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

[Maxime Coquelin]

On 10/15/2018 04:22 AM, Jason Wang wrote:
> On 2018å10æ13æ 01:23, Michael S. Tsirkin wrote:
> > 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
> > > > ÂÂÂÂÂÂÂÂÂ vq->last_avail_idx = s.num;
> > > > ÂÂÂÂÂÂÂÂÂ /* Forget the cached index value. */
> > > > ÂÂÂÂÂÂÂÂÂ vq->avail_idx = vq->last_avail_idx;
> > > > +ÂÂÂÂÂÂÂ if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ vq->last_avail_wrap_counter = wrap_counter;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ vq->avail_wrap_counter = vq->last_avail_wrap_counter;
> > > > +ÂÂÂÂÂÂÂ }
> > > > ÂÂÂÂÂÂÂÂÂ break;
> > > > ÂÂÂÂÂ case VHOST_GET_VRING_BASE:
> > > > ÂÂÂÂÂÂÂÂÂ s.index = idx;
> > > > ÂÂÂÂÂÂÂÂÂ s.num = vq->last_avail_idx;
> > > > +ÂÂÂÂÂÂÂ if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ s.num |= vq->last_avail_wrap_counter << 31;
> > > > +ÂÂÂÂÂÂÂ if (copy_to_user(argp, &s, sizeof(s)))
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ r = -EFAULT;
> > > > +ÂÂÂÂÂÂÂ break;
> > > > +ÂÂÂ case VHOST_SET_VRING_USED_BASE:
> > > > +ÂÂÂÂÂÂÂ /* Moving base with an active backend?
> > > > +ÂÂÂÂÂÂÂÂ * You don't want to do that.
> > > > +ÂÂÂÂÂÂÂÂ */
> > > > +ÂÂÂÂÂÂÂ if (vq->private_data) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ r = -EBUSY;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ break;
> > > > +ÂÂÂÂÂÂÂ }
> > > > +ÂÂÂÂÂÂÂ if (copy_from_user(&s, argp, sizeof(s))) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ r = -EFAULT;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ break;
> > > > +ÂÂÂÂÂÂÂ }
> > > > +ÂÂÂÂÂÂÂ if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ wrap_counter = s.num >> 31;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ s.num &= ~(1 << 31);
> > > > +ÂÂÂÂÂÂÂ }
> > > > +ÂÂÂÂÂÂÂ if (s.num > 0xffff) {
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ r = -EINVAL;
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ break;
> > > > +ÂÂÂÂÂÂÂ }
> > > 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.
>
> Ok, I assumes packed virtqueue support 64K but looks not. I can change 
> it to bit 15 and GET_VRING_BASE need to be changed as well.
>
> > > 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.
>
> Do we need to clarify this in the spec?
>
> > > > +ÂÂÂÂÂÂÂ vq->last_used_idx = s.num;
> > > > +ÂÂÂÂÂÂÂ if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
> > > > +ÂÂÂÂÂÂÂÂÂÂÂ vq->last_used_wrap_counter = wrap_counter;
> > > > +ÂÂÂÂÂÂÂ break;
> > > > +ÂÂÂ 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.
>
> If we agree that 64K virtqueue won't be supported, I'm ok with either.

I'm fine to put wrap_counter at bit 15.
I will post a new version of the DPDK series soon.

> Btw the code assumes used_wrap_counter is equal to avail_wrap_counter 
> which looks wrong?

For split ring, we used to set the last_used_idx to the same value as
last_avail_idx as VHOST_USER_GET_VRING_BASE cannot be called while the
ring is being processed, so their value is always the same at the time
the request is handled.

I kept the same behavior for packed ring, and so the wrap counter have
to be the same.

Regards,
Maxime

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