On Wed, Jul 12, 2017 at 09:29:00PM +0800, Wei Wang wrote:
> 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.
I think this just shows this API is too high level.
This policy should live in drivers.
> >
> > 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
>
So the way I see it, there are several issues:
- internal wait - forces multiple APIs like kick/kick_sync
note how kick_sync can fail but your code never checks return code
- need to re-write the last descriptor - might not work
for alternative layouts which always expose descriptors
immediately
- some kind of iterator type would be nicer instead of
maintaining head/prev explicitly
As for the use, it would be better to do
if (!add_next(vq, ...)) {
add_last(vq, ...)
kick
wait
}
Using VIRTQUEUE_DESC_ID_INIT seems to avoid a branch in the driver, but
in fact it merely puts the branch in the virtio code.
--
MST
