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[mirror_ubuntu-bionic-kernel.git] / drivers / vhost / vringh.c

/*
 * Helpers for the host side of a virtio ring.
 *
 * Since these may be in userspace, we use (inline) accessors.
 */
#include <linux/compiler.h>
#include <linux/module.h>
#include <linux/vringh.h>
#include <linux/virtio_ring.h>
#include <linux/kernel.h>
#include <linux/ratelimit.h>
#include <linux/uaccess.h>
#include <linux/slab.h>
#include <linux/export.h>
#include <uapi/linux/virtio_config.h>

static __printf(1,2) __cold void vringh_bad(const char *fmt, ...)
{
        static DEFINE_RATELIMIT_STATE(vringh_rs,
                                      DEFAULT_RATELIMIT_INTERVAL,
                                      DEFAULT_RATELIMIT_BURST);
        if (__ratelimit(&vringh_rs)) {
                va_list ap;
                va_start(ap, fmt);
                printk(KERN_NOTICE "vringh:");
                vprintk(fmt, ap);
                va_end(ap);
        }
}

/* Returns vring->num if empty, -ve on error. */
static inline int __vringh_get_head(const struct vringh *vrh,
                                    int (*getu16)(const struct vringh *vrh,
                                                  u16 *val, const __virtio16 *p),
                                    u16 *last_avail_idx)
{
        u16 avail_idx, i, head;
        int err;

        err = getu16(vrh, &avail_idx, &vrh->vring.avail->idx);
        if (err) {
                vringh_bad("Failed to access avail idx at %p",
                           &vrh->vring.avail->idx);
                return err;
        }

        if (*last_avail_idx == avail_idx)
                return vrh->vring.num;

        /* Only get avail ring entries after they have been exposed by guest. */
        virtio_rmb(vrh->weak_barriers);

        i = *last_avail_idx & (vrh->vring.num - 1);

        err = getu16(vrh, &head, &vrh->vring.avail->ring[i]);
        if (err) {
                vringh_bad("Failed to read head: idx %d address %p",
                           *last_avail_idx, &vrh->vring.avail->ring[i]);
                return err;
        }

        if (head >= vrh->vring.num) {
                vringh_bad("Guest says index %u > %u is available",
                           head, vrh->vring.num);
                return -EINVAL;
        }

        (*last_avail_idx)++;
        return head;
}

/* Copy some bytes to/from the iovec.  Returns num copied. */
static inline ssize_t vringh_iov_xfer(struct vringh_kiov *iov,
                                      void *ptr, size_t len,
                                      int (*xfer)(void *addr, void *ptr,
                                                  size_t len))
{
        int err, done = 0;

        while (len && iov->i < iov->used) {
                size_t partlen;

                partlen = min(iov->iov[iov->i].iov_len, len);
                err = xfer(iov->iov[iov->i].iov_base, ptr, partlen);
                if (err)
                        return err;
                done += partlen;
                len -= partlen;
                ptr += partlen;
                iov->consumed += partlen;
                iov->iov[iov->i].iov_len -= partlen;
                iov->iov[iov->i].iov_base += partlen;

                if (!iov->iov[iov->i].iov_len) {
                        /* Fix up old iov element then increment. */
                        iov->iov[iov->i].iov_len = iov->consumed;
                        iov->iov[iov->i].iov_base -= iov->consumed;
                        
                        iov->consumed = 0;
                        iov->i++;
                }
        }
        return done;
}

/* May reduce *len if range is shorter. */
static inline bool range_check(struct vringh *vrh, u64 addr, size_t *len,
                               struct vringh_range *range,
                               bool (*getrange)(struct vringh *,
                                                u64, struct vringh_range *))
{
        if (addr < range->start || addr > range->end_incl) {
                if (!getrange(vrh, addr, range))
                        return false;
        }
        BUG_ON(addr < range->start || addr > range->end_incl);

        /* To end of memory? */
        if (unlikely(addr + *len == 0)) {
                if (range->end_incl == -1ULL)
                        return true;
                goto truncate;
        }

        /* Otherwise, don't wrap. */
        if (addr + *len < addr) {
                vringh_bad("Wrapping descriptor %zu@0x%llx",
                           *len, (unsigned long long)addr);
                return false;
        }

        if (unlikely(addr + *len - 1 > range->end_incl))
                goto truncate;
        return true;

truncate:
        *len = range->end_incl + 1 - addr;
        return true;
}

static inline bool no_range_check(struct vringh *vrh, u64 addr, size_t *len,
                                  struct vringh_range *range,
                                  bool (*getrange)(struct vringh *,
                                                   u64, struct vringh_range *))
{
        return true;
}

