[RFC] crypto/virtio: add vhost-vdpa backend

[Gowrishankar Muthukrishnan]

Hi,
We are adding support for vDPA user backend for virtio-crypto PMD in DPDK. We have come up with functional changes which is similar to the support available in net:

    commit 6b901437056eed3ed7c9932c333ba24ac5be116f

    net/virtio: introduce vhost-vDPA backend
    vhost-vDPA is a new virtio backend type introduced by vDPA kernel
    framework, which provides abstraction to the vDPA devices and
    exposes an unified control interface through a char dev.

Our current development reuses some code from net/virtio/virtio_user/, and we realize that we could keep a few things in common between net and crypto, such as:

-> vhost_vdpa.c (and its header file) from net/virtio/virtio_user/:
   Except for VHOST_VDPA_GET_DEVICE_ID and enabling queue pairs, virtio_user_backend_ops can be reused.
-> virtio_user_dev.c (and its header file) from net/virtio/virtio_user/:
   virtio_user_dev_init and its capabilities differ.
-> virtio_cvq.c (and its header file) from net/virtio/:
   There is a difference in the usage of the first and last descriptors for the virtio header and status (net vs. crypto).

We need to standardize these codes to ensure they work universally. Therefore, we propose creating a driver/common/virtio/ directory to house them. This approach will help address common issues and extend Virtio functionalities shared between crypto and net. For example, the crypto PMD can benefit from packed ring support. We welcome your valuable feedback and any suggestions.

Thanks,
Gowrishankar
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2.37.1