Re: [PATCH v3 3/3] vhost: Improve vhost_get_avail_idx() with smp_rmb()

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Mar 28, 2024 at 10:21:49AM +1000, Gavin Shan wrote:
> All the callers of vhost_get_avail_idx() are concerned to the memory
> barrier, imposed by smp_rmb() to ensure the order of the available
> ring entry read and avail_idx read.
> 
> Improve vhost_get_avail_idx() so that smp_rmb() is executed when
> the avail_idx is advanced. With it, the callers needn't to worry
> about the memory barrier.
> 
> Suggested-by: Michael S. Tsirkin <mst@redhat.com>
> Signed-off-by: Gavin Shan <gshan@redhat.com>

Previous patches are ok. This one I feel needs more work -
first more code such as sanity checking should go into
this function, second there's actually a difference
between comparing to last_avail_idx and just comparing
to the previous value of avail_idx.
I will pick patches 1-2 and post a cleanup on top so you can
take a look, ok?


> ---
>  drivers/vhost/vhost.c | 75 +++++++++++++++----------------------------
>  1 file changed, 26 insertions(+), 49 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 32686c79c41d..e6882f4f6ce2 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1290,10 +1290,28 @@ static void vhost_dev_unlock_vqs(struct vhost_dev *d)
>  		mutex_unlock(&d->vqs[i]->mutex);
>  }
>  
> -static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq,
> -				      __virtio16 *idx)
> +static inline int vhost_get_avail_idx(struct vhost_virtqueue *vq)
>  {
> -	return vhost_get_avail(vq, *idx, &vq->avail->idx);
> +	__virtio16 avail_idx;
> +	int r;
> +
> +	r = vhost_get_avail(vq, avail_idx, &vq->avail->idx);
> +	if (unlikely(r)) {
> +		vq_err(vq, "Failed to access avail idx at %p\n",
> +		       &vq->avail->idx);
> +		return r;
> +	}
> +
> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> +	if (vq->avail_idx != vq->last_avail_idx) {
> +		/* Ensure the available ring entry read happens
> +		 * before the avail_idx read when the avail_idx
> +		 * is advanced.
> +		 */
> +		smp_rmb();
> +	}
> +
> +	return 0;
>  }
>  
>  static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> @@ -2499,7 +2517,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  	struct vring_desc desc;
>  	unsigned int i, head, found = 0;
>  	u16 last_avail_idx;
> -	__virtio16 avail_idx;
>  	__virtio16 ring_head;
>  	int ret, access;
>  
> @@ -2507,12 +2524,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  	last_avail_idx = vq->last_avail_idx;
>  
>  	if (vq->avail_idx == vq->last_avail_idx) {
> -		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> -			vq_err(vq, "Failed to access avail idx at %p\n",
> -				&vq->avail->idx);
> +		if (unlikely(vhost_get_avail_idx(vq)))
>  			return -EFAULT;
> -		}
> -		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>  
>  		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
>  			vq_err(vq, "Guest moved used index from %u to %u",
> @@ -2525,11 +2538,6 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
>  		 */
>  		if (vq->avail_idx == last_avail_idx)
>  			return vq->num;
> -
> -		/* Only get avail ring entries after they have been
> -		 * exposed by guest.
> -		 */
> -		smp_rmb();
>  	}
>  
>  	/* Grab the next descriptor number they're advertising, and increment
> @@ -2790,35 +2798,19 @@ EXPORT_SYMBOL_GPL(vhost_add_used_and_signal_n);
>  /* return true if we're sure that avaiable ring is empty */
>  bool vhost_vq_avail_empty(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  {
> -	__virtio16 avail_idx;
> -	int r;
> -
>  	if (vq->avail_idx != vq->last_avail_idx)
>  		return false;
>  
> -	r = vhost_get_avail_idx(vq, &avail_idx);
> -	if (unlikely(r))
> -		return false;
> -
> -	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> -	if (vq->avail_idx != vq->last_avail_idx) {
> -		/* Since we have updated avail_idx, the following
> -		 * call to vhost_get_vq_desc() will read available
> -		 * ring entries. Make sure that read happens after
> -		 * the avail_idx read.
> -		 */
> -		smp_rmb();
> +	if (unlikely(vhost_get_avail_idx(vq)))
>  		return false;
> -	}
>  
> -	return true;
> +	return vq->avail_idx == vq->last_avail_idx;
>  }
>  EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);
>  
>  /* OK, now we need to know about added descriptors. */
>  bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  {
> -	__virtio16 avail_idx;
>  	int r;
>  
>  	if (!(vq->used_flags & VRING_USED_F_NO_NOTIFY))
> @@ -2842,25 +2834,10 @@ bool vhost_enable_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
>  	/* They could have slipped one in as we were doing that: make
>  	 * sure it's written, then check again. */
>  	smp_mb();
> -	r = vhost_get_avail_idx(vq, &avail_idx);
> -	if (r) {
> -		vq_err(vq, "Failed to check avail idx at %p: %d\n",
> -		       &vq->avail->idx, r);
> +	if (unlikely(vhost_get_avail_idx(vq)))
>  		return false;
> -	}
> -
> -	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> -	if (vq->avail_idx != vq->last_avail_idx) {
> -		/* Since we have updated avail_idx, the following
> -		 * call to vhost_get_vq_desc() will read available
> -		 * ring entries. Make sure that read happens after
> -		 * the avail_idx read.
> -		 */
> -		smp_rmb();
> -		return true;
> -	}
>  
> -	return false;
> +	return vq->avail_idx != vq->last_avail_idx;
>  }
>  EXPORT_SYMBOL_GPL(vhost_enable_notify);
>  
> -- 
> 2.44.0