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Subject: Re: [PATCH v7] vsock: add vsock device

On Mon, Aug 08, 2016 at 12:58:24PM +0100, Ian Campbell wrote:
> On 4 August 2016 at 16:24, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > The virtio vsock device is a zero-configuration socket communications
> > device.  It is designed as a guest<->host management channel suitable
> > for communicating with guest agents.
> >
> > vsock is designed with the sockets API in mind and the driver is
> > typically implemented as an address family (at the same level as
> > AF_INET).  Applications written for the sockets API can be ported with
> > minimal changes (similar amount of effort as adding IPv6 support to an
> > IPv4 application).
> >
> > Unlike the existing console device, which is also used for guest<->host
> > communication, multiple clients can connect to a server at the same time
> > over vsock.  This limitation requires console-based users to arbitrate
> > access through a single client.  In vsock they can connect directly and
> > do not have to synchronize with each other.
> >
> > Unlike network devices, no configuration is necessary because the device
> > comes with its address in the configuration space.
> >
> > The vsock device was prototyped by Gerd Hoffmann and Asias He.  I picked
> > the code and design up from them.
> >
> > VIRTIO-151
> >
> > Cc: Gerd Hoffmann <kraxel@redhat.com>
> > Cc: Asias He <asias.hejun@gmail.com>
> > Cc: Michael S. Tsirkin <mst@redhat.com>
> > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > ---
> > v7:
> >  * Add virtqueue flow control section to explain how deadlock is avoided
> >    when rings are full [Ian]
> Thanks, would it be possible/easy for you to update
> https://stefanha.github.io/virtio/#x1-2800007 and
> https://github.com/stefanha/virtio? (I think they both still have v6)

I have updated it:

> Is there a more-or-less semi-official git mirror of the master svn
> repo, or does everyone just roll their own with `git-svn`?

Everyone rolls their own with git-svn.

> I think the following quote is the entirety of the change/newly added text:
> > +\subsubsection{Virtqueue Flow Control}\label{sec:Device Types / Socket Device / Device Operation / Virtqueue Flow Control}
> > +
> > +The tx virtqueue carries packets initiated by applications and replies to
> > +received packets.  The rx virtqueue carries packets initiated by the device and
> > +replies to previously transmitted packets.
> > +
> > +If both rx and tx virtqueues are filled by the driver and device at the same
> > +time then it appears that a deadlock is reached.  The driver has no free tx
> > +descriptors to send replies.  The device has no free rx descriptors to send
> > +replies either.  Therefore neither device nor driver can process virtqueues
> > +since that may involve sending new replies.
> > +
> > +This is solved using additional resources outside the virtqueue to hold
> > +packets.  With additional resources, it becomes possible to process incoming
> > +packets even when outgoing packets cannot be sent.
> > +
> > +Eventually even the additional resources will be exhausted and further
> > +processing is not possible until the other side processes the virtqueue that
> > +it has neglected.  This stop to processing prevents one side from causing
> > +unbounded resource consumption in the other side.
> Rather than "This stop to..." did you mean "This is to stop..." or
> something else? I can't parse it as is I'm afraid.

Wow, I thought I'd crafted some pretty fine prose.  Sorry about that.
I'll change it to:

This stops one side from causing unbounded resource consumption.

> > +\drivernormative{\paragraph}{Device Operation: Virtqueue Flow Control}{Device Types / Socket Device / Device Operation / Virtqueue Flow Control}
> > +
> > +The rx virtqueue MUST be processed even when the tx virtqueue is full so long as there are additional resources available to hold packets outside the tx virtqueue.
> > +
> > +\devicenormative{\paragraph}{Device Operation: Virtqueue Flow Control}{Device Types / Socket Device / Device Operation / Virtqueue Flow Control}
> > +
> > +The tx virtqueue MUST be processed even when the rx virtqueue is full so long as there are additional resources available to hold packets outside the rx virtqueue.
> These describe handling the case where additional resources are
> available (by saying "keep going if you have resources to do so",
> which IMHO doesn't really need stating) but they don't describe what
> the acceptable behaviour is when those resources are no longer
> available, so really this is just kicking the deadlock can a bit
> further down the road without addressing the actual problem (i.e. the
> "Eventually even the additional resources..." paragraph from above).

No, there is an important point in there which I didn't state clearly:

Additional resources are *required*, otherwise virtqueue deadlock is
possible under normal operation.

Regarding kicking the can down the road:

It will always be possible to ignore incoming packets to the point where
the peer will have to stop responding, but this isn't a problem.  Under
correct operation this state is never reached.

> IIRC from previous discussions your intention was to say that a
> driver/backend should stop processessing TX requests if the RX ring
> becomes full (or I suppose if the "additional resources" are
> exhausted), I guess with the implications that it should never stop
> processing RX requests (which I think is always possible since from
> the driver end there are no "additional resources" needed for RX
> processing). But the above essentially explicitly states the OK case
> (i.e. the non-deadlock case, resources available) and leaves what to
> do on resource exhaustion implicit (while that is the most important
> bit of information).
> It's unspecified how much "additional resource" an implementation is
> expected/required to provide. Should it be enough for one ring slot?
> Or N*slots-per-ring? Is 0 an acceptable number?

The way I've implemented the resource limit is to only allow queuing N
reply packets when the outgoing virtqueue is full, where N is the size
of the incoming virtqueue.

If this limit is reached then incoming virtqueue processing is paused
until there is space to transmit pending reply packets again.

> BTW our backend implementation is now available in
> https://github.com/docker/hyperkit/blob/af-vsock/src/pci_virtio_sock.c.
> It corresponds to ~v5 of the spec IIIRC (so it still has workarounds
> relating to the OP_SHUTDOWN issue which you resolved in v6 and it
> hasn't yet switched to 64 bit addressing). WRT the above issue the
> "additional resources" are a pending-reply ring
> (https://github.com/docker/hyperkit/blob/af-vsock/src/pci_virtio_sock.c#L281)
> which I sized to be 2x the ring size (since with 1x I still saw
> deadlocks) however people are currently reporting deadlocks (actually
> an assert failure in this implementation) with that, so I am
> considering making it 4x, but that is just pushing the problem a
> further away again without solving.

One detail about the limit I've implemented is that it only applies to
*reply* packets.  It does not count ordinary outgoing packets.  Perhaps
you are counting all outgoing packets.  I'm trusting rlimits, socket
buffer sizes, etc to limit ordinary outgoing packets so they don't
matter here.

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