netdev - Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used

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Date:   Wed, 27 Sep 2017 20:59:53 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>, kvm@...r.kernel.org
Subject: Re: [PATCH net-next RFC 3/5] vhost: introduce vhost_add_used_idx()

>> > > @@ -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.

That adds a layer of information hiding. If the same variable can be
kept close to the computation in a local variable and passed directly
to vhost_add_used_idx_n that is easier to follow.