netback.c

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/*
 * Back-end of the driver for virtual network devices. This portion of the
 * driver exports a 'unified' network-device interface that can be accessed
 * by any operating system that implements a compatible front end. A
 * reference front-end implementation can be found in:
 *  drivers/net/xen-netfront.c
 *
 * Copyright (c) 2002-2005, K A Fraser
 *
 * This program is free software; you can redistribute it and/or
 * modify it under the terms of the GNU General Public License version 2
 * as published by the Free Software Foundation; or, when distributed
 * separately from the Linux kernel or incorporated into other
 * software packages, subject to the following license:
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this source file (the "Software"), to deal in the Software without
 * restriction, including without limitation the rights to use, copy, modify,
 * merge, publish, distribute, sublicense, and/or sell copies of the Software,
 * and to permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice shall be included in
 * all copies or substantial portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
 * FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS
 * IN THE SOFTWARE.
 */

#include "common.h"

#include <linux/kthread.h>
#include <linux/if_vlan.h>
#include <linux/udp.h>

#include <net/tcp.h>

#include <xen/xen.h>
#include <xen/events.h>
#include <xen/interface/memory.h>

#include <asm/xen/hypercall.h>
#include <asm/xen/page.h>

struct pending_tx_info {
    struct xen_netif_tx_request req;
    struct xenvif *vif;
};
typedef unsigned int pending_ring_idx_t;

struct netbk_rx_meta {
    int id;
    int size;
    int gso_size;
};

#define MAX_PENDING_REQS 256

/* Discriminate from any valid pending_idx value. */
#define INVALID_PENDING_IDX 0xFFFF

#define MAX_BUFFER_OFFSET PAGE_SIZE

/* extra field used in struct page */
union page_ext {
    struct {
#if BITS_PER_LONG < 64
#define IDX_WIDTH   8
#define GROUP_WIDTH (BITS_PER_LONG - IDX_WIDTH)
        unsigned int group:GROUP_WIDTH;
        unsigned int idx:IDX_WIDTH;
#else
        unsigned int group, idx;
#endif
    } e;
    void *mapping;
};

struct xen_netbk {
    wait_queue_head_t wq;
    struct task_struct *task;

    struct sk_buff_head rx_queue;
    struct sk_buff_head tx_queue;

    struct timer_list net_timer;

    struct page *mmap_pages[MAX_PENDING_REQS];

    pending_ring_idx_t pending_prod;
    pending_ring_idx_t pending_cons;
    struct list_head net_schedule_list;

    /* Protect the net_schedule_list in netif. */
    spinlock_t net_schedule_list_lock;

    atomic_t netfront_count;

    struct pending_tx_info pending_tx_info[MAX_PENDING_REQS];
    struct gnttab_copy tx_copy_ops[MAX_PENDING_REQS];

    u16 pending_ring[MAX_PENDING_REQS];

    /*
     * Given MAX_BUFFER_OFFSET of 4096 the worst case is that each
     * head/fragment page uses 2 copy operations because it
     * straddles two buffers in the frontend.
     */
    struct gnttab_copy grant_copy_op[2*XEN_NETIF_RX_RING_SIZE];
    struct netbk_rx_meta meta[2*XEN_NETIF_RX_RING_SIZE];
};

static struct xen_netbk *xen_netbk;
static int xen_netbk_group_nr;

void xen_netbk_add_xenvif(struct xenvif *vif)
{
    int i;
    int min_netfront_count;
    int min_group = 0;
    struct xen_netbk *netbk;

    min_netfront_count = atomic_read(&xen_netbk[0].netfront_count);
    for (i = 0; i < xen_netbk_group_nr; i++) {
        int netfront_count = atomic_read(&xen_netbk[i].netfront_count);
        if (netfront_count < min_netfront_count) {
            min_group = i;
            min_netfront_count = netfront_count;
        }
    }

    netbk = &xen_netbk[min_group];

    vif->netbk = netbk;
    atomic_inc(&netbk->netfront_count);
}

void xen_netbk_remove_xenvif(struct xenvif *vif)
{
    struct xen_netbk *netbk = vif->netbk;
    vif->netbk = NULL;
    atomic_dec(&netbk->netfront_count);
}

static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx);
static void make_tx_response(struct xenvif *vif,
                 struct xen_netif_tx_request *txp,
                 s8       st);
static struct xen_netif_rx_response *make_rx_response(struct xenvif *vif,
                         u16      id,
                         s8       st,
                         u16      offset,
                         u16      size,
                         u16      flags);

static inline unsigned long idx_to_pfn(struct xen_netbk *netbk,
                       u16 idx)
{
    return page_to_pfn(netbk->mmap_pages[idx]);
}

static inline unsigned long idx_to_kaddr(struct xen_netbk *netbk,
                     u16 idx)
{
    return (unsigned long)pfn_to_kaddr(idx_to_pfn(netbk, idx));
}

/* extra field used in struct page */
static inline void set_page_ext(struct page *pg, struct xen_netbk *netbk,
                unsigned int idx)
{
    unsigned int group = netbk - xen_netbk;
    union page_ext ext = { .e = { .group = group + 1, .idx = idx } };

    BUILD_BUG_ON(sizeof(ext) > sizeof(ext.mapping));
    pg->mapping = ext.mapping;
}

static int get_page_ext(struct page *pg,
            unsigned int *pgroup, unsigned int *pidx)
{
    union page_ext ext = { .mapping = pg->mapping };
    struct xen_netbk *netbk;
    unsigned int group, idx;

    group = ext.e.group - 1;

    if (group < 0 || group >= xen_netbk_group_nr)
        return 0;

    netbk = &xen_netbk[group];

    idx = ext.e.idx;

    if ((idx < 0) || (idx >= MAX_PENDING_REQS))
        return 0;

    if (netbk->mmap_pages[idx] != pg)
        return 0;

    *pgroup = group;
    *pidx = idx;

    return 1;
}

/*
 * This is the amount of packet we copy rather than map, so that the
 * guest can't fiddle with the contents of the headers while we do
 * packet processing on them (netfilter, routing, etc).
 */
#define PKT_PROT_LEN    (ETH_HLEN + \
             VLAN_HLEN + \
             sizeof(struct iphdr) + MAX_IPOPTLEN + \
             sizeof(struct tcphdr) + MAX_TCP_OPTION_SPACE)

static u16 frag_get_pending_idx(skb_frag_t *frag)
{
    return (u16)frag->page_offset;
}

static void frag_set_pending_idx(skb_frag_t *frag, u16 pending_idx)
{
    frag->page_offset = pending_idx;
}

static inline pending_ring_idx_t pending_index(unsigned i)
{
    return i & (MAX_PENDING_REQS-1);
}

static inline pending_ring_idx_t nr_pending_reqs(struct xen_netbk *netbk)
{
    return MAX_PENDING_REQS -
        netbk->pending_prod + netbk->pending_cons;
}