/* No reason for this code to be inline. */
static int move_to_indirect(const struct vringh *vrh,
                            int *up_next, u16 *i, void *addr,
                            const struct vring_desc *desc,
                            struct vring_desc **descs, int *desc_max)
{
        u32 len;

        /* Indirect tables can't have indirect. */
        if (*up_next != -1) {
                vringh_bad("Multilevel indirect %u->%u", *up_next, *i);
                return -EINVAL;
        }

        len = vringh32_to_cpu(vrh, desc->len);
        if (unlikely(len % sizeof(struct vring_desc))) {
                vringh_bad("Strange indirect len %u", desc->len);
                return -EINVAL;
        }

        /* We will check this when we follow it! */
        if (desc->flags & cpu_to_vringh16(vrh, VRING_DESC_F_NEXT))
                *up_next = vringh16_to_cpu(vrh, desc->next);
        else
                *up_next = -2;
        *descs = addr;
        *desc_max = len / sizeof(struct vring_desc);

        /* Now, start at the first indirect. */
        *i = 0;
        return 0;
}

static int resize_iovec(struct vringh_kiov *iov, gfp_t gfp)
{
        struct kvec *new;
        unsigned int flag, new_num = (iov->max_num & ~VRINGH_IOV_ALLOCATED) * 2;

        if (new_num < 8)
                new_num = 8;

        flag = (iov->max_num & VRINGH_IOV_ALLOCATED);
        if (flag)
                new = krealloc(iov->iov, new_num * sizeof(struct iovec), gfp);
        else {
                new = kmalloc(new_num * sizeof(struct iovec), gfp);
                if (new) {
                        memcpy(new, iov->iov,
                               iov->max_num * sizeof(struct iovec));
                        flag = VRINGH_IOV_ALLOCATED;
                }
        }
        if (!new)
                return -ENOMEM;
        iov->iov = new;
        iov->max_num = (new_num | flag);
        return 0;
}

static u16 __cold return_from_indirect(const struct vringh *vrh, int *up_next,
                                       struct vring_desc **descs, int *desc_max)
{
        u16 i = *up_next;

        *up_next = -1;
        *descs = vrh->vring.desc;
        *desc_max = vrh->vring.num;
        return i;
}

static int slow_copy(struct vringh *vrh, void *dst, const void *src,
                     bool (*rcheck)(struct vringh *vrh, u64 addr, size_t *len,
                                    struct vringh_range *range,
                                    bool (*getrange)(struct vringh *vrh,
                                                     u64,
                                                     struct vringh_range *)),
                     bool (*getrange)(struct vringh *vrh,
                                      u64 addr,
                                      struct vringh_range *r),
                     struct vringh_range *range,
                     int (*copy)(void *dst, const void *src, size_t len))
{
        size_t part, len = sizeof(struct vring_desc);

        do {
                u64 addr;
                int err;

                part = len;
                addr = (u64)(unsigned long)src - range->offset;

                if (!rcheck(vrh, addr, &part, range, getrange))
                        return -EINVAL;

                err = copy(dst, src, part);
                if (err)
                        return err;

                dst += part;
                src += part;
                len -= part;
        } while (len);
        return 0;
}

static inline int
__vringh_iov(struct vringh *vrh, u16 i,
             struct vringh_kiov *riov,
             struct vringh_kiov *wiov,
             bool (*rcheck)(struct vringh *vrh, u64 addr, size_t *len,
                            struct vringh_range *range,
                            bool (*getrange)(struct vringh *, u64,
                                             struct vringh_range *)),
             bool (*getrange)(struct vringh *, u64, struct vringh_range *),
             gfp_t gfp,
             int (*copy)(void *dst, const void *src, size_t len))
{
        int err, count = 0, up_next, desc_max;
        struct vring_desc desc, *descs;
        struct vringh_range range = { -1ULL, 0 }, slowrange;
        bool slow = false;

