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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Fri, 12 Oct 2018 13:23:46 -0400

On Fri, Oct 12, 2018 at 10:32:44PM +0800, Tiwei Bie wrote:
> 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?

I think I second that - seems to be consistent with
e.g. event suppression structure and the proposed
extension to driver notifications.

> 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;
> [...]