static void xen_netbk_kick_thread(struct xen_netbk *netbk)
{
    wake_up(&netbk->wq);
}

static int max_required_rx_slots(struct xenvif *vif)
{
    int max = DIV_ROUND_UP(vif->dev->mtu, PAGE_SIZE);

    if (vif->can_sg || vif->gso || vif->gso_prefix)
        max += MAX_SKB_FRAGS + 1; /* extra_info + frags */

    return max;
}

int xen_netbk_rx_ring_full(struct xenvif *vif)
{
    RING_IDX peek   = vif->rx_req_cons_peek;
    RING_IDX needed = max_required_rx_slots(vif);

    return ((vif->rx.sring->req_prod - peek) < needed) ||
           ((vif->rx.rsp_prod_pvt + XEN_NETIF_RX_RING_SIZE - peek) < needed);
}

int xen_netbk_must_stop_queue(struct xenvif *vif)
{
    if (!xen_netbk_rx_ring_full(vif))
        return 0;

    vif->rx.sring->req_event = vif->rx_req_cons_peek +
        max_required_rx_slots(vif);
    mb(); /* request notification /then/ check the queue */

    return xen_netbk_rx_ring_full(vif);
}

/*
 * Returns true if we should start a new receive buffer instead of
 * adding 'size' bytes to a buffer which currently contains 'offset'
 * bytes.
 */
static bool start_new_rx_buffer(int offset, unsigned long size, int head)
{
    /* simple case: we have completely filled the current buffer. */
    if (offset == MAX_BUFFER_OFFSET)
        return true;

    /*
     * complex case: start a fresh buffer if the current frag
     * would overflow the current buffer but only if:
     *     (i)   this frag would fit completely in the next buffer
     * and (ii)  there is already some data in the current buffer
     * and (iii) this is not the head buffer.
     *
     * Where:
     * - (i) stops us splitting a frag into two copies
     *   unless the frag is too large for a single buffer.
     * - (ii) stops us from leaving a buffer pointlessly empty.
     * - (iii) stops us leaving the first buffer
     *   empty. Strictly speaking this is already covered
     *   by (ii) but is explicitly checked because
     *   netfront relies on the first buffer being
     *   non-empty and can crash otherwise.
     *
     * This means we will effectively linearise small
     * frags but do not needlessly split large buffers
     * into multiple copies tend to give large frags their
     * own buffers as before.
     */
    if ((offset + size > MAX_BUFFER_OFFSET) &&
        (size <= MAX_BUFFER_OFFSET) && offset && !head)
        return true;

    return false;
}

/*
 * Figure out how many ring slots we're going to need to send @skb to
 * the guest. This function is essentially a dry run of
 * netbk_gop_frag_copy.
 */
unsigned int xen_netbk_count_skb_slots(struct xenvif *vif, struct sk_buff *skb)
{
    unsigned int count;
    int i, copy_off;

    count = DIV_ROUND_UP(skb_headlen(skb), PAGE_SIZE);

    copy_off = skb_headlen(skb) % PAGE_SIZE;

    if (skb_shinfo(skb)->gso_size)
        count++;

    for (i = 0; i < skb_shinfo(skb)->nr_frags; i++) {
        unsigned long size = skb_frag_size(&skb_shinfo(skb)->frags[i]);
        unsigned long offset = skb_shinfo(skb)->frags[i].page_offset;
        unsigned long bytes;

        offset &= ~PAGE_MASK;

        while (size > 0) {
            BUG_ON(offset >= PAGE_SIZE);
            BUG_ON(copy_off > MAX_BUFFER_OFFSET);

            bytes = PAGE_SIZE - offset;

            if (bytes > size)
                bytes = size;

            if (start_new_rx_buffer(copy_off, bytes, 0)) {
                count++;
                copy_off = 0;
            }

            if (copy_off + bytes > MAX_BUFFER_OFFSET)
                bytes = MAX_BUFFER_OFFSET - copy_off;

            copy_off += bytes;

            offset += bytes;
            size -= bytes;

            if (offset == PAGE_SIZE)
                offset = 0;
        }
    }
    return count;
}

struct netrx_pending_operations {
    unsigned copy_prod, copy_cons;
    unsigned meta_prod, meta_cons;
    struct gnttab_copy *copy;
    struct netbk_rx_meta *meta;
    int copy_off;
    grant_ref_t copy_gref;
};

static struct netbk_rx_meta *get_next_rx_buffer(struct xenvif *vif,
                        struct netrx_pending_operations *npo)
{
    struct netbk_rx_meta *meta;
    struct xen_netif_rx_request *req;

    req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++);

    meta = npo->meta + npo->meta_prod++;
    meta->gso_size = 0;
    meta->size = 0;
    meta->id = req->id;

    npo->copy_off = 0;
    npo->copy_gref = req->gref;

    return meta;
}

/*
 * Set up the grant operations for this fragment. If it's a flipping
 * interface, we also set up the unmap request from here.
 */
static void netbk_gop_frag_copy(struct xenvif *vif, struct sk_buff *skb,
                struct netrx_pending_operations *npo,
                struct page *page, unsigned long size,
                unsigned long offset, int *head)
{
    struct gnttab_copy *copy_gop;
    struct netbk_rx_meta *meta;
    /*
     * These variables are used iff get_page_ext returns true,
     * in which case they are guaranteed to be initialized.
     */
    unsigned int uninitialized_var(group), uninitialized_var(idx);
    int foreign = get_page_ext(page, &group, &idx);
    unsigned long bytes;

    /* Data must not cross a page boundary. */
    BUG_ON(size + offset > PAGE_SIZE<<compound_order(page));

    meta = npo->meta + npo->meta_prod - 1;

    /* Skip unused frames from start of page */
    page += offset >> PAGE_SHIFT;
    offset &= ~PAGE_MASK;

    while (size > 0) {
        BUG_ON(offset >= PAGE_SIZE);
        BUG_ON(npo->copy_off > MAX_BUFFER_OFFSET);

        bytes = PAGE_SIZE - offset;

        if (bytes > size)
            bytes = size;

        if (start_new_rx_buffer(npo->copy_off, bytes, *head)) {
            /*
             * Netfront requires there to be some data in the head
             * buffer.
             */
            BUG_ON(*head);

            meta = get_next_rx_buffer(vif, npo);
        }

        if (npo->copy_off + bytes > MAX_BUFFER_OFFSET)
            bytes = MAX_BUFFER_OFFSET - npo->copy_off;

        copy_gop = npo->copy + npo->copy_prod++;
        copy_gop->flags = GNTCOPY_dest_gref;
        if (foreign) {
            struct xen_netbk *netbk = &xen_netbk[group];
            struct pending_tx_info *src_pend;

            src_pend = &netbk->pending_tx_info[idx];

            copy_gop->source.domid = src_pend->vif->domid;
            copy_gop->source.u.ref = src_pend->req.gref;
            copy_gop->flags |= GNTCOPY_source_gref;
        } else {
            void *vaddr = page_address(page);
            copy_gop->source.domid = DOMID_SELF;
            copy_gop->source.u.gmfn = virt_to_mfn(vaddr);
        }
        copy_gop->source.offset = offset;
        copy_gop->dest.domid = vif->domid;

        copy_gop->dest.offset = npo->copy_off;
        copy_gop->dest.u.ref = npo->copy_gref;
        copy_gop->len = bytes;

        npo->copy_off += bytes;
        meta->size += bytes;

        offset += bytes;
        size -= bytes;

