[dpdk-dev] [RFC] vhost-user public struct refactor (was Re: [PATCH RFC 2/4] vhost: make buf vector for scatter RX) local.

[Yuanhan Liu <yuanhan.liu@linux.intel.com>]

On Fri, Feb 19, 2016 at 03:06:50PM +0800, Yuanhan Liu wrote:
> On Fri, Feb 19, 2016 at 09:32:41AM +0300, Ilya Maximets wrote:
> > Array of buf_vector's is just an array for temporary storing information
> > about available descriptors. It used only locally in virtio_dev_merge_rx()
> > and there is no reason for that array to be shared.
> > 
> > Fix that by allocating local buf_vec inside virtio_dev_merge_rx().
> > 
> > Signed-off-by: Ilya Maximets <i.maximets@samsung.com>
> > ---
> >  lib/librte_vhost/rte_virtio_net.h |  1 -
> >  lib/librte_vhost/vhost_rxtx.c     | 45 ++++++++++++++++++++-------------------
> >  2 files changed, 23 insertions(+), 23 deletions(-)
> > 
> > diff --git a/lib/librte_vhost/rte_virtio_net.h b/lib/librte_vhost/rte_virtio_net.h
> > index 10dcb90..ae1e4fb 100644
> > --- a/lib/librte_vhost/rte_virtio_net.h
> > +++ b/lib/librte_vhost/rte_virtio_net.h
> > @@ -91,7 +91,6 @@ struct vhost_virtqueue {
> >  	int			kickfd;			/**< Currently unused as polling mode is enabled. */
> >  	int			enabled;
> >  	uint64_t		reserved[16];		/**< Reserve some spaces for future extension. */
> > -	struct buf_vector	buf_vec[BUF_VECTOR_MAX];	/**< for scatter RX. */
> >  } __rte_cache_aligned;
> 
> I like this kind of cleanup, however, it breaks ABI.

So, I was considering to add vhost-user Tx delayed-copy (or zero copy)
support recently, which comes to yet another ABI violation, as we need
add a new field to virtio_memory_regions struct to do guest phys addr
to host phys addr translation. You may ask, however, that why do we need
expose virtio_memory_regions struct to users at all?

You are right, we don't have to. And here is the thing: we exposed way
too many fields (or even structures) than necessary. Say, vhost_virtqueue
struct should NOT be exposed to user at all: application just need to
tell the right queue id to locate a specific queue, and that's all.
The structure should be defined in an internal header file. With that,
we could do any changes to it we want, without worrying about that we
may offense the painful ABI rules.

Similar changes could be done to virtio_net struct as well, just exposing
very few fields that are necessary and moving all others to an internal
structure.

Huawei then suggested a more radical yet much cleaner one: just exposing
a virtio_net handle to application, just like the way kernel exposes an
fd to user for locating a specific file. However, it's more than an ABI
change; it's also an API change: some fields are referenced by applications,
such as flags, virt_qp_nb. We could expose some new functions to access
them though.

I'd vote for this one, as it sounds very clean to me. This would also
solve the block issue of this patch. Though it would break OVS, I'm thinking
that'd be okay, as OVS has dependence on DPDK version: what we need to
do is just to send few patches to OVS, and let it points to next release,
say DPDK v16.07. Flavio, please correct me if I'm wrong.

Thoughts/comments?

	--yliu