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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 Apr 2018 17:04:59 +0300

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.

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MST