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

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Date:   Fri, 16 Mar 2018 16:30:02 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     Tiwei Bie <tiwei.bie@...el.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [PATCH RFC 2/2] virtio_ring: support packed ring

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.

-- 
MST