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    SubjectRe: [RFC v2 1/7] vhost: expose used buffers
    On Wed, Aug 17, 2022 at 3:58 PM Guo Zhi <qtxuning1999@sjtu.edu.cn> wrote:
    >
    > Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
    > of descriptors.
    >
    > Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
    > ---
    > drivers/vhost/vhost.c | 14 ++++++++++++--
    > drivers/vhost/vhost.h | 1 +
    > 2 files changed, 13 insertions(+), 2 deletions(-)
    >
    > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
    > index 40097826cff0..7b20fa5a46c3 100644
    > --- a/drivers/vhost/vhost.c
    > +++ b/drivers/vhost/vhost.c
    > @@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
    > vring_used_elem_t __user *used;
    > u16 old, new;
    > int start;
    > + int copy_n = count;
    >
    > + /**
    > + * If in order feature negotiated, devices can notify the use of a batch of buffers to
    > + * the driver by only writing out a single used ring entry with the id corresponding
    > + * to the head entry of the descriptor chain describing the last buffer in the batch.
    > + */
    > + if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
    > + copy_n = 1;
    > + heads = &heads[count - 1];
    > + }
    > start = vq->last_used_idx & (vq->num - 1);
    > used = vq->used->ring + start;
    > - if (vhost_put_used(vq, heads, start, count)) {
    > + if (vhost_put_used(vq, heads, start, copy_n)) {
    > vq_err(vq, "Failed to write used");
    > return -EFAULT;
    > }
    > @@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
    >
    > start = vq->last_used_idx & (vq->num - 1);
    > n = vq->num - start;
    > - if (n < count) {
    > + if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
    > r = __vhost_add_used_n(vq, heads, n);
    > if (r < 0)
    > return r;

    It would be simpler to use vhost_add_used in vsock/vhost_test to add a
    batch of used descriptors, and leave vhost_add_used_n untouched.

    Since it's the upper layer the one that manages the in_order in this
    version, we could:
    * Always call vhost_add_used(vq, last_head_of_batch, ...) for the tx
    queue, that does not need used length info.
    * Call vhost_add_used_n for the rx queue, since the driver needs the
    used length info.

    > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
    > index d9109107af08..0d5c49a30421 100644
    > --- a/drivers/vhost/vhost.h
    > +++ b/drivers/vhost/vhost.h
    > @@ -236,6 +236,7 @@ enum {
    > VHOST_FEATURES = (1ULL << VIRTIO_F_NOTIFY_ON_EMPTY) |
    > (1ULL << VIRTIO_RING_F_INDIRECT_DESC) |
    > (1ULL << VIRTIO_RING_F_EVENT_IDX) |
    > + (1ULL << VIRTIO_F_IN_ORDER) |
    > (1ULL << VHOST_F_LOG_ALL) |
    > (1ULL << VIRTIO_F_ANY_LAYOUT) |
    > (1ULL << VIRTIO_F_VERSION_1)
    > --
    > 2.17.1
    >

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