linux-kernel - Re: [PATCH v2 3/4] vhost: Improve vhost_get_avail

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Date: Mon, 29 Apr 2024 14:48:15 -0400
From: "Michael S. Tsirkin" <mst@...hat.com>
To: Gavin Shan <gshan@...hat.com>
Cc: virtualization@...ts.linux.dev, linux-kernel@...r.kernel.org,
	jasowang@...hat.com, shan.gavin@...il.com
Subject: Re: [PATCH v2 3/4] vhost: Improve vhost_get_avail_head()

On Mon, Apr 29, 2024 at 08:13:59PM +1000, Gavin Shan wrote:
> Improve vhost_get_avail_head() so that the head or errno is returned.
> With it, the relevant sanity checks are squeezed to vhost_get_avail_head()
> and vhost_get_vq_desc() is further simplified.
> 
> No functional change intended.
> 
> Signed-off-by: Gavin Shan <gshan@...hat.com>

I don't see what does this moving code around achieve.

> ---
>  drivers/vhost/vhost.c | 50 ++++++++++++++++++++++---------------------
>  1 file changed, 26 insertions(+), 24 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index b278c0333a66..4ddb9ec2fe46 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1322,11 +1322,27 @@ static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
>  	return 1;
>  }
>  
> -static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> -				       __virtio16 *head, int idx)
> +static inline int vhost_get_avail_head(struct vhost_virtqueue *vq)
>  {
> -	return vhost_get_avail(vq, *head,
> -			       &vq->avail->ring[idx & (vq->num - 1)]);
> +	__virtio16 head;
> +	int r;
> +
> +	r = vhost_get_avail(vq, head,
> +			    &vq->avail->ring[vq->last_avail_idx & (vq->num - 1)]);
> +	if (unlikely(r)) {
> +		vq_err(vq, "Failed to read head: index %u address %p\n",
> +		       vq->last_avail_idx,
> +		       &vq->avail->ring[vq->last_avail_idx & (vq->num - 1)]);
> +		return r;
> +	}
> +
> +	r = vhost16_to_cpu(vq, head);
> +	if (unlikely(r >= vq->num)) {
> +		vq_err(vq, "Invalid head %d (%u)\n", r, vq->num);
> +		return -EINVAL;
> +	}
> +
> +	return r;
>  }
>  
>  static inline int vhost_get_avail_flags(struct vhost_virtqueue *vq,
> @@ -2523,9 +2539,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  		      struct vhost_log *log, unsigned int *log_num)
>  {
>  	struct vring_desc desc;
> -	unsigned int i, head, found = 0;
> -	__virtio16 ring_head;
> -	int ret, access;
> +	unsigned int i, found = 0;
> +	int head, ret, access;
>  
>  	if (vq->avail_idx == vq->last_avail_idx) {
>  		ret = vhost_get_avail_idx(vq);
> @@ -2536,23 +2551,10 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  			return vq->num;
>  	}
>  
> -	/* Grab the next descriptor number they're advertising, and increment
> -	 * the index we've seen. */
> -	if (unlikely(vhost_get_avail_head(vq, &ring_head, vq->last_avail_idx))) {
> -		vq_err(vq, "Failed to read head: idx %d address %p\n",
> -		       vq->last_avail_idx,
> -		       &vq->avail->ring[vq->last_avail_idx % vq->num]);
> -		return -EFAULT;
> -	}
> -
> -	head = vhost16_to_cpu(vq, ring_head);
> -
> -	/* If their number is silly, that's an error. */
> -	if (unlikely(head >= vq->num)) {
> -		vq_err(vq, "Guest says index %u > %u is available",
> -		       head, vq->num);
> -		return -EINVAL;
> -	}
> +	/* Grab the next descriptor number they're advertising */
> +	head = vhost_get_avail_head(vq);
> +	if (unlikely(head < 0))
> +		return head;
>  
>  	/* When we start there are none of either input nor output. */
>  	*out_num = *in_num = 0;
> -- 
> 2.44.0