        /* Next frame */
        if (offset == PAGE_SIZE && size) {
            BUG_ON(!PageCompound(page));
            page++;
            offset = 0;
        }

        /* Leave a gap for the GSO descriptor. */
        if (*head && skb_shinfo(skb)->gso_size && !vif->gso_prefix)
            vif->rx.req_cons++;

        *head = 0; /* There must be something in this buffer now. */

    }
}

/*
 * Prepare an SKB to be transmitted to the frontend.
 *
 * This function is responsible for allocating grant operations, meta
 * structures, etc.
 *
 * It returns the number of meta structures consumed. The number of
 * ring slots used is always equal to the number of meta slots used
 * plus the number of GSO descriptors used. Currently, we use either
 * zero GSO descriptors (for non-GSO packets) or one descriptor (for
 * frontend-side LRO).
 */
static int netbk_gop_skb(struct sk_buff *skb,
             struct netrx_pending_operations *npo)
{
    struct xenvif *vif = netdev_priv(skb->dev);
    int nr_frags = skb_shinfo(skb)->nr_frags;
    int i;
    struct xen_netif_rx_request *req;
    struct netbk_rx_meta *meta;
    unsigned char *data;
    int head = 1;
    int old_meta_prod;

    old_meta_prod = npo->meta_prod;

    /* Set up a GSO prefix descriptor, if necessary */
    if (skb_shinfo(skb)->gso_size && vif->gso_prefix) {
        req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++);
        meta = npo->meta + npo->meta_prod++;
        meta->gso_size = skb_shinfo(skb)->gso_size;
        meta->size = 0;
        meta->id = req->id;
    }

    req = RING_GET_REQUEST(&vif->rx, vif->rx.req_cons++);
    meta = npo->meta + npo->meta_prod++;

    if (!vif->gso_prefix)
        meta->gso_size = skb_shinfo(skb)->gso_size;
    else
        meta->gso_size = 0;

    meta->size = 0;
    meta->id = req->id;
    npo->copy_off = 0;
    npo->copy_gref = req->gref;

    data = skb->data;
    while (data < skb_tail_pointer(skb)) {
        unsigned int offset = offset_in_page(data);
        unsigned int len = PAGE_SIZE - offset;

        if (data + len > skb_tail_pointer(skb))
            len = skb_tail_pointer(skb) - data;

        netbk_gop_frag_copy(vif, skb, npo,
                    virt_to_page(data), len, offset, &head);
        data += len;
    }

    for (i = 0; i < nr_frags; i++) {
        netbk_gop_frag_copy(vif, skb, npo,
                    skb_frag_page(&skb_shinfo(skb)->frags[i]),
                    skb_frag_size(&skb_shinfo(skb)->frags[i]),
                    skb_shinfo(skb)->frags[i].page_offset,
                    &head);
    }

    return npo->meta_prod - old_meta_prod;
}

/*
 * This is a twin to netbk_gop_skb.  Assume that netbk_gop_skb was
 * used to set up the operations on the top of
 * netrx_pending_operations, which have since been done.  Check that
 * they didn't give any errors and advance over them.
 */
static int netbk_check_gop(struct xenvif *vif, int nr_meta_slots,
               struct netrx_pending_operations *npo)
{
    struct gnttab_copy     *copy_op;
    int status = XEN_NETIF_RSP_OKAY;
    int i;

    for (i = 0; i < nr_meta_slots; i++) {
        copy_op = npo->copy + npo->copy_cons++;
        if (copy_op->status != GNTST_okay) {
            netdev_dbg(vif->dev,
                   "Bad status %d from copy to DOM%d.\n",
                   copy_op->status, vif->domid);
            status = XEN_NETIF_RSP_ERROR;
        }
    }

    return status;
}

static void netbk_add_frag_responses(struct xenvif *vif, int status,
                     struct netbk_rx_meta *meta,
                     int nr_meta_slots)
{
    int i;
    unsigned long offset;

    /* No fragments used */
    if (nr_meta_slots <= 1)
        return;

    nr_meta_slots--;

    for (i = 0; i < nr_meta_slots; i++) {
        int flags;
        if (i == nr_meta_slots - 1)
            flags = 0;
        else
            flags = XEN_NETRXF_more_data;

        offset = 0;
        make_rx_response(vif, meta[i].id, status, offset,
                 meta[i].size, flags);
    }
}

struct skb_cb_overlay {
    int meta_slots_used;
};

static void xen_netbk_rx_action(struct xen_netbk *netbk)
{
    struct xenvif *vif = NULL, *tmp;
    s8 status;
    u16 irq, flags;
    struct xen_netif_rx_response *resp;
    struct sk_buff_head rxq;
    struct sk_buff *skb;
    LIST_HEAD(notify);
    int ret;
    int nr_frags;
    int count;
    unsigned long offset;
    struct skb_cb_overlay *sco;

    struct netrx_pending_operations npo = {
        .copy  = netbk->grant_copy_op,
        .meta  = netbk->meta,
    };

    skb_queue_head_init(&rxq);

    count = 0;

    while ((skb = skb_dequeue(&netbk->rx_queue)) != NULL) {
        vif = netdev_priv(skb->dev);
        nr_frags = skb_shinfo(skb)->nr_frags;

        sco = (struct skb_cb_overlay *)skb->cb;
        sco->meta_slots_used = netbk_gop_skb(skb, &npo);

        count += nr_frags + 1;

        __skb_queue_tail(&rxq, skb);

        /* Filled the batch queue? */
        if (count + MAX_SKB_FRAGS >= XEN_NETIF_RX_RING_SIZE)
            break;
    }

    BUG_ON(npo.meta_prod > ARRAY_SIZE(netbk->meta));

    if (!npo.copy_prod)
        return;

