Subject: Re: [PATCH net-next V2 6/8] vhost: packed ring support

From: Tiwei Bie <[email-masked]>
Date: Fri, 12 Oct 2018 22:32:44 +0800

On Mon, Jul 16, 2018 at 11:28:09AM +0800, Jason Wang wrote:
[...]
> @@ -1367,10 +1397,48 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
>  		vq->last_avail_idx = s.num;
>  		/* Forget the cached index value. */
>  		vq->avail_idx = vq->last_avail_idx;
> +		if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
> +			vq->last_avail_wrap_counter = wrap_counter;
> +			vq->avail_wrap_counter = vq->last_avail_wrap_counter;
> +		}
>  		break;
>  	case VHOST_GET_VRING_BASE:
>  		s.index = idx;
>  		s.num = vq->last_avail_idx;
> +		if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> +			s.num |= vq->last_avail_wrap_counter << 31;
> +		if (copy_to_user(argp, &s, sizeof(s)))
> +			r = -EFAULT;
> +		break;
> +	case VHOST_SET_VRING_USED_BASE:
> +		/* Moving base with an active backend?
> +		 * You don't want to do that.
> +		 */
> +		if (vq->private_data) {
> +			r = -EBUSY;
> +			break;
> +		}
> +		if (copy_from_user(&s, argp, sizeof(s))) {
> +			r = -EFAULT;
> +			break;
> +		}
> +		if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
> +			wrap_counter = s.num >> 31;
> +			s.num &= ~(1 << 31);
> +		}
> +		if (s.num > 0xffff) {
> +			r = -EINVAL;
> +			break;
> +		}

Do we want to put wrap_counter at bit 15?

If put wrap_counter at bit 31, the check (s.num > 0xffff)
won't be able to catch the illegal index 0x8000~0xffff for
packed ring.

> +		vq->last_used_idx = s.num;
> +		if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> +			vq->last_used_wrap_counter = wrap_counter;
> +		break;
> +	case VHOST_GET_VRING_USED_BASE:

Do we need the new VHOST_GET_VRING_USED_BASE and
VHOST_SET_VRING_USED_BASE ops?

We are going to merge below series in DPDK:

http://patches.dpdk.org/patch/45874/

We may need to reach an agreement first.

> +		s.index = idx;
> +		s.num = vq->last_used_idx;
> +		if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> +			s.num |= vq->last_used_wrap_counter << 31;
>  		if (copy_to_user(argp, &s, sizeof s))
>  			r = -EFAULT;
>  		break;
[...]