Re: [PATCH v2 1/4] vhost: Improve vhost_get_avail_idx() with smp_rmb()

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

On Mon, Apr 29, 2024 at 08:13:57PM +1000, Gavin Shan wrote:
> From: "Michael S. Tsirkin" <mst@redhat.com>
> 
> All the callers of vhost_get_avail_idx() are concerned to the memory

*with* the memory barrier

> 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.

accessed, not advanced. guest advances it.

> With it, the callers needn't to worry
> about the memory barrier.
> 
> No functional change intended.

I'd add:

As a side benefit, we also validate the index on all paths now, which
will hopefully help catch future errors earlier.

Note: current code is inconsistent in how it handles errors:
some places treat it as an empty ring, others - non empty.
This patch does not attempt to change the existing behaviour.



> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> [gshan: repainted vhost_get_avail_idx()]

?repainted?

> Reviewed-by: Gavin Shan <gshan@redhat.com>
> Acked-by: Will Deacon <will@kernel.org>
> ---
>  drivers/vhost/vhost.c | 106 +++++++++++++++++-------------------------
>  1 file changed, 42 insertions(+), 64 deletions(-)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 8995730ce0bf..7aa623117aab 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -1290,10 +1290,36 @@ 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 idx;
> +	int r;
> +
> +	r = vhost_get_avail(vq, idx, &vq->avail->idx);
> +	if (unlikely(r < 0)) {
> +		vq_err(vq, "Failed to access available index at %p (%d)\n",
> +		       &vq->avail->idx, r);
> +		return r;
> +	}
> +
> +	/* Check it isn't doing very strange thing with available indexes */
> +	vq->avail_idx = vhost16_to_cpu(vq, idx);
> +	if (unlikely((u16)(vq->avail_idx - vq->last_avail_idx) > vq->num)) {
> +		vq_err(vq, "Invalid available index change from %u to %u",
> +		       vq->last_avail_idx, vq->avail_idx);
> +		return -EINVAL;
> +	}
> +
> +	/* We're done if there is nothing new */
> +	if (vq->avail_idx == vq->last_avail_idx)
> +		return 0;
> +
> +	/*
> +	 * We updated vq->avail_idx so we need a memory barrier between
> +	 * the index read above and the caller reading avail ring entries.
> +	 */
> +	smp_rmb();
> +	return 1;
>  }
>  
>  static inline int vhost_get_avail_head(struct vhost_virtqueue *vq,
> @@ -2498,38 +2524,17 @@ 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;
> +	u16 last_avail_idx = vq->last_avail_idx;
>  	__virtio16 ring_head;
>  	int ret, access;
>  
> -	/* Check it isn't doing very strange things with descriptor numbers. */
> -	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);
> -			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 avail index from %u to %u",
> -				last_avail_idx, vq->avail_idx);
> -			return -EFAULT;
> -		}
> +		ret = vhost_get_avail_idx(vq);
> +		if (unlikely(ret < 0))
> +			return ret;
>  
> -		/* If there's nothing new since last we looked, return
> -		 * invalid.
> -		 */
> -		if (vq->avail_idx == last_avail_idx)
> +		if (!ret)
>  			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 +2795,20 @@ 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();
> -		return false;
> -	}
> -
> -	return true;
> +	/* Treat error as non-empty here */

If you write the comment like that then put it before "return":
that is where you treat an error like this.
And I feel Note: is better in that the comment does not
explain all of what is going on, just an aspect of it.

> +	r = vhost_get_avail_idx(vq);
> +	return r == 0;
>  }
>  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 +2832,13 @@ 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);
> -		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;
> -	}
> +	/* Treat error as empty here */
> +	r = vhost_get_avail_idx(vq);

If you write the comment like that then put it before "return":
that is where you treat an error like this.
And I feel Note: is better in that the comment does not
explain all of what is going on, just an aspect of it.

> +	if (unlikely(r < 0))
> +		return false;
>  
> -	return false;
> +	return r;
>  }
>  EXPORT_SYMBOL_GPL(vhost_enable_notify);
>  
> -- 
> 2.44.0