    BUG_ON(npo.copy_prod > ARRAY_SIZE(netbk->grant_copy_op));
    gnttab_batch_copy(netbk->grant_copy_op, npo.copy_prod);

    while ((skb = __skb_dequeue(&rxq)) != NULL) {
        sco = (struct skb_cb_overlay *)skb->cb;

        vif = netdev_priv(skb->dev);

        if (netbk->meta[npo.meta_cons].gso_size && vif->gso_prefix) {
            resp = RING_GET_RESPONSE(&vif->rx,
                        vif->rx.rsp_prod_pvt++);

            resp->flags = XEN_NETRXF_gso_prefix | XEN_NETRXF_more_data;

            resp->offset = netbk->meta[npo.meta_cons].gso_size;
            resp->id = netbk->meta[npo.meta_cons].id;
            resp->status = sco->meta_slots_used;

            npo.meta_cons++;
            sco->meta_slots_used--;
        }


        vif->dev->stats.tx_bytes += skb->len;
        vif->dev->stats.tx_packets++;

        status = netbk_check_gop(vif, sco->meta_slots_used, &npo);

        if (sco->meta_slots_used == 1)
            flags = 0;
        else
            flags = XEN_NETRXF_more_data;

        if (skb->ip_summed == CHECKSUM_PARTIAL) /* local packet? */
            flags |= XEN_NETRXF_csum_blank | XEN_NETRXF_data_validated;
        else if (skb->ip_summed == CHECKSUM_UNNECESSARY)
            /* remote but checksummed. */
            flags |= XEN_NETRXF_data_validated;

        offset = 0;
        resp = make_rx_response(vif, netbk->meta[npo.meta_cons].id,
                    status, offset,
                    netbk->meta[npo.meta_cons].size,
                    flags);

        if (netbk->meta[npo.meta_cons].gso_size && !vif->gso_prefix) {
            struct xen_netif_extra_info *gso =
                (struct xen_netif_extra_info *)
                RING_GET_RESPONSE(&vif->rx,
                          vif->rx.rsp_prod_pvt++);

            resp->flags |= XEN_NETRXF_extra_info;

            gso->u.gso.size = netbk->meta[npo.meta_cons].gso_size;
            gso->u.gso.type = XEN_NETIF_GSO_TYPE_TCPV4;
            gso->u.gso.pad = 0;
            gso->u.gso.features = 0;

            gso->type = XEN_NETIF_EXTRA_TYPE_GSO;
            gso->flags = 0;
        }

        netbk_add_frag_responses(vif, status,
                     netbk->meta + npo.meta_cons + 1,
                     sco->meta_slots_used);

        RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&vif->rx, ret);
        irq = vif->irq;
        if (ret && list_empty(&vif->notify_list))
            list_add_tail(&vif->notify_list, &notify);

        xenvif_notify_tx_completion(vif);

        xenvif_put(vif);
        npo.meta_cons += sco->meta_slots_used;
        dev_kfree_skb(skb);
    }

    list_for_each_entry_safe(vif, tmp, &notify, notify_list) {
        notify_remote_via_irq(vif->irq);
        list_del_init(&vif->notify_list);
    }

    /* More work to do? */
    if (!skb_queue_empty(&netbk->rx_queue) &&
            !timer_pending(&netbk->net_timer))
        xen_netbk_kick_thread(netbk);
}

void xen_netbk_queue_tx_skb(struct xenvif *vif, struct sk_buff *skb)
{
    struct xen_netbk *netbk = vif->netbk;

    skb_queue_tail(&netbk->rx_queue, skb);

    xen_netbk_kick_thread(netbk);
}

static void xen_netbk_alarm(unsigned long data)
{
    struct xen_netbk *netbk = (struct xen_netbk *)data;
    xen_netbk_kick_thread(netbk);
}

static int __on_net_schedule_list(struct xenvif *vif)
{
    return !list_empty(&vif->schedule_list);
}

/* Must be called with net_schedule_list_lock held */
static void remove_from_net_schedule_list(struct xenvif *vif)
{
    if (likely(__on_net_schedule_list(vif))) {
        list_del_init(&vif->schedule_list);
        xenvif_put(vif);
    }
}

static struct xenvif *poll_net_schedule_list(struct xen_netbk *netbk)
{
    struct xenvif *vif = NULL;

    spin_lock_irq(&netbk->net_schedule_list_lock);
    if (list_empty(&netbk->net_schedule_list))
        goto out;

    vif = list_first_entry(&netbk->net_schedule_list,
                   struct xenvif, schedule_list);
    if (!vif)
        goto out;

    xenvif_get(vif);

    remove_from_net_schedule_list(vif);
out:
    spin_unlock_irq(&netbk->net_schedule_list_lock);
    return vif;
}

void xen_netbk_schedule_xenvif(struct xenvif *vif)
{
    unsigned long flags;
    struct xen_netbk *netbk = vif->netbk;

    if (__on_net_schedule_list(vif))
        goto kick;

    spin_lock_irqsave(&netbk->net_schedule_list_lock, flags);
    if (!__on_net_schedule_list(vif) &&
        likely(xenvif_schedulable(vif))) {
        list_add_tail(&vif->schedule_list, &netbk->net_schedule_list);
        xenvif_get(vif);
    }
    spin_unlock_irqrestore(&netbk->net_schedule_list_lock, flags);

kick:
    smp_mb();
    if ((nr_pending_reqs(netbk) < (MAX_PENDING_REQS/2)) &&
        !list_empty(&netbk->net_schedule_list))
        xen_netbk_kick_thread(netbk);
}

void xen_netbk_deschedule_xenvif(struct xenvif *vif)
{
    struct xen_netbk *netbk = vif->netbk;
    spin_lock_irq(&netbk->net_schedule_list_lock);
    remove_from_net_schedule_list(vif);
    spin_unlock_irq(&netbk->net_schedule_list_lock);
}

void xen_netbk_check_rx_xenvif(struct xenvif *vif)
{
    int more_to_do;

    RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, more_to_do);

    if (more_to_do)
        xen_netbk_schedule_xenvif(vif);
}

static void tx_add_credit(struct xenvif *vif)
{
    unsigned long max_burst, max_credit;

    /*
     * Allow a burst big enough to transmit a jumbo packet of up to 128kB.
     * Otherwise the interface can seize up due to insufficient credit.
     */
    max_burst = RING_GET_REQUEST(&vif->tx, vif->tx.req_cons)->size;
    max_burst = min(max_burst, 131072UL);
    max_burst = max(max_burst, vif->credit_bytes);

