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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Wed, Mar 17, 2021 at 09:25:26PM +0100, Ilya Maximets wrote:
Hi,
Some questions to understand the problems that SocketPair Broker solves:

> Even more configuration tricks required in order to share some sockets
> between different containers and not only with the host, e.g. to
> create service chains.

How does SocketPair Broker solve this? I guess the idea is that
SocketPair Broker must be started before other containers. That way
applications don't need to sleep and reconnect when a socket isn't
available yet.

On the other hand, the SocketPair Broker might be unavailable (OOM
killer, crash, etc), so applications still need to sleep and reconnect
to the broker itself. I'm not sure the problem has actually been solved
unless there is a reason why the broker is always guaranteed to be
available?

> 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.

> Additionally, all applications (system and user's!) should follow
> naming conventions and place socket files in particular location on a
> file system to make things work.

Does SocketPair Broker solve this? Applications now need to use a naming
convention for keys, so it seems like this issue has not been
eliminated.

> 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.

BTW what is the security model of the broker? Unlike pathname UNIX
domain sockets there is no ownership permission check.

Stefan