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

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

On Mon, Sep 26, 2016 at 03:12:01PM +0200, Thomas Monjalon wrote:
> 2016-09-23 21:23, Wiles, Keith:
> > On Sep 23, 2016, at 12:26 AM, Yuanhan Liu <yuanhan.liu@linux.intel.com> wrote:
> > > On Thu, Sep 22, 2016 at 06:43:55PM +0200, Thomas Monjalon wrote:
> > >>>>>>>> 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.
> > >>> 
> > >>> It's not, instead, it's very generic. The "internal id" is actually the
> > >>> public interface to vhost-user application, like "fd" to file APIs.
> > >>> 
> > >>> Instead of introducing a few specific wrappers/APIs, I'd prefer to
> > >>> introduce a generic one to get the handle, and let the application to
> > >>> call other vhost APIs.
> > >> 
> > >> Yes it makes sense.
> > >> I was thinking of introducing a function to get an internal id from ethdev,
> > >> in order to use it with any driver or underlying library.
> > >> But it would be an opaque pointer and you need an int.
> > >> Note that we can cast an int into a pointer, so I am not sure what is best.
> > > 
> > > Yes, that should work. But I just doubt what the "opaque pointer" could be
> > > for other PMD drivers, and what the application could do with it. For a
> > > typical nic PMD driver, I can think of nothing is valuable to export to
> > > user applications.
> > > 
> > > But maybe it's valuable to other virtual PMD drives as well, like the TAP
> > > pmd from Keith?
> > 
> > I do not see a need in the TAP PMD other then returning the FD for TUN/TAP device. Not sure what any application would need with the FD here, as it could cause some problems.
> > 
> > This feels like we are talking about a IOCTL like generic interface into the PMD. Then we can add new one types and reject types in the PMD that are not supported. Would this not be a better method for all future PMD APIs?
> > 
> > Here is just a thought as to how to solve this problem without a PMD specific API. A number of current ethdev APIs could be removed to use the API below. The APIs would be removed from ethdev structure and have the current APIs use the API below. I know some are not happy with number of APIs in the ethdev structure.
> > 
> > The API could be something like this:
> > struct rte_tlv {		/* Type/Length/Value like structure */
> >     uint16_t type;	/* Type of command */
> >     uint16_t len;         /* Length of data section on input and on output */
> >     uint16_t tlen;        /* Total or max length of data buffer */
> >     uint8_t data[0];
> > };
> > 
> > int rte_eth_dev_ioctl(int pid, int qid, struct rte_tlv *tlv);
> 
> Yes we are talking about having some specific functions per driver which
> are not defined in the generic ethdev layer.
> We need only one function in ethdev to give access to driver-specific API.
> My idea is to convert the port id into an opaque handler.
> Your idea is to use the port id in an ioctl like function.
> 
> About the implementation, these are the 2 differences between my proposal
> and yours:
> - You use the well known port id, whereas I need another handler which is
> understood by the driver.
> - You need to build a message string which will be decoded by the driver.
> I propose to directly offer some specific functions in the drivers which
> are more convenient to use and easier for code review/debug.
> 
> No conclusion here. I just want to make sure that we are on the same page,
> and would like to have feedback from others. Thanks

I personally don't like the idea of having a generic IOCTL in ethdev. If you
want to have NIC-specific functions provided by a driver, that is fine, but
any app using those is going to be limited to working only with that driver.

In that case, since the driver in question is known, I don't see any reason
to go through the ethdev layer. I think it would be much clearer to have the
app instead include the driver's header file and call the driver function
directly. The #include at the top of the file makes the dependency very clear,
and having a function name instead of IOCTL with magic command numbers allows
the action take by the function to be clearer too.

Regards,
/Bruce