Subject: Re: [PATCH RFC 07/13] vhost: format-independent API for used buffers

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 4 Jun 2020 05:03:20 -0400

On Wed, Jun 03, 2020 at 03:58:26PM +0800, Jason Wang wrote:
> 
> On 2020/6/2 下午9:06, Michael S. Tsirkin wrote:
> > Add a new API that doesn't assume used ring, heads, etc.
> > For now, we keep the old APIs around to make it easier
> > to convert drivers.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >   drivers/vhost/vhost.c | 52 ++++++++++++++++++++++++++++++++++---------
> >   drivers/vhost/vhost.h | 17 +++++++++++++-
> >   2 files changed, 58 insertions(+), 11 deletions(-)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index b4a6e44d56a8..be822f0c9428 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -2292,13 +2292,12 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> >    * number of output then some number of input descriptors, it's actually two
> >    * iovecs, but we pack them into one and note how many of each there were.
> >    *
> > - * This function returns the descriptor number found, or vq->num (which is
> > - * never a valid descriptor number) if none was found.  A negative code is
> > - * returned on error. */
> > -int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> > -		      struct iovec iov[], unsigned int iov_size,
> > -		      unsigned int *out_num, unsigned int *in_num,
> > -		      struct vhost_log *log, unsigned int *log_num)
> > + * This function returns a value > 0 if a descriptor was found, or 0 if none were found.
> > + * A negative code is returned on error. */
> > +int vhost_get_avail_buf(struct vhost_virtqueue *vq, struct vhost_buf *buf,
> > +			struct iovec iov[], unsigned int iov_size,
> > +			unsigned int *out_num, unsigned int *in_num,
> > +			struct vhost_log *log, unsigned int *log_num)
> >   {
> >   	int ret = fetch_descs(vq);
> >   	int i;
> > @@ -2311,6 +2310,8 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   	*out_num = *in_num = 0;
> >   	if (unlikely(log))
> >   		*log_num = 0;
> > +	buf->in_len = buf->out_len = 0;
> > +	buf->descs = 0;
> >   	for (i = vq->first_desc; i < vq->ndescs; ++i) {
> >   		unsigned iov_count = *in_num + *out_num;
> > @@ -2340,6 +2341,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   			/* If this is an input descriptor,
> >   			 * increment that count. */
> >   			*in_num += ret;
> > +			buf->in_len += desc->len;
> >   			if (unlikely(log && ret)) {
> >   				log[*log_num].addr = desc->addr;
> >   				log[*log_num].len = desc->len;
> > @@ -2355,9 +2357,11 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   				goto err;
> >   			}
> >   			*out_num += ret;
> > +			buf->out_len += desc->len;
> >   		}
> > -		ret = desc->id;
> > +		buf->id = desc->id;
> > +		++buf->descs;
> >   		if (!(desc->flags & VRING_DESC_F_NEXT))
> >   			break;
> > @@ -2365,7 +2369,7 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   	vq->first_desc = i + 1;
> > -	return ret;
> > +	return 1;
> >   err:
> >   	for (i = vq->first_desc; i < vq->ndescs; ++i)
> > @@ -2375,7 +2379,15 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   	return ret;
> >   }
> > -EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> > +EXPORT_SYMBOL_GPL(vhost_get_avail_buf);
> > +
> > +/* Reverse the effect of vhost_get_avail_buf. Useful for error handling. */
> > +void vhost_discard_avail_bufs(struct vhost_virtqueue *vq,
> > +			      struct vhost_buf *buf, unsigned count)
> > +{
> > +	vhost_discard_vq_desc(vq, count);
> > +}
> > +EXPORT_SYMBOL_GPL(vhost_discard_avail_bufs);
> >   static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> >   			    struct vring_used_elem *heads,
> > @@ -2459,6 +2471,26 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
> >   }
> >   EXPORT_SYMBOL_GPL(vhost_add_used);
> > +int vhost_put_used_buf(struct vhost_virtqueue *vq, struct vhost_buf *buf)
> > +{
> > +	return vhost_add_used(vq, buf->id, buf->in_len);
> > +}
> > +EXPORT_SYMBOL_GPL(vhost_put_used_buf);
> > +
> > +int vhost_put_used_n_bufs(struct vhost_virtqueue *vq,
> > +			  struct vhost_buf *bufs, unsigned count)
> > +{
> > +	unsigned i;
> > +
> > +	for (i = 0; i < count; ++i) {
> > +		vq->heads[i].id = cpu_to_vhost32(vq, bufs[i].id);
> > +		vq->heads[i].len = cpu_to_vhost32(vq, bufs[i].in_len);
> > +	}
> > +
> > +	return vhost_add_used_n(vq, vq->heads, count);
> > +}
> > +EXPORT_SYMBOL_GPL(vhost_put_used_n_bufs);
> > +
> >   static bool vhost_notify(struct vhost_dev *dev, struct vhost_virtqueue *vq)
> >   {
> >   	__u16 old, new;
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index a67bda9792ec..6c10e99ff334 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -67,6 +67,13 @@ struct vhost_desc {
> >   	u16 id;
> >   };
> > +struct vhost_buf {
> > +	u32 out_len;
> > +	u32 in_len;
> > +	u16 descs;
> > +	u16 id;
> > +};
> 
> 
> So it looks to me the struct vhost_buf can work for both split ring and
> packed ring.
> 
> If this is true, we'd better make struct vhost_desc work for both.
> 
> Thanks

Both vhost_desc and vhost_buf can work for split and packed.

Do you mean we should add packed ring support based on this?
For sure, this is one of the motivators for the patchset.

> 
> > +
> >   /* The virtqueue structure describes a queue attached to a device. */
> >   struct vhost_virtqueue {
> >   	struct vhost_dev *dev;
> > @@ -193,7 +200,12 @@ int vhost_get_vq_desc(struct vhost_virtqueue *,
> >   		      unsigned int *out_num, unsigned int *in_num,
> >   		      struct vhost_log *log, unsigned int *log_num);
> >   void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
> > -
> > +int vhost_get_avail_buf(struct vhost_virtqueue *, struct vhost_buf *buf,
> > +			struct iovec iov[], unsigned int iov_count,
> > +			unsigned int *out_num, unsigned int *in_num,
> > +			struct vhost_log *log, unsigned int *log_num);
> > +void vhost_discard_avail_bufs(struct vhost_virtqueue *,
> > +			      struct vhost_buf *, unsigned count);
> >   int vhost_vq_init_access(struct vhost_virtqueue *);
> >   int vhost_add_used(struct vhost_virtqueue *, unsigned int head, int len);
> >   int vhost_add_used_n(struct vhost_virtqueue *, struct vring_used_elem *heads,
> > @@ -202,6 +214,9 @@ void vhost_add_used_and_signal(struct vhost_dev *, struct vhost_virtqueue *,
> >   			       unsigned int id, int len);
> >   void vhost_add_used_and_signal_n(struct vhost_dev *, struct vhost_virtqueue *,
> >   			       struct vring_used_elem *heads, unsigned count);
> > +int vhost_put_used_buf(struct vhost_virtqueue *, struct vhost_buf *buf);
> > +int vhost_put_used_n_bufs(struct vhost_virtqueue *,
> > +			  struct vhost_buf *bufs, unsigned count);
> >   void vhost_signal(struct vhost_dev *, struct vhost_virtqueue *);
> >   void vhost_disable_notify(struct vhost_dev *, struct vhost_virtqueue *);
> >   bool vhost_vq_avail_empty(struct vhost_dev *, struct vhost_virtqueue *);