Set correctly the offload flags of TAP device based on feature negotiation with the guest

[roypat]

Firecracker networking relies on virtio-net [1] virtualized ethernet cards, backed by TAP devices on the host. Both TAP and virtio-net support several offloading features [1][2]. In order for these to play correctly together, each offloading feature needs to be supported by both the guest driver and the TAP device on the host.

The Linux virtio-net driver always (at least in the configuration we use it) supports the same offloading features the TAP devices does. As a result, during device initialization, Firecracker informs the guest about the offloads that are possible (the ones supported by the TAP device) and configures the TAP device to use them trusting that the driver will also support the same features.

However, other operating systems, such as FreeBSD might make other decisions (see also #3905). Moreover, Linux in the future might decide that (at least in some cases) will not support.

We should fix the Firecracker virtio-net implementation to validate that the features it offered to the guest are actually supported by the driver and only configure the TAP device with the subset of the features that the driver acknowledged.

• Change the functionality of virtio-net device to do the right thing
• Add a unit-test that ensures we set the correct features to the TAP device

These is some preliminary work in #4386 on which a PR could be based

[yuluo-zy]

I think I can try this question！！

[roypat]

Hi @yuluo-zy,
that sounds awesome, feel free to have a stab at it :)