Subject: Re: [RFC v2] virtio: support packed ring

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 Apr 2018 18:54:51 +0300

On Tue, Apr 17, 2018 at 10:56:26PM +0800, Tiwei Bie wrote:
> On Tue, Apr 17, 2018 at 05:04:59PM +0300, Michael S. Tsirkin wrote:
> > On Tue, Apr 17, 2018 at 08:47:16PM +0800, Tiwei Bie wrote:
> > > On Tue, Apr 17, 2018 at 03:17:41PM +0300, Michael S. Tsirkin wrote:
> > > > On Tue, Apr 17, 2018 at 10:51:33AM +0800, Tiwei Bie wrote:
> > > > > On Tue, Apr 17, 2018 at 10:11:58AM +0800, Jason Wang wrote:
> > > > > > On 2018年04月13日 15:15, Tiwei Bie wrote:
> > > > > > > On Fri, Apr 13, 2018 at 12:30:24PM +0800, Jason Wang wrote:
> > > > > > > > On 2018年04月01日 22:12, Tiwei Bie wrote:
> > > > > [...]
> > > > > > > > > +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
> > > > > > > > > +			      void **ctx)
> > > > > > > > > +{
> > > > > > > > > +	struct vring_packed_desc *desc;
> > > > > > > > > +	unsigned int i, j;
> > > > > > > > > +
> > > > > > > > > +	/* Clear data ptr. */
> > > > > > > > > +	vq->desc_state[head].data = NULL;
> > > > > > > > > +
> > > > > > > > > +	i = head;
> > > > > > > > > +
> > > > > > > > > +	for (j = 0; j < vq->desc_state[head].num; j++) {
> > > > > > > > > +		desc = &vq->vring_packed.desc[i];
> > > > > > > > > +		vring_unmap_one_packed(vq, desc);
> > > > > > > > > +		desc->flags = 0x0;
> > > > > > > > Looks like this is unnecessary.
> > > > > > > It's safer to zero it. If we don't zero it, after we
> > > > > > > call virtqueue_detach_unused_buf_packed() which calls
> > > > > > > this function, the desc is still available to the
> > > > > > > device.
> > > > > > 
> > > > > > Well detach_unused_buf_packed() should be called after device is stopped,
> > > > > > otherwise even if you try to clear, there will still be a window that device
> > > > > > may use it.
> > > > > 
> > > > > This is not about whether the device has been stopped or
> > > > > not. We don't have other places to re-initialize the ring
> > > > > descriptors and wrap_counter. So they need to be set to
> > > > > the correct values when doing detach_unused_buf.
> > > > > 
> > > > > Best regards,
> > > > > Tiwei Bie
> > > > 
> > > > find vqs is the time to do it.
> > > 
> > > The .find_vqs() will call .setup_vq() which will eventually
> > > call vring_create_virtqueue(). It's a different case. Here
> > > we're talking about re-initializing the descs and updating
> > > the wrap counter when detaching the unused descs (In this
> > > case, split ring just needs to decrease vring.avail->idx).
> > > 
> > > Best regards,
> > > Tiwei Bie
> > 
> > There's no requirement that  virtqueue_detach_unused_buf re-initializes
> > the descs. It happens on cleanup path just before drivers delete the
> > vqs.
> 
> Cool, I wasn't aware of it. I saw split ring decrease
> vring.avail->idx after detaching an unused desc, so I
> thought detaching unused desc also needs to make sure
> that the ring state will be updated correspondingly.

Hmm. You are right. Seems to be out console driver being out of spec.
Will have to look at how to fix that :(

It was done here:

Commit b3258ff1d6086bd2b9eeb556844a868ad7d49bc8
Author: Amit Shah <[email-masked]>
Date:   Wed Mar 16 19:12:10 2011 +0530

    virtio: Decrement avail idx on buffer detach
    
    When detaching a buffer from a vq, the avail.idx value should be
    decremented as well.
    
    This was noticed by hot-unplugging a virtio console port and then
    plugging in a new one on the same number (re-using the vqs which were
    just 'disowned').  qemu reported
    
       'Guest moved used index from 0 to 256'
    
    when any IO was attempted on the new port.
    
    CC: [email-masked]
    Reported-by: juzhang <[email-masked]>
    Signed-off-by: Amit Shah <[email-masked]>
    Signed-off-by: Rusty Russell <[email-masked]>

The spec is quite explicit though:
	A driver MUST NOT decrement the available idx on a live virtqueue (ie. there is no way to “unexpose”
	buffers).

> If there is no such requirement, do you think it's OK
> to remove below two lines:
> 
> vq->avail_idx_shadow--;
> vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev, vq->avail_idx_shadow);
> 
> from virtqueue_detach_unused_buf(), and we could have
> one generic function to handle both rings:
> 
> void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> {
> 	struct vring_virtqueue *vq = to_vvq(_vq);
> 	unsigned int num, i;
> 	void *buf;
> 
> 	START_USE(vq);
> 
> 	num = vq->packed ? vq->vring_packed.num : vq->vring.num;
> 
> 	for (i = 0; i < num; i++) {
> 		if (!vq->desc_state[i].data)
> 			continue;
> 		/* detach_buf clears data, so grab it now. */
> 		buf = vq->desc_state[i].data;
> 		detach_buf(vq, i, NULL);
> 		END_USE(vq);
> 		return buf;
> 	}
> 	/* That should have freed everything. */
> 	BUG_ON(vq->vq.num_free != num);
> 
> 	END_USE(vq);
> 	return NULL;
> }
> 
> Best regards,
> Tiwei Bie