    /* Take care that adding a new chunk of credit doesn't wrap to zero. */
    max_credit = vif->remaining_credit + vif->credit_bytes;
    if (max_credit < vif->remaining_credit)
        max_credit = ULONG_MAX; /* wrapped: clamp to ULONG_MAX */

    vif->remaining_credit = min(max_credit, max_burst);
}

static void tx_credit_callback(unsigned long data)
{
    struct xenvif *vif = (struct xenvif *)data;
    tx_add_credit(vif);
    xen_netbk_check_rx_xenvif(vif);
}

static void netbk_tx_err(struct xenvif *vif,
             struct xen_netif_tx_request *txp, RING_IDX end)
{
    RING_IDX cons = vif->tx.req_cons;

    do {
        make_tx_response(vif, txp, XEN_NETIF_RSP_ERROR);
        if (cons >= end)
            break;
        txp = RING_GET_REQUEST(&vif->tx, cons++);
    } while (1);
    vif->tx.req_cons = cons;
    xen_netbk_check_rx_xenvif(vif);
    xenvif_put(vif);
}

static int netbk_count_requests(struct xenvif *vif,
                struct xen_netif_tx_request *first,
                struct xen_netif_tx_request *txp,
                int work_to_do)
{
    RING_IDX cons = vif->tx.req_cons;
    int frags = 0;

    if (!(first->flags & XEN_NETTXF_more_data))
        return 0;

    do {
        if (frags >= work_to_do) {
            netdev_dbg(vif->dev, "Need more frags\n");
            return -frags;
        }

        if (unlikely(frags >= MAX_SKB_FRAGS)) {
            netdev_dbg(vif->dev, "Too many frags\n");
            return -frags;
        }

        memcpy(txp, RING_GET_REQUEST(&vif->tx, cons + frags),
               sizeof(*txp));
        if (txp->size > first->size) {
            netdev_dbg(vif->dev, "Frags galore\n");
            return -frags;
        }

        first->size -= txp->size;
        frags++;

        if (unlikely((txp->offset + txp->size) > PAGE_SIZE)) {
            netdev_dbg(vif->dev, "txp->offset: %x, size: %u\n",
                 txp->offset, txp->size);
            return -frags;
        }
    } while ((txp++)->flags & XEN_NETTXF_more_data);
    return frags;
}

static struct page *xen_netbk_alloc_page(struct xen_netbk *netbk,
                     struct sk_buff *skb,
                     u16 pending_idx)
{
    struct page *page;
    page = alloc_page(GFP_KERNEL|__GFP_COLD);
    if (!page)
        return NULL;
    set_page_ext(page, netbk, pending_idx);
    netbk->mmap_pages[pending_idx] = page;
    return page;
}

static struct gnttab_copy *xen_netbk_get_requests(struct xen_netbk *netbk,
                          struct xenvif *vif,
                          struct sk_buff *skb,
                          struct xen_netif_tx_request *txp,
                          struct gnttab_copy *gop)
{
    struct skb_shared_info *shinfo = skb_shinfo(skb);
    skb_frag_t *frags = shinfo->frags;
    u16 pending_idx = *((u16 *)skb->data);
    int i, start;

    /* Skip first skb fragment if it is on same page as header fragment. */
    start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);

    for (i = start; i < shinfo->nr_frags; i++, txp++) {
        struct page *page;
        pending_ring_idx_t index;
        struct pending_tx_info *pending_tx_info =
            netbk->pending_tx_info;

        index = pending_index(netbk->pending_cons++);
        pending_idx = netbk->pending_ring[index];
        page = xen_netbk_alloc_page(netbk, skb, pending_idx);
        if (!page)
            return NULL;

        gop->source.u.ref = txp->gref;
        gop->source.domid = vif->domid;
        gop->source.offset = txp->offset;

        gop->dest.u.gmfn = virt_to_mfn(page_address(page));
        gop->dest.domid = DOMID_SELF;
        gop->dest.offset = txp->offset;

        gop->len = txp->size;
        gop->flags = GNTCOPY_source_gref;

        gop++;

        memcpy(&pending_tx_info[pending_idx].req, txp, sizeof(*txp));
        xenvif_get(vif);
        pending_tx_info[pending_idx].vif = vif;
        frag_set_pending_idx(&frags[i], pending_idx);
    }

    return gop;
}

static int xen_netbk_tx_check_gop(struct xen_netbk *netbk,
                  struct sk_buff *skb,
                  struct gnttab_copy **gopp)
{
    struct gnttab_copy *gop = *gopp;
    u16 pending_idx = *((u16 *)skb->data);
    struct pending_tx_info *pending_tx_info = netbk->pending_tx_info;
    struct xenvif *vif = pending_tx_info[pending_idx].vif;
    struct xen_netif_tx_request *txp;
    struct skb_shared_info *shinfo = skb_shinfo(skb);
    int nr_frags = shinfo->nr_frags;
    int i, err, start;

    /* Check status of header. */
    err = gop->status;
    if (unlikely(err)) {
        pending_ring_idx_t index;
        index = pending_index(netbk->pending_prod++);
        txp = &pending_tx_info[pending_idx].req;
        make_tx_response(vif, txp, XEN_NETIF_RSP_ERROR);
        netbk->pending_ring[index] = pending_idx;
        xenvif_put(vif);
    }

    /* Skip first skb fragment if it is on same page as header fragment. */
    start = (frag_get_pending_idx(&shinfo->frags[0]) == pending_idx);

    for (i = start; i < nr_frags; i++) {
        int j, newerr;
        pending_ring_idx_t index;

        pending_idx = frag_get_pending_idx(&shinfo->frags[i]);

        /* Check error status: if okay then remember grant handle. */
        newerr = (++gop)->status;
        if (likely(!newerr)) {
            /* Had a previous error? Invalidate this fragment. */
            if (unlikely(err))
                xen_netbk_idx_release(netbk, pending_idx);
            continue;
        }

        /* Error on this fragment: respond to client with an error. */
        txp = &netbk->pending_tx_info[pending_idx].req;
        make_tx_response(vif, txp, XEN_NETIF_RSP_ERROR);
        index = pending_index(netbk->pending_prod++);
        netbk->pending_ring[index] = pending_idx;
        xenvif_put(vif);

        /* Not the first error? Preceding frags already invalidated. */
        if (err)
            continue;

        /* First error: invalidate header and preceding fragments. */
        pending_idx = *((u16 *)skb->data);
        xen_netbk_idx_release(netbk, pending_idx);
        for (j = start; j < i; j++) {
            pending_idx = frag_get_pending_idx(&shinfo->frags[j]);
            xen_netbk_idx_release(netbk, pending_idx);
        }

        /* Remember the error: invalidate all subsequent fragments. */
        err = newerr;
    }

    *gopp = gop + 1;
    return err;
}

static void xen_netbk_fill_frags(struct xen_netbk *netbk, struct sk_buff *skb)
{
    struct skb_shared_info *shinfo = skb_shinfo(skb);
    int nr_frags = shinfo->nr_frags;
    int i;

    for (i = 0; i < nr_frags; i++) {
        skb_frag_t *frag = shinfo->frags + i;
        struct xen_netif_tx_request *txp;
        struct page *page;
        u16 pending_idx;

        pending_idx = frag_get_pending_idx(frag);

        txp = &netbk->pending_tx_info[pending_idx].req;
        page = virt_to_page(idx_to_kaddr(netbk, pending_idx));
        __skb_fill_page_desc(skb, i, page, txp->offset, txp->size);
        skb->len += txp->size;
        skb->data_len += txp->size;
        skb->truesize += txp->size;

