Re: [RFC v3 1/7] vhost: expose used buffers

[Eugenio Perez Martin]

On Thu, Sep 1, 2022 at 7:55 AM Guo Zhi <qtxuning1999@xxxxxxxxxxx> wrote:
>
> Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
> of descriptors.
>
> Signed-off-by: Guo Zhi <qtxuning1999@xxxxxxxxxxx>
> ---
>  drivers/vhost/vhost.c | 16 +++++++++++++---
>  1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 40097826cff0..26862c8bf751 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;
>         }
> @@ -2388,7 +2398,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>                 smp_wmb();
>                 /* Log used ring entry write. */
>                 log_used(vq, ((void __user *)used - (void __user *)vq->used),
> -                        count * sizeof *used);
> +                        copy_n * sizeof(*used));

log_used reports to the VMM the modified memory by the device. It
iterates over used descriptors translating them to do so.

We need to either report here all the descriptors or to modify
log_used so it reports all the batch with in_order feature. The latter
has an extra advantage: no need to report these non-existent writes to
the used ring of the skipped buffers. Although it probably does not
make a difference in performance.

With the current code, we could iterate the heads[] array too, calling
. However, I think it would be a waste. More on that later.

Thanks!

>         }
>         old = vq->last_used_idx;
>         new = (vq->last_used_idx += count);
> @@ -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;
> --
> 2.17.1
>