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    Subject[PATCH RFC 01/13] vhost: option to fetch descriptors through an independent struct
    The idea is to support multiple ring formats by converting
    to a format-independent array of descriptors.

    This costs extra cycles, but we gain in ability
    to fetch a batch of descriptors in one go, which
    is good for code cache locality.

    When used, this causes a minor performance degradation,
    it's been kept as simple as possible for ease of review.
    A follow-up patch gets us back the performance by adding batching.

    To simplify benchmarking, I kept the old code around so one can switch
    back and forth between old and new code. This will go away in the final
    submission.

    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
    Link: https://lore.kernel.org/r/20200401183118.8334-2-eperezma@redhat.com
    Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
    ---
    drivers/vhost/vhost.c | 297 +++++++++++++++++++++++++++++++++++++++++-
    drivers/vhost/vhost.h | 16 +++
    2 files changed, 312 insertions(+), 1 deletion(-)

    diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
    index 96d9871fa0cb..105fc97af2c8 100644
    --- a/drivers/vhost/vhost.c
    +++ b/drivers/vhost/vhost.c
    @@ -298,6 +298,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
    struct vhost_virtqueue *vq)
    {
    vq->num = 1;
    + vq->ndescs = 0;
    vq->desc = NULL;
    vq->avail = NULL;
    vq->used = NULL;
    @@ -368,6 +369,9 @@ static int vhost_worker(void *data)

    static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
    {
    + kfree(vq->descs);
    + vq->descs = NULL;
    + vq->max_descs = 0;
    kfree(vq->indirect);
    vq->indirect = NULL;
    kfree(vq->log);
    @@ -384,6 +388,10 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)

    for (i = 0; i < dev->nvqs; ++i) {
    vq = dev->vqs[i];
    + vq->max_descs = dev->iov_limit;
    + vq->descs = kmalloc_array(vq->max_descs,
    + sizeof(*vq->descs),
    + GFP_KERNEL);
    vq->indirect = kmalloc_array(UIO_MAXIOV,
    sizeof(*vq->indirect),
    GFP_KERNEL);
    @@ -391,7 +399,7 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
    GFP_KERNEL);
    vq->heads = kmalloc_array(dev->iov_limit, sizeof(*vq->heads),
    GFP_KERNEL);
    - if (!vq->indirect || !vq->log || !vq->heads)
    + if (!vq->indirect || !vq->log || !vq->heads || !vq->descs)
    goto err_nomem;
    }
    return 0;
    @@ -2277,6 +2285,293 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
    }
    EXPORT_SYMBOL_GPL(vhost_get_vq_desc);

