Re: [dpdk-dev] [RFC 0/4] SocketPair Broker support for vhost and virtio-user.

[Ilya Maximets <i.maximets@ovn.org>]

On 3/19/21 3:05 PM, Stefan Hajnoczi wrote:
> On Thu, Mar 18, 2021 at 08:47:12PM +0100, Ilya Maximets wrote:
>> On 3/18/21 6:52 PM, Stefan Hajnoczi wrote:
>>> On Wed, Mar 17, 2021 at 09:25:26PM +0100, Ilya Maximets wrote:
>>>> And some housekeeping usually required for applications in case the
>>>> socket server terminated abnormally and socket files left on a file
>>>> system:
>>>>  "failed to bind to vhu: Address already in use; remove it and try again"
>>>
>>> QEMU avoids this by unlinking before binding. The drawback is that users
>>> might accidentally hijack an existing listen socket, but that can be
>>> solved with a pidfile.
>>
>> How exactly this could be solved with a pidfile?
> 
> A pidfile prevents two instances of the same service from running at the
> same time.
> 
> The same effect can be achieved by the container orchestrator, systemd,
> etc too because it refuses to run the same service twice.

Sure. I understand that.  My point was that these could be 2 different
applications and they might not know which process to look for.

> 
>> And what if this is
>> a different application that tries to create a socket on a same path?
>> e.g. QEMU creates a socket (started in a server mode) and user
>> accidentally created dpdkvhostuser port in Open vSwitch instead of
>> dpdkvhostuserclient.  This way rte_vhost library will try to bind
>> to an existing socket file and will fail.  Subsequently port creation
>> in OVS will fail.   We can't allow OVS to unlink files because this
>> way OVS users will have ability to unlink random sockets that OVS has
>> access to and we also has no idea if it's a QEMU that created a file
>> or it was a virtio-user application or someone else.
> 
> If rte_vhost unlinks the socket then the user will find that networking
> doesn't work. They can either hot unplug the QEMU vhost-user-net device
> or restart QEMU, depending on whether they need to keep the guest
> running or not. This is a misconfiguration that is recoverable.

True, it's recoverable, but with a high cost.  Restart of a VM is rarely
desirable.  And the application inside the guest might not feel itself
well after hot re-plug of a device that it actively used.  I'd expect
a DPDK application that runs inside a guest on some virtio-net device
to crash after this kind of manipulations.  Especially, if it uses some
older versions of DPDK.

> 
> Regarding letting OVS unlink files, I agree that it shouldn't if this
> create a security issue. I don't know the security model of OVS.

In general privileges of a ovs-vswitchd daemon might be completely
different from privileges required to invoke control utilities or
to access the configuration database.  SO, yes, we should not allow
that.

> 
>> There are, probably, ways to detect if there is any alive process that
>> has this socket open, but that sounds like too much for this purpose,
>> also I'm not sure if it's possible if actual user is in a different
>> container.
>> So I don't see a good reliable way to detect these conditions.  This
>> falls on shoulders of a higher level management software or a user to
>> clean these socket files up before adding ports.
> 
> Does OVS always run in the same net namespace (pod) as the DPDK
> application? If yes, then abstract AF_UNIX sockets can be used. Abstract
> AF_UNIX sockets don't have a filesystem path and the socket address
> disappears when there is no process listening anymore.

OVS is usually started right on the host in a main network namespace.
In case it's started in a pod, it will run in a separate container but
configured with a host network.  Applications almost exclusively runs
in separate pods.

