier(std::make_unique<notifier_vhost>(std::move(_rxq_kick)));
    _txq.set_notifier(std::make_unique<notifier_vhost>(std::move(_txq_kick)));

    _vhost_fd.ioctl(VHOST_NET_SET_BACKEND, vhost_vring_file{0, tap_fd.get()});
    _vhost_fd.ioctl(VHOST_NET_SET_BACKEND, vhost_vring_file{1, tap_fd.get()});
}

#ifdef HAVE_OSV
class qp_osv : public qp {
private:
    ethernet_address _mac;
    osv::assigned_virtio &_virtio;
public:
    qp_osv(osv::assigned_virtio &virtio,
            boost::program_options::variables_map opts);
    virtual ethernet_address hw_address() override {
        return _mac;
    }
};

qp_osv::qp_osv(osv::assigned_virtio &virtio,
        boost::program_options::variables_map opts)
        : qp(opts, virtio.queue_size(0), virtio.queue_size(1))
        , _virtio(virtio)
{
    // Read the host's virtio supported feature bitmask, AND it with the
    // features we want to use, and tell the host of the result:
    uint32_t subset = _virtio.init_features(_dev->features());
    if (subset & VIRTIO_NET_F_MRG_RXBUF) {
        _header_len = sizeof(net_hdr_mrg);
    } else {
        _header_len = sizeof(net_hdr);
    }

    // TODO: save bits from "subset" in _hw_features?
//    bool _mergeable_bufs = subset & VIRTIO_NET_F_MRG_RXBUF;
//    bool _status = subset & VIRTIO_NET_F_STATUS;
//    bool _tso_ecn = subset & VIRTIO_NET_F_GUEST_ECN;
//    bool _host_tso_ecn = subset & VIRTIO_NET_F_HOST_ECN;
//    bool _csum = subset & VIRTIO_NET_F_CSUM;
//    bool _guest_csum = subset & VIRTIO_NET_F_GUEST_CSUM;
//    bool _guest_tso4 = subset & VIRTIO_NET_F_GUEST_TSO4;
//    bool _host_tso4 = subset & VIRTIO_NET_F_HOST_TSO4;
//    bool _guest_ufo = subset & VIRTIO_NET_F_GUEST_UFO;

    // Get the MAC address set by the host
    assert(subset & VIRTIO_NET_F_MAC);
    struct net_config {
        /* The ring_config defining mac address (if VIRTIO_NET_F_MAC) */
        uint8_t mac[6];
        /* See VIRTIO_NET_F_STATUS and VIRTIO_NET_S_* */
        uint16_t status;
        /* Maximum number of each of transmit and receive queues;
         * see VIRTIO_NET_F_MQ and VIRTIO_NET_CTRL_MQ.
         * Legal values are between 1 and 0x8000
         */
        uint16_t max_virtqueue_pairs;
    } __attribute__((packed)) host_config;
    _virtio.conf_read(&host_config, sizeof(host_config));
    _mac = { host_config.mac[0], host_config.mac[1], host_config.mac[2],
             host_config.mac[3], host_config.mac[4], host_config.mac[5] };

    // Setup notifiers
    _rxq.set_notifier(std::make_unique<notifier_osv>(_virtio, 0));
    _txq.set_notifier(std::make_unique<notifier_osv>(_virtio, 1));


    // Tell the host where we put the rings (we already allocated them earlier)
    _virtio.set_queue_pfn(
            0, virt_to_phys(_rxq.getconfig().descs));
    _virtio.set_queue_pfn(
            1, virt_to_phys(_txq.getconfig().descs));

    _txq.run();

    // Set up interrupts
    // FIXME: in OSv, the first thing we do in the handler is to call
    // _rqx.disable_interrupts(). Here in seastar, we only do it much later
    // in the main engine. Probably needs to do it like in osv - in the beginning of the handler.
    _virtio.enable_interrupt(
            0, [&] { _rxq.wake_notifier_wait(); } );
    _virtio.enable_interrupt(
            1, [&] { _txq.wake_notifier_wait(); } );

    _virtio.set_driver_ok();
}
#endif

std::unique_ptr<net::qp> device::init_local_queue(boost::program_options::variables_map opts, uint16_t qid) {
    static bool called = false;
    assert(!qid);
    assert(!called);
    called = true;

#ifdef HAVE_OSV
    if (osv::assigned_virtio::get && osv::assigned_virtio::get()) {
        std::cout << "In OSv and assigned host's virtio device\n";
        return std::make_unique<qp_osv>(*osv::assigned_virtio::get(), opts);
    }
#endif
    return std::make_unique<qp_vhost>(this, opts);
}

}

boost::program_options::options_description
get_virtio_net_options_description()
{
    boost::program_options::options_description opts(
            "Virtio net options");
    opts.add_options()
        ("event-index",
                boost::program_options::value<std::string>()->default_value("on"),
                "Enable event-index feature (on / off)")
        ("csum-offload",
                boost::program_options::value<std::string>()->default_value("on"),
                "Enable checksum offload feature (on / off)")
        ("tso",
                boost::program_options::value<std::string>()->default_value("on"),
                "Enable TCP segment offload feature (on / off)")
        ("ufo",
                boost::program_options::value<std::string>()->default_value("on"),
                "Enable UDP fragmentation offload feature (on / off)")
        ("virtio-ring-size",
                boost::program_options::value<unsigned>()->default_value(256),
                "Virtio ring size (must be power-of-two)")
        ;
    return opts;
}

std::unique_ptr<net::device> create_virtio_net_device(boost::program_options::variables_map opts) {
    return std::make_unique<virtio::device>(opts);
}

// Locks the shared object in memory and forces on-load function resolution.
// Needed if the function passed to enable_interrupt() is run at interrupt
// time.
// TODO: Instead of doing this, _virtio.enable_interrupt() could take a
// pollable to wake instead of a function, then this won't be needed.
asm(".pushsection .note.osv-mlock, \"a\"; .long 0, 0, 0; .popsection");