        /* We start traversing vring's descriptor table. */
        descs = vrh->vring.desc;
        desc_max = vrh->vring.num;
        up_next = -1;

        if (riov)
                riov->i = riov->used = 0;
        else if (wiov)
                wiov->i = wiov->used = 0;
        else
                /* You must want something! */
                BUG();

        for (;;) {
                void *addr;
                struct vringh_kiov *iov;
                size_t len;

                if (unlikely(slow))
                        err = slow_copy(vrh, &desc, &descs[i], rcheck, getrange,
                                        &slowrange, copy);
                else
                        err = copy(&desc, &descs[i], sizeof(desc));
                if (unlikely(err))
                        goto fail;

                if (unlikely(desc.flags &
                             cpu_to_vringh16(vrh, VRING_DESC_F_INDIRECT))) {
                        u64 a = vringh64_to_cpu(vrh, desc.addr);

                        /* Make sure it's OK, and get offset. */
                        len = vringh32_to_cpu(vrh, desc.len);
                        if (!rcheck(vrh, a, &len, &range, getrange)) {
                                err = -EINVAL;
                                goto fail;
                        }

                        if (unlikely(len != vringh32_to_cpu(vrh, desc.len))) {
                                slow = true;
                                /* We need to save this range to use offset */
                                slowrange = range;
                        }

                        addr = (void *)(long)(a + range.offset);
                        err = move_to_indirect(vrh, &up_next, &i, addr, &desc,
                                               &descs, &desc_max);
                        if (err)
                                goto fail;
                        continue;
                }

                if (count++ == vrh->vring.num) {
                        vringh_bad("Descriptor loop in %p", descs);
                        err = -ELOOP;
                        goto fail;
                }

                if (desc.flags & cpu_to_vringh16(vrh, VRING_DESC_F_WRITE))
                        iov = wiov;
                else {
                        iov = riov;
                        if (unlikely(wiov && wiov->i)) {
                                vringh_bad("Readable desc %p after writable",
                                           &descs[i]);
                                err = -EINVAL;
                                goto fail;
                        }
                }

                if (!iov) {
                        vringh_bad("Unexpected %s desc",
                                   !wiov ? "writable" : "readable");
                        err = -EPROTO;
                        goto fail;
                }

        again:
                /* Make sure it's OK, and get offset. */
                len = vringh32_to_cpu(vrh, desc.len);
                if (!rcheck(vrh, vringh64_to_cpu(vrh, desc.addr), &len, &range,
                            getrange)) {
                        err = -EINVAL;
                        goto fail;
                }
                addr = (void *)(unsigned long)(vringh64_to_cpu(vrh, desc.addr) +
                                               range.offset);

                if (unlikely(iov->used == (iov->max_num & ~VRINGH_IOV_ALLOCATED))) {
                        err = resize_iovec(iov, gfp);
                        if (err)
                                goto fail;
                }

                iov->iov[iov->used].iov_base = addr;
                iov->iov[iov->used].iov_len = len;
                iov->used++;

                if (unlikely(len != vringh32_to_cpu(vrh, desc.len))) {
                        desc.len = cpu_to_vringh32(vrh,
                                   vringh32_to_cpu(vrh, desc.len) - len);
                        desc.addr = cpu_to_vringh64(vrh,
                                    vringh64_to_cpu(vrh, desc.addr) + len);
                        goto again;
                }

                if (desc.flags & cpu_to_vringh16(vrh, VRING_DESC_F_NEXT)) {
                        i = vringh16_to_cpu(vrh, desc.next);
                } else {
                        /* Just in case we need to finish traversing above. */
                        if (unlikely(up_next > 0)) {
                                i = return_from_indirect(vrh, &up_next,
                                                         &descs, &desc_max);
                                slow = false;
                        } else
                                break;
                }

                if (i >= desc_max) {
                        vringh_bad("Chained index %u > %u", i, desc_max);
                        err = -EINVAL;
                        goto fail;
                }
        }

        return 0;

fail:
        return err;
}

static inline int __vringh_complete(struct vringh *vrh,
                                    const struct vring_used_elem *used,
                                    unsigned int num_used,
                                    int (*putu16)(const struct vringh *vrh,
                                                  __virtio16 *p, u16 val),
                                    int (*putused)(struct vring_used_elem *dst,
                                                   const struct vring_used_elem
                                                   *src, unsigned num))
{
        struct vring_used *used_ring;
        int err;
        u16 used_idx, off;

        used_ring = vrh->vring.used;
        used_idx = vrh->last_used_idx + vrh->completed;

        off = used_idx % vrh->vring.num;

