Re: [PATCH net-next RFC 2/5] vhost: introduce helper to prefetch desc index

[Jason Wang]

On 2017å09æ28æ 08:47, Willem de Bruijn wrote:
> On Fri, Sep 22, 2017 at 4:02 AM, Jason Wang <jasowang@xxxxxxxxxx> wrote:
> > This patch introduces vhost_prefetch_desc_indices() which could batch
> > descriptor indices fetching and used ring updating. This intends to
> > reduce the cache misses of indices fetching and updating and reduce
> > cache line bounce when virtqueue is almost full. copy_to_user() was
> > used in order to benefit from modern cpus that support fast string
> > copy. Batched virtqueue processing will be the first user.
> >
> > Signed-off-by: Jason Wang <jasowang@xxxxxxxxxx>
> > ---
> >   drivers/vhost/vhost.c | 55 +++++++++++++++++++++++++++++++++++++++++++++++++++
> >   drivers/vhost/vhost.h |  3 +++
> >   2 files changed, 58 insertions(+)
> >
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index f87ec75..8424166d 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct vhost_dev *dev,
> >   }
> >   EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
> >
> > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> > +                               struct vring_used_elem *heads,
> > +                               u16 num, bool used_update)
> > +{
> > +       int ret, ret2;
> > +       u16 last_avail_idx, last_used_idx, total, copied;
> > +       __virtio16 avail_idx;
> > +       struct vring_used_elem __user *used;
> > +       int i;
> > +
> > +       if (unlikely(vhost_get_avail(vq, avail_idx, &vq->avail->idx))) {
> > +               vq_err(vq, "Failed to access avail idx at %p\n",
> > +                      &vq->avail->idx);
> > +               return -EFAULT;
> > +       }
> > +       last_avail_idx = vq->last_avail_idx & (vq->num - 1);
> > +       vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > +       total = vq->avail_idx - vq->last_avail_idx;
> > +       ret = total = min(total, num);
> > +
> > +       for (i = 0; i < ret; i++) {
> > +               ret2 = vhost_get_avail(vq, heads[i].id,
> > +                                     &vq->avail->ring[last_avail_idx]);
> > +               if (unlikely(ret2)) {
> > +                       vq_err(vq, "Failed to get descriptors\n");
> > +                       return -EFAULT;
> > +               }
> > +               last_avail_idx = (last_avail_idx + 1) & (vq->num - 1);
> > +       }
> This is understandably very similar to the existing logic in vhost_get_vq_desc.
> Can that be extracted to a helper to avoid code duplication?
>
> Perhaps one helper to update vq->avail_idx and return num, and
> another to call vhost_get_avail one or more times.

Yes it can.

> > +
> > +       if (!used_update)
> > +               return ret;
> > +
> > +       last_used_idx = vq->last_used_idx & (vq->num - 1);
> > +       while (total) {
> > +               copied = min((u16)(vq->num - last_used_idx), total);
> > +               ret2 = vhost_copy_to_user(vq,
> > +                                         &vq->used->ring[last_used_idx],
> > +                                         &heads[ret - total],
> > +                                         copied * sizeof(*used));
> > +
> > +               if (unlikely(ret2)) {
> > +                       vq_err(vq, "Failed to update used ring!\n");
> > +                       return -EFAULT;
> > +               }
> > +
> > +               last_used_idx = 0;
> > +               total -= copied;
> > +       }
> This second part seems unrelated and could be a separate function?

Yes.

> Also, no need for ret2 and double assignment "ret = total =" if not
> modifying total
> in the the second loop:
>
>    for (i = 0; i < total; ) {
>      ...
>      i += copied;
>    }

Right, will do this in V2.

Thanks