Re: [Qemu-devel] [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG

[Wei Wang]

On 07/12/2017 09:06 PM, Michael S. Tsirkin wrote:
> On Wed, Jul 12, 2017 at 08:40:18PM +0800, Wei Wang wrote:
> > diff --git a/include/linux/virtio.h b/include/linux/virtio.h
> > index 28b0e96..9f27101 100644
> > --- a/include/linux/virtio.h
> > +++ b/include/linux/virtio.h
> > @@ -57,8 +57,28 @@ int virtqueue_add_sgs(struct virtqueue *vq,
> >                       void *data,
> >                       gfp_t gfp);
> >   
> > +/* A desc with this init id is treated as an invalid desc */
> > +#define VIRTQUEUE_DESC_ID_INIT UINT_MAX
> > +int virtqueue_add_chain_desc(struct virtqueue *_vq,
> > +                            uint64_t addr,
> > +                            uint32_t len,
> > +                            unsigned int *head_id,
> > +                            unsigned int *prev_id,
> > +                            bool in);
> > +
> > +int virtqueue_add_chain(struct virtqueue *_vq,
> > +                       unsigned int head,
> > +                       bool indirect,
> > +                       struct vring_desc *indirect_desc,
> > +                       void *data,
> > +                       void *ctx);
> > +
> >   bool virtqueue_kick(struct virtqueue *vq);
> >   
> > +bool virtqueue_kick_sync(struct virtqueue *vq);
> > +
> > +bool virtqueue_kick_async(struct virtqueue *vq, wait_queue_head_t wq);
> > +
> >   bool virtqueue_kick_prepare(struct virtqueue *vq);
> >   
> >   bool virtqueue_notify(struct virtqueue *vq);
> I don't much care for this API. It does exactly what balloon needs,
> but at cost of e.g. transparently busy-waiting. Unlikely to be
> a good fit for anything else.

If you were referring to this API - virtqueue_add_chain_desc():

Busy waiting only happens when the vq is full (i.e. no desc left). If
necessary, I think we can add an input parameter like
"bool busywaiting", then the caller can decide to simply get a -ENOSPC
or busy wait to add when no desc is available.

> If you don't like my original _first/_next/_last, you will
> need to come up with something else.

I thought the above virtqueue_add_chain_des() performs the same
functionality as _first/next/last, which are used to grab descs from the
vq and chain them together. If not, could you please elaborate the
usage of the original proposal?

Best,
Wei