Re: [dpdk-dev] [RFC v3 0/7] vhost2: new librte_vhost2 proposal

[Thomas Monjalon <thomas@monjalon.net>]

26/06/2018 10:22, Tiwei Bie:
> On Mon, Jun 25, 2018 at 08:17:08PM +0800, Stojaczyk, DariuszX wrote:
> > From: Tiwei Bie
> > > 
> > > Hi Dariusz,
> > 
> > Hi Tiwei,
> > 
> > > Thank you for putting efforts in making the DPDK
> > > vhost more generic!
> > > 
> > > From my understanding, your proposal is that:
> > > 
> > > 1) Introduce rte_vhost2 to provide the APIs which
> > >    allow users to implement vhost backends like
> > >    SCSI, net, crypto, ..
> > 
> > That's right.
> > 
> > > 2) Refactor the existing rte_vhost to use rte_vhost2.
> > >    The rte_vhost will still provide below existing
> > >    sets of APIs:
> > >     1. The APIs which allow users to implement
> > >        external vhost backends (these APIs were
> > >        designed for SPDK previously)
> > >     2. The APIs provided by the net backend
> > >     3. The APIs provided by the crypto backend
> > >    And above APIs in rte_vhost won't be changed.
> > 
> > That's correct. Rte_vhost would register its own rte_vhost2_tgt_ops underneath and will call existing vhost_device_ops for e.g. starting the device once all queues are started.
> 
> Currently I have below concerns and questions:
> 
> - The rte_vhost's problem is still there. Even though
>   rte_vhost2 is introduced, the net and crypto backends
>   in rte_vhost won't benefit from the new callbacks.
> 
>   The existing rte_vhost in DPDK not only provides the
>   APIs for DPDK applications to implement the external
>   backends. But also provides high performance net and
>   crypto backends implementation (maybe more in the
>   future). So it's important that besides the DPDK
>   applications which implement their external backends,
>   the DPDK applications which use the builtin backends
>   will also benefit from the new callbacks.
> 
>   So we should have a clear plan on how will the legacy
>   callbacks in rte_vhost be dealt with in the next step.
> 
>   Besides, the new library's name is a bit misleading.
>   It makes the existing rte_vhost library sound like an
>   obsolete library. But actually the existing rte_vhost
>   isn't an obsolete library. It will still provide the
>   net and crypto backends. So if we want to introduce
>   this new library, we should give it a better name.
> 
> - It's possible to solve rte_vhost's problem you met
>   by refactoring the existing vhost library directly
>   instead of re-implementing a new vhost library from
>   scratch and keeping the old one's problem as is.

+1

>   In this way, it will solve the problem you met and
>   also solve the problem for rte_vhost. Why not go
>   this way? Something like:
> 
>   Below is the existing callbacks set in rte_vhost.h:
> 
>   /**
>    * Device and vring operations.
>    */
>   struct vhost_device_ops {
>           ......
>   };
> 
>   It's a legacy implementation, and doesn't really
>   follow the DPDK API design (e.g. no rte_ prefix).
>   We can design and implement a new message handling
>   and a new set of callbacks for rte_vhost to solve
>   the problem you met without changing the old one.
>   Something like:
> 
>   struct rte_vhost_device_ops {
>           ......
>   }
> 
>   int
>   vhost_user_msg_handler(struct vhost_dev *vdev, struct vhost_user_msg *msg)
>   {
>           ......
> 
>           if (!vdev->is_using_new_device_ops) {
>                   // Call the existing message handler
>                   return vhost_user_msg_handler_legacy(vdev, msg);
>           }
> 
>           // Implement the new logic here
>           ......
>   }
> 
>   A vhost application is allowed to register only struct
>   rte_vhost_device_ops or struct vhost_device_ops (which
>   should be deprecated in the future). The two ops cannot
>   be registered at the same time.
> 
>   The existing applications could use the old ops. And
>   if an application registers struct rte_vhost_device_ops,
>   the new callbacks and message handler will be used.