Re: [RFC v3 1/7] vhost: expose used buffers

[Jason Wang <jasowang@redhat.com>]

在 2022/9/1 13:54, Guo Zhi 写道:
> Follow VIRTIO 1.1 spec, only writing out a single used ring for a batch
> of descriptors.
>
> Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
> ---
>   drivers/vhost/vhost.c | 16 +++++++++++++---
>   1 file changed, 13 insertions(+), 3 deletions(-)
>
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index 40097826cff0..26862c8bf751 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2376,10 +2376,20 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>   	vring_used_elem_t __user *used;
>   	u16 old, new;
>   	int start;
> +	int copy_n = count;
>   
> +	/**
> +	 * If in order feature negotiated, devices can notify the use of a batch of buffers to
> +	 * the driver by only writing out a single used ring entry with the id corresponding
> +	 * to the head entry of the descriptor chain describing the last buffer in the batch.
> +	 */
> +	if (vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {
> +		copy_n = 1;
> +		heads = &heads[count - 1];
> +	}


Would it better to have a dedicated helper like 
vhost_add_used_in_order() here?


>   	start = vq->last_used_idx & (vq->num - 1);
>   	used = vq->used->ring + start;
> -	if (vhost_put_used(vq, heads, start, count)) {
> +	if (vhost_put_used(vq, heads, start, copy_n)) {
>   		vq_err(vq, "Failed to write used");
>   		return -EFAULT;
>   	}
> @@ -2388,7 +2398,7 @@ static int __vhost_add_used_n(struct vhost_virtqueue *vq,
>   		smp_wmb();
>   		/* Log used ring entry write. */
>   		log_used(vq, ((void __user *)used - (void __user *)vq->used),
> -			 count * sizeof *used);
> +			 copy_n * sizeof(*used));
>   	}
>   	old = vq->last_used_idx;
>   	new = (vq->last_used_idx += count);
> @@ -2410,7 +2420,7 @@ int vhost_add_used_n(struct vhost_virtqueue *vq, struct vring_used_elem *heads,
>   
>   	start = vq->last_used_idx & (vq->num - 1);
>   	n = vq->num - start;
> -	if (n < count) {
> +	if (n < count && !vhost_has_feature(vq, VIRTIO_F_IN_ORDER)) {


This seems strange, any reason for this? (Actually if we support 
in-order we only need one used slot which fit for the case here)

Thanks


>   		r = __vhost_add_used_n(vq, heads, n);
>   		if (r < 0)
>   			return r;

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