        /* Compiler knows num_used == 1 sometimes, hence extra check */
        if (num_used > 1 && unlikely(off + num_used >= vrh->vring.num)) {
                u16 part = vrh->vring.num - off;
                err = putused(&used_ring->ring[off], used, part);
                if (!err)
                        err = putused(&used_ring->ring[0], used + part,
                                      num_used - part);
        } else
                err = putused(&used_ring->ring[off], used, num_used);

        if (err) {
                vringh_bad("Failed to write %u used entries %u at %p",
                           num_used, off, &used_ring->ring[off]);
                return err;
        }

        /* Make sure buffer is written before we update index. */
        virtio_wmb(vrh->weak_barriers);

        err = putu16(vrh, &vrh->vring.used->idx, used_idx + num_used);
        if (err) {
                vringh_bad("Failed to update used index at %p",
                           &vrh->vring.used->idx);
                return err;
        }

        vrh->completed += num_used;
        return 0;
}


static inline int __vringh_need_notify(struct vringh *vrh,
                                       int (*getu16)(const struct vringh *vrh,
                                                     u16 *val,
                                                     const __virtio16 *p))
{
        bool notify;
        u16 used_event;
        int err;

        /* Flush out used index update. This is paired with the
         * barrier that the Guest executes when enabling
         * interrupts. */
        virtio_mb(vrh->weak_barriers);

        /* Old-style, without event indices. */
        if (!vrh->event_indices) {
                u16 flags;
                err = getu16(vrh, &flags, &vrh->vring.avail->flags);
                if (err) {
                        vringh_bad("Failed to get flags at %p",
                                   &vrh->vring.avail->flags);
                        return err;
                }
                return (!(flags & VRING_AVAIL_F_NO_INTERRUPT));
        }

        /* Modern: we know when other side wants to know. */
        err = getu16(vrh, &used_event, &vring_used_event(&vrh->vring));
        if (err) {
                vringh_bad("Failed to get used event idx at %p",
                           &vring_used_event(&vrh->vring));
                return err;
        }

        /* Just in case we added so many that we wrap. */
        if (unlikely(vrh->completed > 0xffff))
                notify = true;
        else
                notify = vring_need_event(used_event,
                                          vrh->last_used_idx + vrh->completed,
                                          vrh->last_used_idx);

        vrh->last_used_idx += vrh->completed;
        vrh->completed = 0;
        return notify;
}

static inline bool __vringh_notify_enable(struct vringh *vrh,
                                          int (*getu16)(const struct vringh *vrh,
                                                        u16 *val, const __virtio16 *p),
                                          int (*putu16)(const struct vringh *vrh,
                                                        __virtio16 *p, u16 val))
{
        u16 avail;

        if (!vrh->event_indices) {
                /* Old-school; update flags. */
                if (putu16(vrh, &vrh->vring.used->flags, 0) != 0) {
                        vringh_bad("Clearing used flags %p",
                                   &vrh->vring.used->flags);
                        return true;
                }
        } else {
                if (putu16(vrh, &vring_avail_event(&vrh->vring),
                           vrh->last_avail_idx) != 0) {
                        vringh_bad("Updating avail event index %p",
                                   &vring_avail_event(&vrh->vring));
                        return true;
                }
        }

        /* They could have slipped one in as we were doing that: make
         * sure it's written, then check again. */
        virtio_mb(vrh->weak_barriers);

        if (getu16(vrh, &avail, &vrh->vring.avail->idx) != 0) {
                vringh_bad("Failed to check avail idx at %p",
                           &vrh->vring.avail->idx);
                return true;
        }

        /* This is unlikely, so we just leave notifications enabled
         * (if we're using event_indices, we'll only get one
         * notification anyway). */
        return avail == vrh->last_avail_idx;
}

static inline void __vringh_notify_disable(struct vringh *vrh,
                                           int (*putu16)(const struct vringh *vrh,
                                                         __virtio16 *p, u16 val))
{
        if (!vrh->event_indices) {
                /* Old-school; update flags. */
                if (putu16(vrh, &vrh->vring.used->flags,
                           VRING_USED_F_NO_NOTIFY)) {
                        vringh_bad("Setting used flags %p",
                                   &vrh->vring.used->flags);
                }
        }
}

