Re: [PATCH RFC 2/2] virtio_ring: support packed ring

[Tiwei Bie]

On Fri, Mar 16, 2018 at 04:30:02PM +0200, Michael S. Tsirkin wrote:
> On Fri, Mar 16, 2018 at 07:36:47PM +0800, Jason Wang wrote:
> > > > @@ -1096,17 +1599,21 @@ struct virtqueue *vring_create_virtqueue(
> > > > > > > > >      	if (!queue) {
> > > > > > > > >      		/* Try to get a single page. You are my only hope! */
> > > > > > > > > -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> > > > > > > > > +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> > > > > > > > > +							     packed),
> > > > > > > > >      					  &dma_addr, GFP_KERNEL|__GFP_ZERO);
> > > > > > > > >      	}
> > > > > > > > >      	if (!queue)
> > > > > > > > >      		return NULL;
> > > > > > > > > -	queue_size_in_bytes = vring_size(num, vring_align);
> > > > > > > > > -	vring_init(&vring, num, queue, vring_align);
> > > > > > > > > +	queue_size_in_bytes = __vring_size(num, vring_align, packed);
> > > > > > > > > +	if (packed)
> > > > > > > > > +		vring_packed_init(&vring.vring_packed, num, queue, vring_align);
> > > > > > > > > +	else
> > > > > > > > > +		vring_init(&vring.vring_split, num, queue, vring_align);
> > > > > > > > Let's rename vring_init to vring_init_split() like other helpers?
> > > > > > > The vring_init() is a public API in include/uapi/linux/virtio_ring.h.
> > > > > > > I don't think we can rename it.
> > > > > > I see, then this need more thoughts to unify the API.
> > > > > My thought is to keep the old API as is, and introduce
> > > > > new types and helpers for packed ring.
> > > > I admit it's not a fault of this patch. But we'd better think of this in the
> > > > future, consider we may have new kinds of ring.
> > > > 
> > > > > More details can be found in this patch:
> > > > > https://lkml.org/lkml/2018/2/23/243
> > > > > (PS. The type which has bit fields is just for reference,
> > > > >    and will be changed in next version.)
> > > > > 
> > > > > Do you have any other suggestions?
> > > > No.
> > > Hmm.. Sorry, I didn't describe my question well.
> > > I mean do you have any suggestions about the API
> > > design for packed ring in uapi header? Currently
> > > I introduced below two new helpers:
> > > 
> > > static inline void vring_packed_init(struct vring_packed *vr, unsigned int num,
> > > 				     void *p, unsigned long align);
> > > static inline unsigned vring_packed_size(unsigned int num, unsigned long align);
> > > 
> > > When new rings are introduced in the future, above
> > > helpers can't be reused. Maybe we should make the
> > > helpers be able to determine the ring type?
> > 
> > Let's wait for Michael's comment here. Generally, I fail to understand why
> > vring_init() become a part of uapi. Git grep shows the only use cases are
> > virtio_test/vringh_test.
> > 
> > Thanks
> 
> For init - I think it's a mistake that stems from lguest which sometimes
> made it less than obvious which code is where.  I don't see a reason to
> add to it.

Got it! I'll move vring_packed_init() out of uapi. Many thanks! :)

Best regards,
Tiwei Bie

> 
> -- 
> MST