Re: [dpdk-dev] [PATCH v2] vhost: add support for dynamic vhost PMD creation

[Bruce Richardson <bruce.richardson@intel.com>]

On Tue, May 24, 2016 at 01:11:26PM +0800, Yuanhan Liu wrote:
> On Mon, May 23, 2016 at 06:06:21PM +0100, Ferruh Yigit wrote:
> > On 5/23/2016 2:24 PM, Yuanhan Liu wrote:
> > > On Fri, May 20, 2016 at 11:37:47AM +0100, Bruce Richardson wrote:
> > >> On Thu, May 19, 2016 at 06:44:44PM +0200, Thomas Monjalon wrote:
> > >>> 2016-05-19 17:28, Ferruh Yigit:
> > >>>> On 5/19/2016 9:33 AM, Thomas Monjalon wrote:
> > >>>>> 2016-05-18 18:10, Ferruh Yigit:
> > >>>>>> Add rte_eth_from_vhost() API to create vhost PMD dynamically from
> > >>>>>> applications.
> > >>>>>
> > >>>>> How is it different from rte_eth_dev_attach() calling rte_eal_vdev_init()?
> > >>>>>
> > >>>>
> > >>>> When used rte_eth_dev_attach(), application also needs to do:
> > >>>> rte_eth_dev_configure()
> > >>>> rte_eth_rx_queue_setup()
> > >>>> rte_eth_tx_queue_setup()
> > >>>> rte_eth_dev_start()
> > >>>>
> > >>>> rte_eth_from_vhost() does these internally, easier to use for applications.
> > >>>
> > >>> This argument is not sufficient.
> > >>> We are not going to add new APIs just for wrapping others.
> > >>
> > >> Why not - if there is a sufficient increase in developer usability by doing so?
> > >> Having one API that saves an app from having to call 5 other APIs looks like
> > >> something that should always be given fair consideration.
> > > 
> > > Good point. Judging that vhost is not the only virtual device we
> > > support, and it may also look reasonable to add something similar
> > > for others in future (say, IIRC, you proposed two more internally
> > > that also introduced similar APIs). So, instead of introducing a
> > > new API for each such vdev, may we introduce a common one? Say,
> > > a refined rte_eth_dev_attach(), including dev_configure(),
> > > queue_setup(), etc.
> > > 
> > 
> > This sounds good to me. If there is not objection, I will send a patch
> > and we can discuss based on patch.
> 
> Let's wait and gather some comments first?
> 
I'm not sure that such a general approach is likely to work, as the parameters
needed for each individual driver are going to be different. For some devices,
much of the parameters can be implied, while for others they may not be and still
others needed additional setup parameters. For the simplest case, take the
rte_eth_from_ring API, which creates an ethdev backed by a single rte_ring. The
number of rx and tx queues and their sizes are all determined by the actual
underlying ring, as is the numa node and all other parameters. On the other
hand, we have something like a pcap PMD, where again none of the queue sizes
need to be specified, but we do need additional parameters to provide the
underlying pcap file/device to use. Other devices will similarly need different
options, including in some cases queue counts and sizes.

Therefore, I think trying to generalise the function is pointless. If you have
to write your code to build up a specific set of parameters to pass to a general
API, then you are no better off than just calling a specific API directly. In
both cases you need different code for each device type.

Regards,
/Bruce