Re: [RFC v5 0/5] Add virtio transport for AF_VSOCK

Some how Stefan's reply disapeared from my INBOX (although I did see
it) so replying here.

On Mon, 2016-04-11 at 15:54 +0300, Michael S. Tsirkin wrote:

On Mon, Apr 11, 2016 at 11:45:48AM +0100, Stefan Hajnoczi wrote:

quoted

On Fri, Apr 08, 2016 at 04:35:05PM +0100, Ian Campbell wrote:

quoted

On Fri, 2016-04-01 at 15:23 +0100, Stefan Hajnoczi wrote:

quoted

This series is based on Michael Tsirkin's vhost branch (v4.5-rc6).

I'm about to process Claudio Imbrenda's locking fixes for virtio-vsock but
first I want to share the latest version of the code.  Several people are
playing with vsock now so sharing the latest code should avoid duplicate work.

Thanks for this, I've been using it in my project and it mostly seems
fine.

One wrinkle I came across, which I'm not sure if it is by design or a
problem is that I can see this sequence coming from the guest (with
other activity in between):

    1) OP_SHUTDOWN w/ flags == SHUTDOWN_RX
    2) OP_SHUTDOWN w/ flags == SHUTDOWN_TX
    3) OP_SHUTDOWN w/ flags == SHUTDOWN_TX|SHUTDOWN_RX

I orignally had my backend close things down at #2, however this meant
that when #3 arrived it was for a non-existent socket (or, worse, an
active one if the ports got reused). I checked v5 of the spec
proposal[0] which says:
    If these bits are set and there are no more virtqueue buffers
    pending the socket is disconnected.

but I'm not entirely sure if this behaviour contradicts this or not
(the bits have both been set at #2, but not at the same time).

BTW, how does one tell if there are no more virtqueue buffers pending
or not while processing the op?

#2 is odd.  The shutdown bits are sticky so they cannot be cleared once
set.  I would have expected just #1 and #3.  The behavior you observe
look like a bug.

The spec text does not convey the meaning of OP_SHUTDOWN well.
OP_SHUTDOWN SHUTDOWN_TX|SHUTDOWN_RX means no further rx/tx is possible
for this connection.  "there are no more virtqueue buffers pending the
socket" really means that this isn't an immediate close from the
perspective of the application.  If the application still has unread rx
buffers then the socket stays readable until the rx data has been fully
read.

Thanks, distinguishing the local buffer to the application from the
vring would make that clearer. Perhaps by not talking about "virtqueue
buffers" since they sound like a vring thing.

However, as Michael observes I'm not sure that's the whole story.

Yes but you also wrote:
	If these bits are set and there are no more virtqueue buffers
	pending the socket is disconnected.

how does remote know that there are no buffers pending and so it's safe
to reuse the same source/destination address now?

Indeed this is one of the things I struggled with. e.g. If I send a
SHUTDOWN_RX to my peer am I supposed to wait for that buffer to come
back (so I know the peer has seen it) and then wait for an entire
"cycle" of the TX ring to know there is nothing still in flight? That's
some tricky book-keeping.

  Maybe destination
should send RST at that point?

i.e. upon receipt of SHUTDOWN_RX|SHUTDOWN_TX from the peer you are
expected to send a RST. When the peer observes that then they know
there is no further data in that connection on the ring?

That sounds like it would be helpful.

quoted

quoted

Another thing I noticed, which is really more to do with the generic
AF_VSOCK bits than anything to do with your patches is that there is no
limitations on which vsock ports a non-privileged user can bind to and
relatedly that there is no netns support so e.g. users in unproivileged
containers can bind to any vsock port and talk to the host, which might
be undesirable. For my use for now I just went with the big hammer
approach of denying access from anything other than init_net
namespace[1] while I consider what the right answer is.

From the vhost point of view each netns should have its own AF_VSOCK
namespace.  This way two containers could act as "the host" (CID 2) for
their respective guests.

When you say "should" you mean that's the intended design as opposed to
what the current code is actually doing, right?

Ian.
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