/* Userspace access helpers: in this case, addresses are really userspace. */
static inline int getu16_user(const struct vringh *vrh, u16 *val, const __virtio16 *p)
{
        __virtio16 v = 0;
        int rc = get_user(v, (__force __virtio16 __user *)p);
        *val = vringh16_to_cpu(vrh, v);
        return rc;
}

static inline int putu16_user(const struct vringh *vrh, __virtio16 *p, u16 val)
{
        __virtio16 v = cpu_to_vringh16(vrh, val);
        return put_user(v, (__force __virtio16 __user *)p);
}

static inline int copydesc_user(void *dst, const void *src, size_t len)
{
        return copy_from_user(dst, (__force void __user *)src, len) ?
                -EFAULT : 0;
}

static inline int putused_user(struct vring_used_elem *dst,
                               const struct vring_used_elem *src,
                               unsigned int num)
{
        return copy_to_user((__force void __user *)dst, src,
                            sizeof(*dst) * num) ? -EFAULT : 0;
}

static inline int xfer_from_user(void *src, void *dst, size_t len)
{
        return copy_from_user(dst, (__force void __user *)src, len) ?
                -EFAULT : 0;
}

static inline int xfer_to_user(void *dst, void *src, size_t len)
{
        return copy_to_user((__force void __user *)dst, src, len) ?
                -EFAULT : 0;
}

/**
 * vringh_init_user - initialize a vringh for a userspace vring.
 * @vrh: the vringh to initialize.
 * @features: the feature bits for this ring.
 * @num: the number of elements.
 * @weak_barriers: true if we only need memory barriers, not I/O.
 * @desc: the userpace descriptor pointer.
 * @avail: the userpace avail pointer.
 * @used: the userpace used pointer.
 *
 * Returns an error if num is invalid: you should check pointers
 * yourself!
 */
int vringh_init_user(struct vringh *vrh, u64 features,
                     unsigned int num, bool weak_barriers,
                     struct vring_desc __user *desc,
                     struct vring_avail __user *avail,
                     struct vring_used __user *used)
{
        /* Sane power of 2 please! */
        if (!num || num > 0xffff || (num & (num - 1))) {
                vringh_bad("Bad ring size %u", num);
                return -EINVAL;
        }

        vrh->little_endian = (features & (1ULL << VIRTIO_F_VERSION_1));
        vrh->event_indices = (features & (1 << VIRTIO_RING_F_EVENT_IDX));
        vrh->weak_barriers = weak_barriers;
        vrh->completed = 0;
        vrh->last_avail_idx = 0;
        vrh->last_used_idx = 0;
        vrh->vring.num = num;
        /* vring expects kernel addresses, but only used via accessors. */
        vrh->vring.desc = (__force struct vring_desc *)desc;
        vrh->vring.avail = (__force struct vring_avail *)avail;
        vrh->vring.used = (__force struct vring_used *)used;
        return 0;
}
EXPORT_SYMBOL(vringh_init_user);

/**
 * vringh_getdesc_user - get next available descriptor from userspace ring.
 * @vrh: the userspace vring.
 * @riov: where to put the readable descriptors (or NULL)
 * @wiov: where to put the writable descriptors (or NULL)
 * @getrange: function to call to check ranges.
 * @head: head index we received, for passing to vringh_complete_user().
 *
 * Returns 0 if there was no descriptor, 1 if there was, or -errno.
 *
 * Note that on error return, you can tell the difference between an
 * invalid ring and a single invalid descriptor: in the former case,
 * *head will be vrh->vring.num.  You may be able to ignore an invalid
 * descriptor, but there's not much you can do with an invalid ring.
 *
 * Note that you may need to clean up riov and wiov, even on error!
 */
int vringh_getdesc_user(struct vringh *vrh,
                        struct vringh_iov *riov,
                        struct vringh_iov *wiov,
                        bool (*getrange)(struct vringh *vrh,
                                         u64 addr, struct vringh_range *r),
                        u16 *head)
{
        int err;

        *head = vrh->vring.num;
        err = __vringh_get_head(vrh, getu16_user, &vrh->last_avail_idx);
        if (err < 0)
                return err;

        /* Empty... */
        if (err == vrh->vring.num)
                return 0;

