Re: [Qemu-devel] [PATCH 0/9] introduce virtio net dataplane

[Michael S. Tsirkin]

On Thu, Feb 21, 2013 at 11:02:31AM -0600, Anthony Liguori wrote:
> "Michael S. Tsirkin" <address@hidden> writes:
> 
> > On Thu, Feb 21, 2013 at 10:15:58AM -0600, Anthony Liguori wrote:
> >> "Michael S. Tsirkin" <address@hidden> writes:
> >> 
> >> > On Thu, Feb 21, 2013 at 08:54:44PM +0800, Liu Ping Fan wrote:
> >> >> This is a emulation to virtio-blk dataplane, which push the data
> >> >> handling out of biglock. And it is a try to implement this process
> >> >> in userspace, while vhost-net in kernel.
> >> >
> >> > What's the motivation for doing this?
> >> 
> >> I haven't looked carefully at the patches, but I don't think we should
> >> "cheat" when it comes to virtio-net.  I think it's useful to have a
> >> baseline proof of concept but I think we should focus on making the
> >> network layer re-entrant.
> >> 
> >> In terms of why even bother, if we can make virtio-net a viable
> >> alternative to vhost-net, it's a huge win.  Being in userspace is a huge
> >> advantage.
> >> 
> >> I understand the challenges with zero-copy but we really need a proper
> >> userspace implementation to determine how much it really matters.
> >> Zero-copy tap is also not entirely outside the realm of possibility.
> >
> > This is exactly what we have, vhost-net is the interface we use
> > for asynchronous communication. If you want to add yet another
> > asynchronous interface in kernel, it's certainly doable but then what's
> > the point?
> >
> >> >> The iperf's result show it improving the perfermance of base line,
> >> >> but still slower than vhost-net (maybe the rx path need optimized).
> >> >> 
> >> >> Todo:
> >> >> implement fine lock for net-subsystem
> >> >
> >> > vhost-net is currently the only way to get zero copy transmit
> >> > on linux (and zero copy receive in the future)
> >> > so at least in theory it'll always be faster.
> >> 
> >> It might always be faster.  But that doesn't mean we should limit the
> >> performance of virtio-net in userspace.  Some people may be willing to
> >> take a small performance hit in order to obtain the security offered by
> >> being in userspace.
> >> 
> >> Regards,
> >> 
> >> Anthony Liguori
> >
> > By 'being in userspace' I presume you mean the ring processing code.
> 
> I mean that:
> 
> 1) the same code path is used regardless of network backend
> 
> 2) QEMU has the ability to manipulate the traffic if it needs/wants to
> 
> 3) the state of the device model is userspace at all times
> 
> 4) QEMU is always the front-line interface communicating with the guest
> 
> Yes, this could be a model where -netdev vhost-net was the backend used
> by QEMU.
> 
> > Ring processing can be done in userspace if you are ready
> > to use the synchronous read/write operations, skipping
> > this bunch of code might improve security. But it requires a data
> > copy almost by definition.
> 
> Not sure what you mean here.  If you are talking about read/write on a
> tap file descriptor, I'm not even suggesting that.
> 
> I'm perfectly fine with vhost-net being a -netdev backend.  But that's a
> very different model than how we use it today.
> 
> Regards,
> 
> Anthony Liguori

It's not hard to make the first step there.  Basically just make -netdev
vhost-net an alias for -netdev tap, just making vhost=on the default.

For non virtio frontends, it needs a driver for vhost-net in userspace,
I started a work on this at some point as it would let me
write unit tests for virtio, but never finished.
Any takers?


> >
> >
> >> >
> >> >> Liu Ping Fan (9):
> >> >>   vring: split the modification and publish of used ring
> >> >>   vring: introduce vring_restore() to restore from img
> >> >>   event poll: make epoll work for normal fd
> >> >>   event poll: pass event type to event callback
> >> >>   event poll: enable event poll handle more than one event each time
> >> >>   virtio net: introduce dataplane for virtio net
> >> >>   tap: make tap peer work on dedicated data-plane thread
> >> >>   virtio net: enable dataplane for virtio net
> >> >>   configure: make virtio net dataplane configurable
> >> >> 
> >> >>  configure                  |   12 ++
> >> >>  hw/dataplane/Makefile.objs |    4 +-
> >> >>  hw/dataplane/event-poll.c  |   69 +++++--
> >> >>  hw/dataplane/event-poll.h  |   14 ++-
> >> >>  hw/dataplane/virtio-blk.c  |    8 +-
> >> >>  hw/dataplane/virtio-net.c  |  444 
> >> >> ++++++++++++++++++++++++++++++++++++++++++++
> >> >>  hw/dataplane/virtio-net.h  |   32 ++++
> >> >>  hw/dataplane/vring.c       |   37 ++++
> >> >>  hw/dataplane/vring.h       |    3 +
> >> >>  hw/virtio-net.c            |   94 +++++-----
> >> >>  hw/virtio-net.h            |   64 +++++++
> >> >>  hw/virtio-pci.c            |    2 +-
> >> >>  include/net/tap.h          |    6 +
> >> >>  net/tap.c                  |   92 +++++++++-
> >> >>  14 files changed, 806 insertions(+), 75 deletions(-)
> >> >>  create mode 100644 hw/dataplane/virtio-net.c
> >> >>  create mode 100644 hw/dataplane/virtio-net.h
> >> >> 
> >> >> -- 
> >> >> 1.7.4.4
> >> >>