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

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Message-ID: <20170922090257.GB9243@stefanha-x1.localdomain>
Date:   Fri, 22 Sep 2017 10:02:57 +0100
From:   Stefan Hajnoczi <stefanha@...il.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Subject: Re: [PATCH net-next RFC 2/5] vhost: introduce helper to prefetch
 desc index

On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang wrote:
> 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)

Missing doc comment.

> +{
> +	int ret, ret2;
> +	u16 last_avail_idx, last_used_idx, total, copied;
> +	__virtio16 avail_idx;
> +	struct vring_used_elem __user *used;
> +	int i;

The following variable names are a little confusing:

last_avail_idx vs vq->last_avail_idx.  last_avail_idx is a wrapped
avail->ring[] index, vq->last_avail_idx is a free-running counter.  The
same for last_used_idx vs vq->last_used_idx.

num argument vs vq->num.  The argument could be called nheads instead to
make it clear that this is heads[] and not the virtqueue size.

Not a bug but it took me a while to figure out what was going on.