        /* We need the layouts to be the identical for this to work */
        BUILD_BUG_ON(sizeof(struct vringh_kiov) != sizeof(struct vringh_iov));
        BUILD_BUG_ON(offsetof(struct vringh_kiov, iov) !=
                     offsetof(struct vringh_iov, iov));
        BUILD_BUG_ON(offsetof(struct vringh_kiov, i) !=
                     offsetof(struct vringh_iov, i));
        BUILD_BUG_ON(offsetof(struct vringh_kiov, used) !=
                     offsetof(struct vringh_iov, used));
        BUILD_BUG_ON(offsetof(struct vringh_kiov, max_num) !=
                     offsetof(struct vringh_iov, max_num));
        BUILD_BUG_ON(sizeof(struct iovec) != sizeof(struct kvec));
        BUILD_BUG_ON(offsetof(struct iovec, iov_base) !=
                     offsetof(struct kvec, iov_base));
        BUILD_BUG_ON(offsetof(struct iovec, iov_len) !=
                     offsetof(struct kvec, iov_len));
        BUILD_BUG_ON(sizeof(((struct iovec *)NULL)->iov_base)
                     != sizeof(((struct kvec *)NULL)->iov_base));
        BUILD_BUG_ON(sizeof(((struct iovec *)NULL)->iov_len)
                     != sizeof(((struct kvec *)NULL)->iov_len));

        *head = err;
        err = __vringh_iov(vrh, *head, (struct vringh_kiov *)riov,
                           (struct vringh_kiov *)wiov,
                           range_check, getrange, GFP_KERNEL, copydesc_user);
        if (err)
                return err;

        return 1;
}
EXPORT_SYMBOL(vringh_getdesc_user);

/**
 * vringh_iov_pull_user - copy bytes from vring_iov.
 * @riov: the riov as passed to vringh_getdesc_user() (updated as we consume)
 * @dst: the place to copy.
 * @len: the maximum length to copy.
 *
 * Returns the bytes copied <= len or a negative errno.
 */
ssize_t vringh_iov_pull_user(struct vringh_iov *riov, void *dst, size_t len)
{
        return vringh_iov_xfer((struct vringh_kiov *)riov,
                               dst, len, xfer_from_user);
}
EXPORT_SYMBOL(vringh_iov_pull_user);

/**
 * vringh_iov_push_user - copy bytes into vring_iov.
 * @wiov: the wiov as passed to vringh_getdesc_user() (updated as we consume)
 * @dst: the place to copy.
 * @len: the maximum length to copy.
 *
 * Returns the bytes copied <= len or a negative errno.
 */
ssize_t vringh_iov_push_user(struct vringh_iov *wiov,
                             const void *src, size_t len)
{
        return vringh_iov_xfer((struct vringh_kiov *)wiov,
                               (void *)src, len, xfer_to_user);
}
EXPORT_SYMBOL(vringh_iov_push_user);

/**
 * vringh_abandon_user - we've decided not to handle the descriptor(s).
 * @vrh: the vring.
 * @num: the number of descriptors to put back (ie. num
 *       vringh_get_user() to undo).
 *
 * The next vringh_get_user() will return the old descriptor(s) again.
 */
void vringh_abandon_user(struct vringh *vrh, unsigned int num)
{
        /* We only update vring_avail_event(vr) when we want to be notified,
         * so we haven't changed that yet. */
        vrh->last_avail_idx -= num;
}
EXPORT_SYMBOL(vringh_abandon_user);

/**
 * vringh_complete_user - we've finished with descriptor, publish it.
 * @vrh: the vring.
 * @head: the head as filled in by vringh_getdesc_user.
 * @len: the length of data we have written.
 *
 * You should check vringh_need_notify_user() after one or more calls
 * to this function.
 */
int vringh_complete_user(struct vringh *vrh, u16 head, u32 len)
{
        struct vring_used_elem used;

        used.id = cpu_to_vringh32(vrh, head);
        used.len = cpu_to_vringh32(vrh, len);
        return __vringh_complete(vrh, &used, 1, putu16_user, putused_user);
}
EXPORT_SYMBOL(vringh_complete_user);

/**
 * vringh_complete_multi_user - we've finished with many descriptors.
 * @vrh: the vring.
 * @used: the head, length pairs.
 * @num_used: the number of used elements.
 *
 * You should check vringh_need_notify_user() after one or more calls
 * to this function.
 */
int vringh_complete_multi_user(struct vringh *vrh,
                               const struct vring_used_elem used[],
                               unsigned num_used)
{
        return __vringh_complete(vrh, used, num_used,
                                 putu16_user, putused_user);
}
EXPORT_SYMBOL(vringh_complete_multi_user);

