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

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

2016-09-23 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