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SubjectRe: [PATCH RFC 01/13] vhost: option to fetch descriptors through an independent struct
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On 2020/6/2 下午9:05, Michael S. Tsirkin wrote:
> 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);


The error is ignored.


> + } 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);


Note that this means we don't supported chained indirect descriptors
which complies the spec but we support this in vhost_get_vq_desc().

We probably need either fail early or just support that.

Thanks


> + 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,

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