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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 28 Sep 2017 01:57:20 +0300

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 <[email-masked]>
> > > ---
> > >   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
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