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

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Date:   Thu, 4 Jun 2020 06:17:33 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Eugenio Pérez <eperezma@...hat.com>,
        kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org
Subject: Re: [PATCH RFC 07/13] vhost: format-independent API for used buffers

On Thu, Jun 04, 2020 at 05:18:00PM +0800, Jason Wang wrote:
> 
> On 2020/6/4 下午5:03, Michael S. Tsirkin wrote:
> > > >    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.
> > 
> 
> Somehow. But the reason I ask is that I see "split" suffix is used in patch
> 1 as:
> 
> peek_split_desc()
> pop_split_desc()
> push_split_desc()
> 
> But that suffix is not used for the new used ring API invented in this
> patch.
> 
> Thanks
> 

And that is intentional: split is *not* part of API. The whole idea is
that ring APIs are format agnostic using "buffer" terminology from spec.
The split things are all static within vhost.c

OK so where I had to add a bunch of new format specific code, that was
tagged as "split" to make it easier to spot that they only support a
specific format.  At the same time, I did not rename existing code
adding "split" in the name. I agree it's a useful additional step for
packed ring format support, and it's fairly easy. I just didn't want
to do it automatically.



-- 
MST