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

[Jason Wang]

On 2017å09æ27æ 03:19, Michael S. Tsirkin wrote:
> On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang 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)
> why do you need to combine used update with prefetch?

For better performance and I believe we don't care about the overhead 
when we meet errors in tx.

> > +{
> > +	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);
> > +	}
> > +
> > +	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;
> > +	}
> > +
> > +	/* Only get avail ring entries after they have been exposed by guest. */
> > +	smp_rmb();
> Barrier before return is a very confusing API. I guess it's designed to
> be used in a specific way to make it necessary - but what is it?

Looks like a and we need do this after reading avail_idx.

Thanks

> > +	return ret;
> > +}
> > +EXPORT_SYMBOL(vhost_prefetch_desc_indices);
> >   
> >   static int __init vhost_init(void)
> >   {
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 39ff897..16c2cb6 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -228,6 +228,9 @@ ssize_t vhost_chr_read_iter(struct vhost_dev *dev, struct iov_iter *to,
> >   ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
> >   			     struct iov_iter *from);
> >   int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
> > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> > +				struct vring_used_elem *heads,
> > +				u16 num, bool used_update);
> >   
> >   #define vq_err(vq, fmt, ...) do {                                  \
> >   		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> > --
> > 2.7.4