[dpdk-dev] [RFC PATCH v2] vhost: Add VHOST PMD

[Loftus, Ciara]

> On 2015/10/16 21:52, Bruce Richardson wrote:
> > On Mon, Aug 31, 2015 at 12:55:26PM +0900, Tetsuya Mukawa wrote:
> >> The patch introduces a new PMD. This PMD is implemented as thin
> wrapper
> >> of librte_vhost. It means librte_vhost is also needed to compile the PMD.
> >> The PMD can have 'iface' parameter like below to specify a path to
> connect
> >> to a virtio-net device.
> >>
> >> $ ./testpmd -c f -n 4 --vdev 'eth_vhost0,iface=/tmp/sock0' -- -i
> >>
> >> To connect above testpmd, here is qemu command example.
> >>
> >> $ qemu-system-x86_64 \
> >>         <snip>
> >>         -chardev socket,id=chr0,path=/tmp/sock0 \
> >>         -netdev vhost-user,id=net0,chardev=chr0,vhostforce \
> >>         -device virtio-net-pci,netdev=net0
> >>
> >> Signed-off-by: Tetsuya Mukawa <mukawa at igel.co.jp>
> > With this PMD in place, is there any need to keep the existing vhost library
> > around as a separate entity? Can the existing library be
> subsumed/converted into
> > a standard PMD?
> >
> > /Bruce
> 
> Hi Bruce,
> 
> I concern about whether the PMD has all features of librte_vhost,
> because librte_vhost provides more features and freedom than ethdev API
> provides.
> In some cases, user needs to choose limited implementation without
> librte_vhost.
> I am going to eliminate such cases while implementing the PMD.
> But I don't have strong belief that we can remove librte_vhost now.
> 
> So how about keeping current separation in next DPDK?
> I guess people will try to replace librte_vhost to vhost PMD, because
> apparently using ethdev APIs will be useful in many cases.
> And we will get feedbacks like "vhost PMD needs to support like this usage".
> (Or we will not have feedbacks, but it's also OK.)
> Then, we will be able to merge librte_vhost and vhost PMD.

I agree with the above. One the concerns I had when reviewing the patch was that the PMD removes some freedom that is available with the library. Eg. Ability to implement the new_device and destroy_device callbacks. If using the PMD you are constrained to the implementations of these in the PMD driver, but if using librte_vhost, you can implement your own with whatever functionality you like - a good example of this can be seen in the vhost sample app.
On the other hand, the PMD is useful in that it removes a lot of complexity for the user and may work for some more general use cases. So I would be in favour of having both options available too.

Ciara

> 
> Thanks,
> Tetsuya