/**
 * vringh_notify_enable_user - we want to know if something changes.
 * @vrh: the vring.
 *
 * This always enables notifications, but returns false if there are
 * now more buffers available in the vring.
 */
bool vringh_notify_enable_user(struct vringh *vrh)
{
        return __vringh_notify_enable(vrh, getu16_user, putu16_user);
}
EXPORT_SYMBOL(vringh_notify_enable_user);

/**
 * vringh_notify_disable_user - don't tell us if something changes.
 * @vrh: the vring.
 *
 * This is our normal running state: we disable and then only enable when
 * we're going to sleep.
 */
void vringh_notify_disable_user(struct vringh *vrh)
{
        __vringh_notify_disable(vrh, putu16_user);
}
EXPORT_SYMBOL(vringh_notify_disable_user);

/**
 * vringh_need_notify_user - must we tell the other side about used buffers?
 * @vrh: the vring we've called vringh_complete_user() on.
 *
 * Returns -errno or 0 if we don't need to tell the other side, 1 if we do.
 */
int vringh_need_notify_user(struct vringh *vrh)
{
        return __vringh_need_notify(vrh, getu16_user);
}
EXPORT_SYMBOL(vringh_need_notify_user);

/* Kernelspace access helpers. */
static inline int getu16_kern(const struct vringh *vrh,
                              u16 *val, const __virtio16 *p)
{
        *val = vringh16_to_cpu(vrh, READ_ONCE(*p));
        return 0;
}

static inline int putu16_kern(const struct vringh *vrh, __virtio16 *p, u16 val)
{
        WRITE_ONCE(*p, cpu_to_vringh16(vrh, val));
        return 0;
}

static inline int copydesc_kern(void *dst, const void *src, size_t len)
{
        memcpy(dst, src, len);
        return 0;
}

static inline int putused_kern(struct vring_used_elem *dst,
                               const struct vring_used_elem *src,
                               unsigned int num)
{
        memcpy(dst, src, num * sizeof(*dst));
        return 0;
}

static inline int xfer_kern(void *src, void *dst, size_t len)
{
        memcpy(dst, src, len);
        return 0;
}

/**
 * vringh_init_kern - initialize a vringh for a kernelspace vring.
 * @vrh: the vringh to initialize.
 * @features: the feature bits for this ring.
 * @num: the number of elements.
 * @weak_barriers: true if we only need memory barriers, not I/O.
 * @desc: the userpace descriptor pointer.
 * @avail: the userpace avail pointer.
 * @used: the userpace used pointer.
 *
 * Returns an error if num is invalid.
 */
int vringh_init_kern(struct vringh *vrh, u64 features,
                     unsigned int num, bool weak_barriers,
                     struct vring_desc *desc,
                     struct vring_avail *avail,
                     struct vring_used *used)
{
        /* Sane power of 2 please! */
        if (!num || num > 0xffff || (num & (num - 1))) {
                vringh_bad("Bad ring size %u", num);
                return -EINVAL;
        }

        vrh->little_endian = (features & (1ULL << VIRTIO_F_VERSION_1));
        vrh->event_indices = (features & (1 << VIRTIO_RING_F_EVENT_IDX));
        vrh->weak_barriers = weak_barriers;
        vrh->completed = 0;
        vrh->last_avail_idx = 0;
        vrh->last_used_idx = 0;
        vrh->vring.num = num;
        vrh->vring.desc = desc;
        vrh->vring.avail = avail;
        vrh->vring.used = used;
        return 0;
}
EXPORT_SYMBOL(vringh_init_kern);

/**
 * vringh_getdesc_kern - get next available descriptor from kernelspace ring.
 * @vrh: the kernelspace vring.
 * @riov: where to put the readable descriptors (or NULL)
 * @wiov: where to put the writable descriptors (or NULL)
 * @head: head index we received, for passing to vringh_complete_kern().
 * @gfp: flags for allocating larger riov/wiov.
 *
 * Returns 0 if there was no descriptor, 1 if there was, or -errno.
 *
 * Note that on error return, you can tell the difference between an
 * invalid ring and a single invalid descriptor: in the former case,
 * *head will be vrh->vring.num.  You may be able to ignore an invalid
 * descriptor, but there's not much you can do with an invalid ring.
 *
 * Note that you may need to clean up riov and wiov, even on error!
 */
int vringh_getdesc_kern(struct vringh *vrh,
                        struct vringh_kiov *riov,
                        struct vringh_kiov *wiov,
                        u16 *head,
                        gfp_t gfp)
{
        int err;

        err = __vringh_get_head(vrh, getu16_kern, &vrh->last_avail_idx);
        if (err < 0)
                return err;

