Subject: Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used_idx()

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 28 Sep 2017 01:58:40 +0300

On Wed, Sep 27, 2017 at 08:38:24AM +0800, Jason Wang wrote:
> 
> 
> On 2017年09月27日 03:13, Michael S. Tsirkin wrote:
> > On Fri, Sep 22, 2017 at 04:02:33PM +0800, Jason Wang wrote:
> > > This patch introduces a helper which just increase the used idx. This
> > > will be used in pair with vhost_prefetch_desc_indices() by batching
> > > code.
> > > 
> > > Signed-off-by: Jason Wang <[email-masked]>
> > > ---
> > >   drivers/vhost/vhost.c | 33 +++++++++++++++++++++++++++++++++
> > >   drivers/vhost/vhost.h |  1 +
> > >   2 files changed, 34 insertions(+)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index 8424166d..6532cda 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -2178,6 +2178,39 @@ int vhost_add_used(struct vhost_virtqueue *vq, unsigned int head, int len)
> > >   }
> > >   EXPORT_SYMBOL_GPL(vhost_add_used);
> > > +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n)
> > > +{
> > > +	u16 old, new;
> > > +
> > > +	old = vq->last_used_idx;
> > > +	new = (vq->last_used_idx += n);
> > > +	/* If the driver never bothers to signal in a very long while,
> > > +	 * used index might wrap around. If that happens, invalidate
> > > +	 * signalled_used index we stored. TODO: make sure driver
> > > +	 * signals at least once in 2^16 and remove this.
> > > +	 */
> > > +	if (unlikely((u16)(new - vq->signalled_used) < (u16)(new - old)))
> > > +		vq->signalled_used_valid = false;
> > > +
> > > +	/* Make sure buffer is written before we update index. */
> > > +	smp_wmb();
> > > +	if (vhost_put_user(vq, cpu_to_vhost16(vq, vq->last_used_idx),
> > > +			   &vq->used->idx)) {
> > > +		vq_err(vq, "Failed to increment used idx");
> > > +		return -EFAULT;
> > > +	}
> > > +	if (unlikely(vq->log_used)) {
> > > +		/* Log used index update. */
> > > +		log_write(vq->log_base,
> > > +			  vq->log_addr + offsetof(struct vring_used, idx),
> > > +			  sizeof(vq->used->idx));
> > > +		if (vq->log_ctx)
> > > +			eventfd_signal(vq->log_ctx, 1);
> > > +	}
> > > +	return 0;
> > > +}
> > > +EXPORT_SYMBOL_GPL(vhost_add_used_idx);
> > > +
> > >   static int __vhost_add_used_n(struct vhost_virtqueue *vq,
> > >   			    struct vring_used_elem *heads,
> > >   			    unsigned count)
> > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > index 16c2cb6..5dd6c05 100644
> > > --- a/drivers/vhost/vhost.h
> > > +++ b/drivers/vhost/vhost.h
> > > @@ -199,6 +199,7 @@ int __vhost_get_vq_desc(struct vhost_virtqueue *vq,
> > >   void vhost_discard_vq_desc(struct vhost_virtqueue *, int n);
> > >   int vhost_vq_init_access(struct vhost_virtqueue *);
> > > +int vhost_add_used_idx(struct vhost_virtqueue *vq, int n);
> > >   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,
> > >   		     unsigned count);
> > Please change the API to hide the fact that there's an index that needs
> > to be updated.
> 
> In fact, an interesting optimization on top is just call
> vhost_add_used_idx(vq, n) instead of n vhost_add_used_idx(vq, 1). That's the
> reason I leave n in the API.
> 
> Thanks

Right but you could increment some internal counter in the vq
structure then update the used index using some api
with a generic name, e.g.  add_used_complete or something like this.

> > > -- 
> > > 2.7.4