Re: [RFC v2] virtio: support packed ring

[Jason Wang]

On 2018å04æ24æ 09:05, Michael S. Tsirkin wrote:
> > > > > +	if (vq->indirect) {
> > > > > +		u32 len;
> > > > > +
> > > > > +		desc = vq->desc_state[head].indir_desc;
> > > > > +		/* Free the indirect table, if any, now that it's unmapped. */
> > > > > +		if (!desc)
> > > > > +			goto out;
> > > > > +
> > > > > +		len = virtio32_to_cpu(vq->vq.vdev,
> > > > > +				      vq->vring_packed.desc[head].len);
> > > > > +
> > > > > +		BUG_ON(!(vq->vring_packed.desc[head].flags &
> > > > > +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
> > > > It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So we
> > > > can safely remove this BUG_ON() here.
> > > >
> > > > > +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
> > > > Len could be ignored for used descriptor according to the spec, so we need
> > > > remove this BUG_ON() too.
> > > Yeah, you're right! The BUG_ON() isn't right. I'll remove it.
> > > And I think something related to this in the spec isn't very
> > > clear currently.
> > >
> > > In the spec, there are below words:
> > >
> > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272
> > > """
> > > In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE
> > > is reserved and is ignored by the device.
> > > """
> > >
> > > So when device writes back an used descriptor in this case,
> > > device may not set the VIRTQ_DESC_F_WRITE flag as the flag
> > > is reserved and should be ignored.
> > >
> > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170
> > > """
> > > Element Length is reserved for used descriptors without the
> > > VIRTQ_DESC_F_WRITE flag, and is ignored by drivers.
> > > """
> > >
> > > And this is the way how driver ignores the `len` in an used
> > > descriptor.
> > >
> > > https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241
> > > """
> > > To increase ring capacity the driver can store a (read-only
> > > by the device) table of indirect descriptors anywhere in memory,
> > > and insert a descriptor in the main virtqueue (with \field{Flags}
> > > bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element
> > > containing this indirect descriptor table;
> > > """
> > >
> > > So the indirect descriptors in the table are read-only by
> > > the device. And the only descriptor which is writeable by
> > > the device is the descriptor in the main virtqueue (with
> > > Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the
> > > `len` in this descriptor, we won't be able to get the
> > > length of the data written by the device.
> > >
> > > So I think the `len` in this descriptor will carry the
> > > length of the data written by the device (if the buffers
> > > are writable to the device) even if the VIRTQ_DESC_F_WRITE
> > > isn't set by the device. How do you think?
> > Yes I think so. But we'd better need clarification from Michael.
> I think if you use a descriptor, and you want to supply len
> to guest, you set VIRTQ_DESC_F_WRITE in the used descriptor.
> Spec also says you must not set VIRTQ_DESC_F_INDIRECT then.
> If that's a problem we could look at relaxing that last requirement -
> does driver want INDIRECT in used descriptor to match
> the value in the avail descriptor for some reason?

Looks not, so what I get it:

- device and set VIRTQ_DESC_F_WRITE flag for used descriptor when needed
- there no need to keep INDIRECT flag in used descriptor

So for the above case, we can just have a used descriptor with _F_WRITE 
but without INDIRECT flag.

Thanks