Re: [virtio-comment] Seeking guidance for custom virtIO device

[Stefano Garzarella <sgarzare@redhat.com>]

Hi,

On Fri, Apr 10, 2020 at 09:36:58AM +0000, Eftime, Petre wrote:
> Hi all,
> 
> I am looking for guidance on how to proceed with regards to either reserving a virtio device ID for a specific device for a particular usecase  or for formalizing a device type that could be potentially used by others.
> 
> We have developed a virtio device that acts as a transport for API calls between a guest userspace library and a backend server in the host system.
> Our requirements are:
> * multiple clients in the guest (multiple servers is not required)
> * provide an in-order, reliable datagram transport mechanism
> * datagram size should be either negotiable or large (16k-64k?)
> * performance is not a big concern for our usecase

It looks really close to vsock.

> 
> The reason why we used a special device and not something else is the following:
> * vsock spec does not contain a datagram specification (eg. SOCK_DGRAM, SOCK_SEQPACKET) and the effort of updating the Linux driver and other implementations for this particular purpose  seemed relatively high. The path to approach this problem wasn't clear. Vsock today only works in SOCK_STREAM mode and this is not ideal: the receiver must implement additional state and buffer incoming data,  adding complexity and host resource usage.

AF_VSOCK itself supports SOCK_DGRAM, but virtio-vsock doesn't provide
this feature. (vmci provides SOCK_DGRAM support)

The changes should not be too intrusive in the virtio-vsock specs and
implementation, we already have the "type" field in the packet header
to address this new feature.

We also have the credit-mechanism to provide in-order and reliable
packets delivery.

Maybe the hardest part could be change something in the core to handle
multiple transports that provide SOCK_DGRAM, for nested VMs.
We already did for stream sockets, but we didn't handle the datagram
socket for now.

I am not sure how convenient it is to have two very similar devices...

If you decide to give virtio-vsock a chance to get SOCK_DGRAM, I can try to
give you a more complete list of changes to make. :-)

Cheers,
Stefano