netdev - Re: [RFC v2 1/7] vhost: expose used buffers

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Date:   Thu, 18 Aug 2022 08:16:40 +0200
From:   Eugenio Perez Martin <eperezma@...hat.com>
To:     Guo Zhi <qtxuning1999@...u.edu.cn>
Cc:     Jason Wang <jasowang@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Michael Tsirkin <mst@...hat.com>,
        netdev <netdev@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvm list <kvm@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>
Subject: Re: [RFC v2 1/7] vhost: expose used buffers

On Wed, Aug 17, 2022 at 3:58 PM Guo Zhi <qtxuning1999@...u.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@...u.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
>