    +static struct vhost_desc *peek_split_desc(struct vhost_virtqueue *vq)
    +{
    + BUG_ON(!vq->ndescs);
    + return &vq->descs[vq->ndescs - 1];
    +}
    +
    +static void pop_split_desc(struct vhost_virtqueue *vq)
    +{
    + BUG_ON(!vq->ndescs);
    + --vq->ndescs;
    +}
    +
    +#define VHOST_DESC_FLAGS (VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE | \
    + VRING_DESC_F_NEXT)
    +static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc, u16 id)
    +{
    + struct vhost_desc *h;
    +
    + if (unlikely(vq->ndescs >= vq->max_descs))
    + return -EINVAL;
    + h = &vq->descs[vq->ndescs++];
    + h->addr = vhost64_to_cpu(vq, desc->addr);
    + h->len = vhost32_to_cpu(vq, desc->len);
    + h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
    + h->id = id;
    +
    + return 0;
    +}
    +
    +static int fetch_indirect_descs(struct vhost_virtqueue *vq,
    + struct vhost_desc *indirect,
    + u16 head)
    +{
    + struct vring_desc desc;
    + unsigned int i = 0, count, found = 0;
    + u32 len = indirect->len;
    + struct iov_iter from;
    + int ret;
    +
    + /* Sanity check */
    + if (unlikely(len % sizeof desc)) {
    + vq_err(vq, "Invalid length in indirect descriptor: "
    + "len 0x%llx not multiple of 0x%zx\n",
    + (unsigned long long)len,
    + sizeof desc);
    + return -EINVAL;
    + }
    +
    + ret = translate_desc(vq, indirect->addr, len, vq->indirect,
    + UIO_MAXIOV, VHOST_ACCESS_RO);
    + if (unlikely(ret < 0)) {
    + if (ret != -EAGAIN)
    + vq_err(vq, "Translation failure %d in indirect.\n", ret);
    + return ret;
    + }
    + iov_iter_init(&from, READ, vq->indirect, ret, len);
    +
    + /* We will use the result as an address to read from, so most
    + * architectures only need a compiler barrier here. */
    + read_barrier_depends();
    +
    + count = len / sizeof desc;
    + /* Buffers are chained via a 16 bit next field, so
    + * we can have at most 2^16 of these. */
    + if (unlikely(count > USHRT_MAX + 1)) {
    + vq_err(vq, "Indirect buffer length too big: %d\n",
    + indirect->len);
    + return -E2BIG;
    + }
    + if (unlikely(vq->ndescs + count > vq->max_descs)) {
    + vq_err(vq, "Too many indirect + direct descs: %d + %d\n",
    + vq->ndescs, indirect->len);
    + return -E2BIG;
    + }
    +
    + do {
    + if (unlikely(++found > count)) {
    + vq_err(vq, "Loop detected: last one at %u "
    + "indirect size %u\n",
    + i, count);
    + return -EINVAL;
    + }
    + if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) {
    + vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
    + i, (size_t)indirect->addr + i * sizeof desc);
    + return -EINVAL;
    + }
    + if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
    + vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
    + i, (size_t)indirect->addr + i * sizeof desc);
    + return -EINVAL;
    + }
    +
    + push_split_desc(vq, &desc, head);
    + } while ((i = next_desc(vq, &desc)) != -1);
    + return 0;
    +}
    +
    +static int fetch_descs(struct vhost_virtqueue *vq)
    +{
    + unsigned int i, head, found = 0;
    + struct vhost_desc *last;
    + struct vring_desc desc;
    + __virtio16 avail_idx;
    + __virtio16 ring_head;
    + u16 last_avail_idx;
    + int ret;
    +
    + /* Check it isn't doing very strange things with descriptor numbers. */
    + last_avail_idx = vq->last_avail_idx;
    +
    + if (vq->avail_idx == vq->last_avail_idx) {
    + if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
    + vq_err(vq, "Failed to access avail idx at %p\n",
    + &vq->avail->idx);
    + return -EFAULT;
    + }
    + vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
    +
    + if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
    + vq_err(vq, "Guest moved used index from %u to %u",
    + last_avail_idx, vq->avail_idx);
    + return -EFAULT;
    + }
    +
    + /* If there's nothing new since last we looked, return
    + * invalid.
    + */
    + if (vq->avail_idx == last_avail_idx)
    + return vq->num;
    +
    + /* Only get avail ring entries after they have been
    + * exposed by guest.
    + */
    + smp_rmb();
    + }
    +
    + /* Grab the next descriptor number they're advertising */
    + if (unlikely(vhost_get_avail_head(vq, &ring_head, last_avail_idx))) {
    + vq_err(vq, "Failed to read head: idx %d address %p\n",
    + last_avail_idx,
    + &vq->avail->ring[last_avail_idx % vq->num]);
    + return -EFAULT;
    + }
    +
    + head = vhost16_to_cpu(vq, ring_head);
    +
    + /* If their number is silly, that's an error. */
    + if (unlikely(head >= vq->num)) {
    + vq_err(vq, "Guest says index %u > %u is available",
    + head, vq->num);
    + return -EINVAL;
    + }
    +
    + i = head;
    + do {
    + if (unlikely(i >= vq->num)) {
    + vq_err(vq, "Desc index is %u > %u, head = %u",
    + i, vq->num, head);
    + return -EINVAL;
    + }
    + if (unlikely(++found > vq->num)) {
    + vq_err(vq, "Loop detected: last one at %u "
    + "vq size %u head %u\n",
    + i, vq->num, head);
    + return -EINVAL;
    + }
    + ret = vhost_get_desc(vq, &desc, i);
    + if (unlikely(ret)) {
    + vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
    + i, vq->desc + i);
    + return -EFAULT;
    + }
    + ret = push_split_desc(vq, &desc, head);
    + if (unlikely(ret)) {
    + vq_err(vq, "Failed to save descriptor: idx %d\n", i);
    + return -EINVAL;
    + }
    + } while ((i = next_desc(vq, &desc)) != -1);
    +
    + last = peek_split_desc(vq);
    + if (unlikely(last->flags & VRING_DESC_F_INDIRECT)) {
    + pop_split_desc(vq);
    + ret = fetch_indirect_descs(vq, last, head);
    + if (unlikely(ret < 0)) {
    + if (ret != -EAGAIN)
    + vq_err(vq, "Failure detected "
    + "in indirect descriptor at idx %d\n", head);
    + return ret;
    + }
    + }
    +
    + /* Assume notifications from guest are disabled at this point,
    + * if they aren't we would need to update avail_event index. */
    + BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
    +
    + /* On success, increment avail index. */
    + vq->last_avail_idx++;
    +
    + return 0;
    +}
    +
    +/* This looks in the virtqueue and for the first available buffer, and converts
    + * it to an iovec for convenient access. Since descriptors consist of some
    + * number of output then some number of input descriptors, it's actually two
    + * iovecs, but we pack them into one and note how many of each there were.
    + *
    + * This function returns the descriptor number found, or vq->num (which is
    + * never a valid descriptor number) if none was found. A negative code is
    + * returned on error. */
    +int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
    + struct iovec iov[], unsigned int iov_size,
    + unsigned int *out_num, unsigned int *in_num,
    + struct vhost_log *log, unsigned int *log_num)
    +{
    + int ret = fetch_descs(vq);
    + int i;
    +
    + if (ret)
    + return ret;
    +
    + /* Now convert to IOV */
    + /* When we start there are none of either input nor output. */
    + *out_num = *in_num = 0;
    + if (unlikely(log))
    + *log_num = 0;
    +
    + for (i = 0; i < vq->ndescs; ++i) {
    + unsigned iov_count = *in_num + *out_num;
    + struct vhost_desc *desc = &vq->descs[i];
    + int access;
    +
    + if (desc->flags & ~VHOST_DESC_FLAGS) {
    + vq_err(vq, "Unexpected flags: 0x%x at descriptor id 0x%x\n",
    + desc->flags, desc->id);
    + ret = -EINVAL;
    + goto err;
    + }
    + if (desc->flags & VRING_DESC_F_WRITE)
    + access = VHOST_ACCESS_WO;
    + else
    + access = VHOST_ACCESS_RO;
    + ret = translate_desc(vq, desc->addr,
    + desc->len, iov + iov_count,
    + iov_size - iov_count, access);
    + if (unlikely(ret < 0)) {
    + if (ret != -EAGAIN)
    + vq_err(vq, "Translation failure %d descriptor idx %d\n",
    + ret, i);
    + goto err;
    + }
    + if (access == VHOST_ACCESS_WO) {
    + /* If this is an input descriptor,
    + * increment that count. */
    + *in_num += ret;
    + if (unlikely(log && ret)) {
    + log[*log_num].addr = desc->addr;
    + log[*log_num].len = desc->len;
    + ++*log_num;
    + }
    + } else {
    + /* If it's an output descriptor, they're all supposed
    + * to come before any input descriptors. */
    + if (unlikely(*in_num)) {
    + vq_err(vq, "Descriptor has out after in: "
    + "idx %d\n", i);
    + ret = -EINVAL;
    + goto err;
    + }
    + *out_num += ret;
    + }
    +
    + ret = desc->id;
    + }
    +
    + vq->ndescs = 0;
    +
    + return ret;
    +
    +err:
    + vhost_discard_vq_desc(vq, 1);
    + vq->ndescs = 0;
    +
    + return ret;
    +}
    +EXPORT_SYMBOL_GPL(vhost_get_vq_desc_batch);
    +
    /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
    void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
    {
    diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
    index 60cab4c78229..0976a2853935 100644
    --- a/drivers/vhost/vhost.h
    +++ b/drivers/vhost/vhost.h
    @@ -60,6 +60,13 @@ enum vhost_uaddr_type {
    VHOST_NUM_ADDRS = 3,
    };

