Re: [PATCH v1] virtio-net: Fix to avoid using reserved feature bits

[Paolo Abeni <pabeni@redhat.com>]

On 4/23/25 8:07 PM, Michael S. Tsirkin wrote:
> On Wed, Apr 23, 2025 at 09:29:11AM -0700, Daniel Verkamp wrote:
>> On Tue, Apr 22, 2025 at 10:46 PM Michael S. Tsirkin <mst@redhat.com> wrote:
>>> I'm afraid we'll have to bite the bullet.
>>
>> One other issue with bits > 63 is that the vhost-user protocol
>> VHOST_USER_GET_FEATURES and VHOST_USER_SET_FEATURES messages use u64
>> to represent the features, so vhost-user-net devices can't query or
>> enable these features. vhost-user is outside the scope of the virtio
>> spec, though, and I think it's reasonable to extend the protocol to
>> enable high feature bits rather than avoiding them forever.
> 
> Yes you would use VHOST_USER_SET_PROTOCOL_FEATURES to make VHOST_USER_GET_FEATURES
> return two u64s, or even a new message returning an array.

I think that additionally the VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET command
will need some clarification, as in the current text looks a bit
inconsistent:

"""
// in Offloads State Configuration / Setting Offloads State:

#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO  46

// ...

The class VIRTIO_NET_CTRL_GUEST_OFFLOADS has one command:
VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET applies the new offloads configuration.

le64 value passed as command data is a bitmask, bits set define
offloads to be enabled, bits cleared - offloads to be disabled.

There is a corresponding device feature for each offload. Upon feature
negotiation corresponding offload gets enabled to preserve backward
compatibility
"""

The "corresponding device feature" has the same numerical value of the
selected offloads, except for UDP tunnels related one (which are mapped
to bits corresponding to reserved features).

It's unclear to me which should be the better way to address this
inconsistency.

/P