> 
>>>> This patch-set aims to eliminate most of the inconveniences by
>>>> leveraging an infrastructure service provided by a SocketPair Broker.
>>>
>>> I don't understand yet why this is useful for vhost-user, where the
>>> creation of the vhost-user device backend and its use by a VMM are
>>> closely managed by one piece of software:
>>>
>>> 1. Unlink the socket path.
>>> 2. Create, bind, and listen on the socket path.
>>> 3. Instantiate the vhost-user device backend (e.g. talk to DPDK/SPDK
>>>    RPC, spawn a process, etc) and pass in the listen fd.
>>> 4. In the meantime the VMM can open the socket path and call connect(2).
>>>    As soon as the vhost-user device backend calls accept(2) the
>>>    connection will proceed (there is no need for sleeping).
>>>
>>> This approach works across containers without a broker.
>>
>> Not sure if I fully understood a question here, but anyway.
>>
>> This approach works fine if you know what application to run.
>> In case of a k8s cluster, it might be a random DPDK application
>> with virtio-user ports running inside a container and want to
>> have a network connection.  Also, this application needs to run
>> virtio-user in server mode, otherwise restart of the OVS will
>> require restart of the application.  So, you basically need to
>> rely on a third-party application to create a socket with a right
>> name and in a correct location that is shared with a host, so
>> OVS can find it and connect.
>>
>> In a VM world everything is much more simple, since you have
>> a libvirt and QEMU that will take care of all of these stuff
>> and which are also under full control of management software
>> and a system administrator.
>> In case of a container with a "random" DPDK application inside
>> there is no such entity that can help.  Of course, some solution
>> might be implemented in docker/podman daemon to create and manage
>> outside-looking sockets for an application inside the container,
>> but that is not available today AFAIK and I'm not sure if it
>> ever will.
> 
> Wait, when you say there is no entity like management software or a
> system administrator, then how does OVS know to instantiate the new
> port? I guess something still needs to invoke ovs-ctl add-port?

I didn't mean that there is no any application that configures
everything.  Of course, there is.  I mean that there is no such
entity that abstracts all that socket machinery from the user's
application that runs inside the container.  QEMU hides all the
details of the connection to vhost backend and presents the device
as a PCI device with a network interface wrapping from the guest
kernel.  So, the application inside VM shouldn't care what actually
there is a socket connected to OVS that implements backend and
forward traffic somewhere.  For the application it's just a usual
network interface.
But in case of a container world, application should handle all
that by creating a virtio-user device that will connect to some
socket, that has an OVS on the other side.

> 
> Can you describe the steps used today (without the broker) for
> instantiating a new DPDK app container and connecting it to OVS?
> Although my interest is in the vhost-user protocol I think it's
> necessary to understand the OVS requirements here and I know little
> about them.

I might describe some things wrong since I worked with k8s and CNI
plugins last time ~1.5 years ago, but the basic schema will look
something like this:

1. user decides to start a new pod and requests k8s to do that
   via cmdline tools or some API calls.

2. k8s scheduler looks for available resources asking resource
   manager plugins, finds an appropriate physical host and asks
   local to that node kubelet daemon to launch a new pod there.

3. kubelet asks local CNI plugin to allocate network resources
   and annotate the pod with required mount points, devices that
   needs to be passed in and environment variables.
   (this is, IIRC, a gRPC connection.   It might be a multus-cni
   or kuryr-kubernetes or any other CNI plugin.  CNI plugin is
   usually deployed as a system DaemonSet, so it runs in a
   separate pod.

4. Assuming that vhost-user connection requested in server mode.
   CNI plugin will:
   4.1 create a directory for a vhost-user socket.
   4.2 add this directory to pod annotations as a mount point.
   4.3 create a port in OVS by invoking 'ovs-vsctl port-add' or
       by connecting to ovsdb-server by JSONRPC directly.
       It will set port type as dpdkvhostuserclient and specify
       socket-path as a path inside the directory it created.
       (OVS will create a port and rte_vhost will enter the
        re-connection loop since socket does not exist yet.)
   4.4 Set up socket file location as environment variable in
       pod annotations.
   4.5 report success to kubelet.

5. kubelet will finish all other preparations and resource
   allocations and will ask docker/podman to start a container
   with all mount points, devices and environment variables from
   the pod annotation.

6. docker/podman starts a container.
   Need to mention here that in many cases initial process of
   a container is not the actual application that will use a
   vhost-user connection, but likely a shell that will invoke
   the actual application.

7. Application starts inside the container, checks the environment
   variables (actually, checking of environment variables usually
   happens in a shell script that invokes the application with
   correct arguments) and creates a net_virtio_user port in server
   mode.  At this point socket file will be created.
   (since we're running third-party application inside the container
    we can only assume that it will do what is written here, it's
    a responsibility of an application developer to do the right
    thing.)

8. OVS successfully re-connects to the newly created socket in a
   shared directory and vhost-user protocol establishes the network
   connectivity.

As you can wee, there are way too many entities and communication
methods involved.  So, passing a pre-opened file descriptor from
CNI all the way down to application is not that easy as it is in
case of QEMU+LibVirt.

Best regards, Ilya Maximets.