Re: [dpdk-dev] [PATCH] net/vhost: Add function to retreive the 'vid' for a given port id

[Thomas Monjalon <thomas.monjalon@6wind.com>]

2016-09-18 16:27, Yuanhan Liu:
> On Wed, Sep 14, 2016 at 10:35:53AM +0200, Thomas Monjalon wrote:
> > 2016-09-14 15:21, Yuanhan Liu:
> > > On Wed, Sep 14, 2016 at 09:10:48AM +0200, Thomas Monjalon wrote:
> > > > 2016-09-14 12:43, Yuanhan Liu:
> > > > > On Tue, Sep 13, 2016 at 05:10:09PM +0200, Thomas Monjalon wrote:
> > > > > > 2016-09-13 14:47, Ciara Loftus:
> > > > > > > In some cases when using the vHost PMD, certain vHost library functions
> > > > > > > may still need to be accessed. One such example is the
> > > > > > > rte_vhost_get_queue_num function which returns the number of virtqueues
> > > > > > > reported by the guest - information which is not exposed by the PMD.
> > > > > > > 
> > > > > > > This commit introduces a new rte_eth_vhost function that returns the
> > > > > > > 'vid' associated with a given port id. This allows the PMD user to call
> > > > > > > vHost library functions which require the 'vid' value.
> > > > > > 
> > > > > > I think we should not add any API to the PMDs.
> > > > > 
> > > > > In general, I agree with you.
> > > > > 
> > > > > > Maybe you are looking for a generic API in ethdev.
> > > > > 
> > > > > But maybe it's a bit hard to define a "right" generic API here. For this
> > > > > case, the generic API I can think of could be:
> > > > > 
> > > > > - an API to get queue num, like rte_eth_get_queue_enabled_num
> > > > >   I barely know NIC pmd drivers, but I doubt it's useful/meaningful for them.
> > > > > 
> > > > > - an API to get a PMD driver private (or specific) data.
> > > > >   For vhost-pmd, it's vid. Again, I don't know what it could be for other nic
> > > > >   drivers.
> > > > > 
> > > > >   This one may be a better option here, because it expose a key field to
> > > > >   the application, vid, with which the application can invoke more vhost
> > > > >   APIs. And apparently, it's not feasible to try to define a generic API
> > > > >   for some (if not each) vhost APIs.
> > > > 
> > > > There could be a similar need in other PMD.
> > > > If we can get an opaque identifier of the device which is not the port id,
> > > > we could call some specific functions of the driver not implemented in
> > > > the generic ethdev API.
> > > 
> > > That means you have to add/export the PMD API first. Isn't it against what
> > > you are proposing -- "I think we should not add any API to the PMDs" ;)
> > 
> > Yes you are totally right :)
> > Except that in vhost case, we would not have any API in the PMD.
> > But it would allow to have some specific API in other PMDs for the features
> > which do not fit in a generic API.
> 
> So, does that mean you are okay with this patch now? I mean, okay to introduce
> a vhost PMD API?

It means I would be in favor of introducing API in drivers for very specific
features.
In this case, I am not sure that retrieving an internal id is very specific.
Opinions are welcome