        /* Take an extra reference to offset xen_netbk_idx_release */
        get_page(netbk->mmap_pages[pending_idx]);
        xen_netbk_idx_release(netbk, pending_idx);
    }
}

static int xen_netbk_get_extras(struct xenvif *vif,
                struct xen_netif_extra_info *extras,
                int work_to_do)
{
    struct xen_netif_extra_info extra;
    RING_IDX cons = vif->tx.req_cons;

    do {
        if (unlikely(work_to_do-- <= 0)) {
            netdev_dbg(vif->dev, "Missing extra info\n");
            return -EBADR;
        }

        memcpy(&extra, RING_GET_REQUEST(&vif->tx, cons),
               sizeof(extra));
        if (unlikely(!extra.type ||
                 extra.type >= XEN_NETIF_EXTRA_TYPE_MAX)) {
            vif->tx.req_cons = ++cons;
            netdev_dbg(vif->dev,
                   "Invalid extra type: %d\n", extra.type);
            return -EINVAL;
        }

        memcpy(&extras[extra.type - 1], &extra, sizeof(extra));
        vif->tx.req_cons = ++cons;
    } while (extra.flags & XEN_NETIF_EXTRA_FLAG_MORE);

    return work_to_do;
}

static int netbk_set_skb_gso(struct xenvif *vif,
                 struct sk_buff *skb,
                 struct xen_netif_extra_info *gso)
{
    if (!gso->u.gso.size) {
        netdev_dbg(vif->dev, "GSO size must not be zero.\n");
        return -EINVAL;
    }

    /* Currently only TCPv4 S.O. is supported. */
    if (gso->u.gso.type != XEN_NETIF_GSO_TYPE_TCPV4) {
        netdev_dbg(vif->dev, "Bad GSO type %d.\n", gso->u.gso.type);
        return -EINVAL;
    }

    skb_shinfo(skb)->gso_size = gso->u.gso.size;
    skb_shinfo(skb)->gso_type = SKB_GSO_TCPV4;

    /* Header must be checked, and gso_segs computed. */
    skb_shinfo(skb)->gso_type |= SKB_GSO_DODGY;
    skb_shinfo(skb)->gso_segs = 0;

    return 0;
}

static int checksum_setup(struct xenvif *vif, struct sk_buff *skb)
{
    struct iphdr *iph;
    unsigned char *th;
    int err = -EPROTO;
    int recalculate_partial_csum = 0;

    /*
     * A GSO SKB must be CHECKSUM_PARTIAL. However some buggy
     * peers can fail to set NETRXF_csum_blank when sending a GSO
     * frame. In this case force the SKB to CHECKSUM_PARTIAL and
     * recalculate the partial checksum.
     */
    if (skb->ip_summed != CHECKSUM_PARTIAL && skb_is_gso(skb)) {
        vif->rx_gso_checksum_fixup++;
        skb->ip_summed = CHECKSUM_PARTIAL;
        recalculate_partial_csum = 1;
    }

    /* A non-CHECKSUM_PARTIAL SKB does not require setup. */
    if (skb->ip_summed != CHECKSUM_PARTIAL)
        return 0;

    if (skb->protocol != htons(ETH_P_IP))
        goto out;

    iph = (void *)skb->data;
    th = skb->data + 4 * iph->ihl;
    if (th >= skb_tail_pointer(skb))
        goto out;

    skb->csum_start = th - skb->head;
    switch (iph->protocol) {
    case IPPROTO_TCP:
        skb->csum_offset = offsetof(struct tcphdr, check);

        if (recalculate_partial_csum) {
            struct tcphdr *tcph = (struct tcphdr *)th;
            tcph->check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
                             skb->len - iph->ihl*4,
                             IPPROTO_TCP, 0);
        }
        break;
    case IPPROTO_UDP:
        skb->csum_offset = offsetof(struct udphdr, check);

        if (recalculate_partial_csum) {
            struct udphdr *udph = (struct udphdr *)th;
            udph->check = ~csum_tcpudp_magic(iph->saddr, iph->daddr,
                             skb->len - iph->ihl*4,
                             IPPROTO_UDP, 0);
        }
        break;
    default:
        if (net_ratelimit())
            netdev_err(vif->dev,
                   "Attempting to checksum a non-TCP/UDP packet, dropping a protocol %d packet\n",
                   iph->protocol);
        goto out;
    }

    if ((th + skb->csum_offset + 2) > skb_tail_pointer(skb))
        goto out;

    err = 0;

out:
    return err;
}

static bool tx_credit_exceeded(struct xenvif *vif, unsigned size)
{
    unsigned long now = jiffies;
    unsigned long next_credit =
        vif->credit_timeout.expires +
        msecs_to_jiffies(vif->credit_usec / 1000);

    /* Timer could already be pending in rare cases. */
    if (timer_pending(&vif->credit_timeout))
        return true;

    /* Passed the point where we can replenish credit? */
    if (time_after_eq(now, next_credit)) {
        vif->credit_timeout.expires = now;
        tx_add_credit(vif);
    }

    /* Still too big to send right now? Set a callback. */
    if (size > vif->remaining_credit) {
        vif->credit_timeout.data     =
            (unsigned long)vif;
        vif->credit_timeout.function =
            tx_credit_callback;
        mod_timer(&vif->credit_timeout,
              next_credit);

        return true;
    }

    return false;
}

static unsigned xen_netbk_tx_build_gops(struct xen_netbk *netbk)
{
    struct gnttab_copy *gop = netbk->tx_copy_ops, *request_gop;
    struct sk_buff *skb;
    int ret;

    while (((nr_pending_reqs(netbk) + MAX_SKB_FRAGS) < MAX_PENDING_REQS) &&
        !list_empty(&netbk->net_schedule_list)) {
        struct xenvif *vif;
        struct xen_netif_tx_request txreq;
        struct xen_netif_tx_request txfrags[MAX_SKB_FRAGS];
        struct page *page;
        struct xen_netif_extra_info extras[XEN_NETIF_EXTRA_TYPE_MAX-1];
        u16 pending_idx;
        RING_IDX idx;
        int work_to_do;
        unsigned int data_len;
        pending_ring_idx_t index;

        /* Get a netif from the list with work to do. */
        vif = poll_net_schedule_list(netbk);
        if (!vif)
            continue;

        RING_FINAL_CHECK_FOR_REQUESTS(&vif->tx, work_to_do);
        if (!work_to_do) {
            xenvif_put(vif);
            continue;
        }

        idx = vif->tx.req_cons;
        rmb(); /* Ensure that we see the request before we copy it. */
        memcpy(&txreq, RING_GET_REQUEST(&vif->tx, idx), sizeof(txreq));

