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

Michael S. Tsirkin
2017-09-27T22:57:20Z

On Wed, Sep 27, 2017 at 08:35:47AM +0800, Jason Wang wrote:
>
>
> On 2017年09月27日 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@redhat.com>
> > > ---
> > > 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

Why is sticking a branch in there better than requesting the update
conditionally from the caller?

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

That's a separate question, I do not really understand how
you can fetch a descriptor and update the used ring at the same
time. This allows the guest to overwrite the buffer.
I might be misunderstanding what is going on here though.

> >
> > > +{
> > > + 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