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

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Message-ID: <20181015062241-mutt-send-email-mst@kernel.org>
Date:   Mon, 15 Oct 2018 06:25:08 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Tiwei Bie <tiwei.bie@...el.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, wexu@...hat.com,
        jfreimann@...hat.com, maxime.coquelin@...hat.com
Subject: Re: [PATCH net-next V2 6/8] vhost: packed ring support

On Mon, Oct 15, 2018 at 10:51:06AM +0800, Jason Wang wrote:
> 
> 
> On 2018年10月15日 10:43, Michael S. Tsirkin wrote:
> > On Mon, Oct 15, 2018 at 10:22:33AM +0800, Jason Wang wrote:
> > > 
> > > On 2018年10月13日 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
> > > > > >    		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.
> > > 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?
> > Isn't this all internal vhost stuff?
> 
> I meant the illegal index 0x8000-0xffff.

It does say packed virtqueues support up to 2 15 entries each.

But yes we can add a requirement that devices do not expose
larger rings. Split does not support 2**16 either, right?
With 2**16 enties avail index becomes 0 and ring looks empty.

> 
> > 
> > > > > > +		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.
> > > If we agree that 64K virtqueue won't be supported, I'm ok with either.
> > Well the spec says right at the beginning:
> > 
> > Packed virtqueues support up to 2 15 entries each.
> 
> Ok. I get it.
> 
> Then I can change vhost to match what dpdk did.
> 
> Thanks
> 
> > 
> > 
> > > Btw the code assumes used_wrap_counter is equal to avail_wrap_counter which
> > > looks wrong?
> > > 
> > > Thanks
> > > 
> > > > > > +		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;
> > > > > [...]