        /* Credit-based scheduling. */
        if (txreq.size > vif->remaining_credit &&
            tx_credit_exceeded(vif, txreq.size)) {
            xenvif_put(vif);
            continue;
        }

        vif->remaining_credit -= txreq.size;

        work_to_do--;
        vif->tx.req_cons = ++idx;

        memset(extras, 0, sizeof(extras));
        if (txreq.flags & XEN_NETTXF_extra_info) {
            work_to_do = xen_netbk_get_extras(vif, extras,
                              work_to_do);
            idx = vif->tx.req_cons;
            if (unlikely(work_to_do < 0)) {
                netbk_tx_err(vif, &txreq, idx);
                continue;
            }
        }

        ret = netbk_count_requests(vif, &txreq, txfrags, work_to_do);
        if (unlikely(ret < 0)) {
            netbk_tx_err(vif, &txreq, idx - ret);
            continue;
        }
        idx += ret;

        if (unlikely(txreq.size < ETH_HLEN)) {
            netdev_dbg(vif->dev,
                   "Bad packet size: %d\n", txreq.size);
            netbk_tx_err(vif, &txreq, idx);
            continue;
        }

        /* No crossing a page as the payload mustn't fragment. */
        if (unlikely((txreq.offset + txreq.size) > PAGE_SIZE)) {
            netdev_dbg(vif->dev,
                   "txreq.offset: %x, size: %u, end: %lu\n",
                   txreq.offset, txreq.size,
                   (txreq.offset&~PAGE_MASK) + txreq.size);
            netbk_tx_err(vif, &txreq, idx);
            continue;
        }

        index = pending_index(netbk->pending_cons);
        pending_idx = netbk->pending_ring[index];

        data_len = (txreq.size > PKT_PROT_LEN &&
                ret < MAX_SKB_FRAGS) ?
            PKT_PROT_LEN : txreq.size;

        skb = alloc_skb(data_len + NET_SKB_PAD + NET_IP_ALIGN,
                GFP_ATOMIC | __GFP_NOWARN);
        if (unlikely(skb == NULL)) {
            netdev_dbg(vif->dev,
                   "Can't allocate a skb in start_xmit.\n");
            netbk_tx_err(vif, &txreq, idx);
            break;
        }

        /* Packets passed to netif_rx() must have some headroom. */
        skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);

        if (extras[XEN_NETIF_EXTRA_TYPE_GSO - 1].type) {
            struct xen_netif_extra_info *gso;
            gso = &extras[XEN_NETIF_EXTRA_TYPE_GSO - 1];

            if (netbk_set_skb_gso(vif, skb, gso)) {
                kfree_skb(skb);
                netbk_tx_err(vif, &txreq, idx);
                continue;
            }
        }

        /* XXX could copy straight to head */
        page = xen_netbk_alloc_page(netbk, skb, pending_idx);
        if (!page) {
            kfree_skb(skb);
            netbk_tx_err(vif, &txreq, idx);
            continue;
        }

        gop->source.u.ref = txreq.gref;
        gop->source.domid = vif->domid;
        gop->source.offset = txreq.offset;

        gop->dest.u.gmfn = virt_to_mfn(page_address(page));
        gop->dest.domid = DOMID_SELF;
        gop->dest.offset = txreq.offset;

        gop->len = txreq.size;
        gop->flags = GNTCOPY_source_gref;

        gop++;

        memcpy(&netbk->pending_tx_info[pending_idx].req,
               &txreq, sizeof(txreq));
        netbk->pending_tx_info[pending_idx].vif = vif;
        *((u16 *)skb->data) = pending_idx;

        __skb_put(skb, data_len);

        skb_shinfo(skb)->nr_frags = ret;
        if (data_len < txreq.size) {
            skb_shinfo(skb)->nr_frags++;
            frag_set_pending_idx(&skb_shinfo(skb)->frags[0],
                         pending_idx);
        } else {
            frag_set_pending_idx(&skb_shinfo(skb)->frags[0],
                         INVALID_PENDING_IDX);
        }

        netbk->pending_cons++;

        request_gop = xen_netbk_get_requests(netbk, vif,
                             skb, txfrags, gop);
        if (request_gop == NULL) {
            kfree_skb(skb);
            netbk_tx_err(vif, &txreq, idx);
            continue;
        }
        gop = request_gop;

        __skb_queue_tail(&netbk->tx_queue, skb);

        vif->tx.req_cons = idx;
        xen_netbk_check_rx_xenvif(vif);

        if ((gop-netbk->tx_copy_ops) >= ARRAY_SIZE(netbk->tx_copy_ops))
            break;
    }

    return gop - netbk->tx_copy_ops;
}

static void xen_netbk_tx_submit(struct xen_netbk *netbk)
{
    struct gnttab_copy *gop = netbk->tx_copy_ops;
    struct sk_buff *skb;

    while ((skb = __skb_dequeue(&netbk->tx_queue)) != NULL) {
        struct xen_netif_tx_request *txp;
        struct xenvif *vif;
        u16 pending_idx;
        unsigned data_len;

        pending_idx = *((u16 *)skb->data);
        vif = netbk->pending_tx_info[pending_idx].vif;
        txp = &netbk->pending_tx_info[pending_idx].req;

        /* Check the remap error code. */
        if (unlikely(xen_netbk_tx_check_gop(netbk, skb, &gop))) {
            netdev_dbg(vif->dev, "netback grant failed.\n");
            skb_shinfo(skb)->nr_frags = 0;
            kfree_skb(skb);
            continue;
        }

        data_len = skb->len;
        memcpy(skb->data,
               (void *)(idx_to_kaddr(netbk, pending_idx)|txp->offset),
               data_len);
        if (data_len < txp->size) {
            /* Append the packet payload as a fragment. */
            txp->offset += data_len;
            txp->size -= data_len;
        } else {
            /* Schedule a response immediately. */
            xen_netbk_idx_release(netbk, pending_idx);
        }

        if (txp->flags & XEN_NETTXF_csum_blank)
            skb->ip_summed = CHECKSUM_PARTIAL;
        else if (txp->flags & XEN_NETTXF_data_validated)
            skb->ip_summed = CHECKSUM_UNNECESSARY;

        xen_netbk_fill_frags(netbk, skb);

        /*
         * If the initial fragment was < PKT_PROT_LEN then
         * pull through some bytes from the other fragments to
         * increase the linear region to PKT_PROT_LEN bytes.
         */
        if (skb_headlen(skb) < PKT_PROT_LEN && skb_is_nonlinear(skb)) {
            int target = min_t(int, skb->len, PKT_PROT_LEN);
            __pskb_pull_tail(skb, target - skb_headlen(skb));
        }

        skb->dev      = vif->dev;
        skb->protocol = eth_type_trans(skb, skb->dev);

        if (checksum_setup(vif, skb)) {
            netdev_dbg(vif->dev,
                   "Can't setup checksum in net_tx_action\n");
            kfree_skb(skb);
            continue;
        }

        vif->dev->stats.rx_bytes += skb->len;
        vif->dev->stats.rx_packets++;

        xenvif_receive_skb(vif, skb);
    }
}

/* Called after netfront has transmitted */
static void xen_netbk_tx_action(struct xen_netbk *netbk)
{
    unsigned nr_gops;

    nr_gops = xen_netbk_tx_build_gops(netbk);

    if (nr_gops == 0)
        return;

    gnttab_batch_copy(netbk->tx_copy_ops, nr_gops);

    xen_netbk_tx_submit(netbk);
}

static void xen_netbk_idx_release(struct xen_netbk *netbk, u16 pending_idx)
{
    struct xenvif *vif;
    struct pending_tx_info *pending_tx_info;
    pending_ring_idx_t index;