    +struct vhost_desc {
    + u64 addr;
    + u32 len;
    + u16 flags; /* VRING_DESC_F_WRITE, VRING_DESC_F_NEXT */
    + u16 id;
    +};
    +
    /* The virtqueue structure describes a queue attached to a device. */
    struct vhost_virtqueue {
    struct vhost_dev *dev;
    @@ -71,6 +78,11 @@ struct vhost_virtqueue {
    vring_avail_t __user *avail;
    vring_used_t __user *used;
    const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS];
    +
    + struct vhost_desc *descs;
    + int ndescs;
    + int max_descs;
    +
    struct file *kick;
    struct eventfd_ctx *call_ctx;
    struct eventfd_ctx *error_ctx;
    @@ -175,6 +187,10 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
    bool vhost_vq_access_ok(struct vhost_virtqueue *vq);
    bool vhost_log_access_ok(struct vhost_dev *);

    +int vhost_get_vq_desc_batch(struct vhost_virtqueue *,
    + struct iovec iov[], unsigned int iov_count,
    + unsigned int *out_num, unsigned int *in_num,
    + struct vhost_log *log, unsigned int *log_num);
    int vhost_get_vq_desc(struct vhost_virtqueue *,
    struct iovec iov[], unsigned int iov_count,
    unsigned int *out_num, unsigned int *in_num,
    --
    MST
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