        /* Empty... */
        if (err == vrh->vring.num)
                return 0;

        *head = err;
        err = __vringh_iov(vrh, *head, riov, wiov, no_range_check, NULL,
                           gfp, copydesc_kern);
        if (err)
                return err;

        return 1;
}
EXPORT_SYMBOL(vringh_getdesc_kern);

/**
 * vringh_iov_pull_kern - copy bytes from vring_iov.
 * @riov: the riov as passed to vringh_getdesc_kern() (updated as we consume)
 * @dst: the place to copy.
 * @len: the maximum length to copy.
 *
 * Returns the bytes copied <= len or a negative errno.
 */
ssize_t vringh_iov_pull_kern(struct vringh_kiov *riov, void *dst, size_t len)
{
        return vringh_iov_xfer(riov, dst, len, xfer_kern);
}
EXPORT_SYMBOL(vringh_iov_pull_kern);

/**
 * vringh_iov_push_kern - copy bytes into vring_iov.
 * @wiov: the wiov as passed to vringh_getdesc_kern() (updated as we consume)
 * @dst: the place to copy.
 * @len: the maximum length to copy.
 *
 * Returns the bytes copied <= len or a negative errno.
 */
ssize_t vringh_iov_push_kern(struct vringh_kiov *wiov,
                             const void *src, size_t len)
{
        return vringh_iov_xfer(wiov, (void *)src, len, xfer_kern);
}
EXPORT_SYMBOL(vringh_iov_push_kern);

/**
 * vringh_abandon_kern - we've decided not to handle the descriptor(s).
 * @vrh: the vring.
 * @num: the number of descriptors to put back (ie. num
 *       vringh_get_kern() to undo).
 *
 * The next vringh_get_kern() will return the old descriptor(s) again.
 */
void vringh_abandon_kern(struct vringh *vrh, unsigned int num)
{
        /* We only update vring_avail_event(vr) when we want to be notified,
         * so we haven't changed that yet. */
        vrh->last_avail_idx -= num;
}
EXPORT_SYMBOL(vringh_abandon_kern);

/**
 * vringh_complete_kern - we've finished with descriptor, publish it.
 * @vrh: the vring.
 * @head: the head as filled in by vringh_getdesc_kern.
 * @len: the length of data we have written.
 *
 * You should check vringh_need_notify_kern() after one or more calls
 * to this function.
 */
int vringh_complete_kern(struct vringh *vrh, u16 head, u32 len)
{
        struct vring_used_elem used;

        used.id = cpu_to_vringh32(vrh, head);
        used.len = cpu_to_vringh32(vrh, len);

        return __vringh_complete(vrh, &used, 1, putu16_kern, putused_kern);
}
EXPORT_SYMBOL(vringh_complete_kern);

/**
 * vringh_notify_enable_kern - we want to know if something changes.
 * @vrh: the vring.
 *
 * This always enables notifications, but returns false if there are
 * now more buffers available in the vring.
 */
bool vringh_notify_enable_kern(struct vringh *vrh)
{
        return __vringh_notify_enable(vrh, getu16_kern, putu16_kern);
}
EXPORT_SYMBOL(vringh_notify_enable_kern);

/**
 * vringh_notify_disable_kern - don't tell us if something changes.
 * @vrh: the vring.
 *
 * This is our normal running state: we disable and then only enable when
 * we're going to sleep.
 */
void vringh_notify_disable_kern(struct vringh *vrh)
{
        __vringh_notify_disable(vrh, putu16_kern);
}
EXPORT_SYMBOL(vringh_notify_disable_kern);

/**
 * vringh_need_notify_kern - must we tell the other side about used buffers?
 * @vrh: the vring we've called vringh_complete_kern() on.
 *
 * Returns -errno or 0 if we don't need to tell the other side, 1 if we do.
 */
int vringh_need_notify_kern(struct vringh *vrh)
{
        return __vringh_need_notify(vrh, getu16_kern);
}
EXPORT_SYMBOL(vringh_need_notify_kern);

MODULE_LICENSE("GPL");