fix(virtio): KVM receive path

[Enigamict]

Description

First, Under QEMU, both transmit and receive paths worked correctly, but under KVM, only transmission was functional reception failed because no notifications were observed in the host’s used ring.

As a cause, vio_populate_rx_mbufs() was not called before setting the DRIVER_OK status bit.

1. From the host’s perspective, the used ring was initialized but the RX buffer queue remained empty.

2. When attaching, the host could not place incoming packets into the RX queue, leading to unstable or nonfunctional RX under KVM.

Reordered the initialization sequence in virtio_attach_finish()

queue setup → RX populate → kick → DRIVER_OK

This ensures that RX buffers are populated before notifying the host (DRIVER_OK), so that incoming data can be received immediately after device setup.

Related links

How was this PR tested?

Notes for reviewers

[veqcc]

@Enigamict
The code looks good.
Do you have a corresponding OpenBSD/FreeBSD implementation?

[Enigamict]

@veqcc
Thank you for your review.
How to FreeBSD prepares the RX queue.

vtnet_init_locked() [1]. That is called when initializing the interface; inside it, vtnet_reinit() [2] handles the Virtio reinitialization. Within vtnet_reinit(), the functions vtnet_init_rxtx_queues() [3] and vtnet_init_rx_queues() [4] prepare the RX buffers, which mirrors the Awkernel path where virtio_attach_finish() sets up each queue via virtio_pci_setup_queue() before touching the RX rings. vtnet_init_rxtx_queues() [5] does the RX/TX virtqueue work.

The key part is the RX-side preparation. vtnet_rxq_populate() [6] corresponds to should_notify.push(rx.vio_populate_rx_mbufs());, i.e., it actually populates the virtqueue with RX buffers via vtnet_rxq_new_buf() [7], just as Awkernel’s rx.vio_populate_rx_mbufs() fills the ring. Once RX buffers have been placed on the ring, virtqueue_notify() [8] is used to notify the host. He same step Awkernel implements by looping over should_notify and calling self.virtio_notify(rx_vq_idx) for the queues that need a kick.

[1]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L3489
[2]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L3452
[3]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L3481
[4]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L3375
[5]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L3371
[6]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L1493
[7]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L1507
[8]https://github.com/freebsd/freebsd-src/blob/9b0102837e305ca75de2bc14d284f786a33f9a6a/sys/dev/virtio/network/if_vtnet.c#L1514

[veqcc]

@Enigamict
Thank you for your explanation 👍