    /* Already complete? */
    if (netbk->mmap_pages[pending_idx] == NULL)
        return;

    pending_tx_info = &netbk->pending_tx_info[pending_idx];

    vif = pending_tx_info->vif;

    make_tx_response(vif, &pending_tx_info->req, XEN_NETIF_RSP_OKAY);

    index = pending_index(netbk->pending_prod++);
    netbk->pending_ring[index] = pending_idx;

    xenvif_put(vif);

    netbk->mmap_pages[pending_idx]->mapping = 0;
    put_page(netbk->mmap_pages[pending_idx]);
    netbk->mmap_pages[pending_idx] = NULL;
}

static void make_tx_response(struct xenvif *vif,
                 struct xen_netif_tx_request *txp,
                 s8       st)
{
    RING_IDX i = vif->tx.rsp_prod_pvt;
    struct xen_netif_tx_response *resp;
    int notify;

    resp = RING_GET_RESPONSE(&vif->tx, i);
    resp->id     = txp->id;
    resp->status = st;

    if (txp->flags & XEN_NETTXF_extra_info)
        RING_GET_RESPONSE(&vif->tx, ++i)->status = XEN_NETIF_RSP_NULL;

    vif->tx.rsp_prod_pvt = ++i;
    RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&vif->tx, notify);
    if (notify)
        notify_remote_via_irq(vif->irq);
}

static struct xen_netif_rx_response *make_rx_response(struct xenvif *vif,
                         u16      id,
                         s8       st,
                         u16      offset,
                         u16      size,
                         u16      flags)
{
    RING_IDX i = vif->rx.rsp_prod_pvt;
    struct xen_netif_rx_response *resp;

    resp = RING_GET_RESPONSE(&vif->rx, i);
    resp->offset     = offset;
    resp->flags      = flags;
    resp->id         = id;
    resp->status     = (s16)size;
    if (st < 0)
        resp->status = (s16)st;

    vif->rx.rsp_prod_pvt = ++i;

    return resp;
}

static inline int rx_work_todo(struct xen_netbk *netbk)
{
    return !skb_queue_empty(&netbk->rx_queue);
}

static inline int tx_work_todo(struct xen_netbk *netbk)
{

    if (((nr_pending_reqs(netbk) + MAX_SKB_FRAGS) < MAX_PENDING_REQS) &&
            !list_empty(&netbk->net_schedule_list))
        return 1;

    return 0;
}

static int xen_netbk_kthread(void *data)
{
    struct xen_netbk *netbk = data;
    while (!kthread_should_stop()) {
        wait_event_interruptible(netbk->wq,
                rx_work_todo(netbk) ||
                tx_work_todo(netbk) ||
                kthread_should_stop());
        cond_resched();

        if (kthread_should_stop())
            break;

        if (rx_work_todo(netbk))
            xen_netbk_rx_action(netbk);

        if (tx_work_todo(netbk))
            xen_netbk_tx_action(netbk);
    }

    return 0;
}

void xen_netbk_unmap_frontend_rings(struct xenvif *vif)
{
    if (vif->tx.sring)
        xenbus_unmap_ring_vfree(xenvif_to_xenbus_device(vif),
                    vif->tx.sring);
    if (vif->rx.sring)
        xenbus_unmap_ring_vfree(xenvif_to_xenbus_device(vif),
                    vif->rx.sring);
}

int xen_netbk_map_frontend_rings(struct xenvif *vif,
                 grant_ref_t tx_ring_ref,
                 grant_ref_t rx_ring_ref)
{
    void *addr;
    struct xen_netif_tx_sring *txs;
    struct xen_netif_rx_sring *rxs;

    int err = -ENOMEM;

    err = xenbus_map_ring_valloc(xenvif_to_xenbus_device(vif),
                     tx_ring_ref, &addr);
    if (err)
        goto err;

    txs = (struct xen_netif_tx_sring *)addr;
    BACK_RING_INIT(&vif->tx, txs, PAGE_SIZE);

    err = xenbus_map_ring_valloc(xenvif_to_xenbus_device(vif),
                     rx_ring_ref, &addr);
    if (err)
        goto err;

    rxs = (struct xen_netif_rx_sring *)addr;
    BACK_RING_INIT(&vif->rx, rxs, PAGE_SIZE);

    vif->rx_req_cons_peek = 0;

    return 0;

err:
    xen_netbk_unmap_frontend_rings(vif);
    return err;
}

static int __init netback_init(void)
{
    int i;
    int rc = 0;
    int group;

    if (!xen_domain())
        return -ENODEV;

    xen_netbk_group_nr = num_online_cpus();
    xen_netbk = vzalloc(sizeof(struct xen_netbk) * xen_netbk_group_nr);
    if (!xen_netbk)
        return -ENOMEM;

    for (group = 0; group < xen_netbk_group_nr; group++) {
        struct xen_netbk *netbk = &xen_netbk[group];
        skb_queue_head_init(&netbk->rx_queue);
        skb_queue_head_init(&netbk->tx_queue);

        init_timer(&netbk->net_timer);
        netbk->net_timer.data = (unsigned long)netbk;
        netbk->net_timer.function = xen_netbk_alarm;

        netbk->pending_cons = 0;
        netbk->pending_prod = MAX_PENDING_REQS;
        for (i = 0; i < MAX_PENDING_REQS; i++)
            netbk->pending_ring[i] = i;

        init_waitqueue_head(&netbk->wq);
        netbk->task = kthread_create(xen_netbk_kthread,
                         (void *)netbk,
                         "netback/%u", group);

        if (IS_ERR(netbk->task)) {
            printk(KERN_ALERT "kthread_create() fails at netback\n");
            del_timer(&netbk->net_timer);
            rc = PTR_ERR(netbk->task);
            goto failed_init;
        }

        kthread_bind(netbk->task, group);

        INIT_LIST_HEAD(&netbk->net_schedule_list);

        spin_lock_init(&netbk->net_schedule_list_lock);

        atomic_set(&netbk->netfront_count, 0);

        wake_up_process(netbk->task);
    }

    rc = xenvif_xenbus_init();
    if (rc)
        goto failed_init;

    return 0;

failed_init:
    while (--group >= 0) {
        struct xen_netbk *netbk = &xen_netbk[group];
        for (i = 0; i < MAX_PENDING_REQS; i++) {
            if (netbk->mmap_pages[i])
                __free_page(netbk->mmap_pages[i]);
        }
        del_timer(&netbk->net_timer);
        kthread_stop(netbk->task);
    }
    vfree(xen_netbk);
    return rc;

}

module_init(netback_init);

MODULE_LICENSE("Dual BSD/GPL");
MODULE_ALIAS("xen-backend:vif");