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

[Paolo Abeni <pabeni@redhat.com>]

Hi all,

On 5/6/25 10:56 AM, Parav Pandit wrote:
>> From: Michael S. Tsirkin <mst@redhat.com>
>> Sent: Tuesday, May 6, 2025 1:26 PM
>>> There are few proposals on table.
>>>
>>> 1. From Paolo,
>>> - 0 to 23, and 50 to 127 Feature bits for the specific device type
>>> + 0 to 23, and 46 to 127 Feature bits for the specific device type
>>>
>>> This does not have good reason of why it should still be 127.
>>>
>>> 2. From me:
>>> - 0 to 23, and 50 to 127 Feature bits for the specific device type
>>> + 0 to 23, and 45 to 64 Feature bits for the specific device type
>>>
>>> This is an extension of Paolo, to justify that implementing feature bits is
>> extremely hard even for experts as pointed by Paolo.
>>> It is worth to not extend it further.
>>> RSS can be negotiated via new bit 44 in future bit as OR of 44 and 64 so that
>> more wider users (Linux, freebsd, qnx, Windows, dpdk pmd) can pick 44.
>>>
>>> 3. From me:
>>> Keep the feature bits encoding as is up to 127 bits, because may be there is
>> (unknown and weird) value in having 127 feature bits.
>>> (unknown because the reasoning of #1 and #3 mismatch).
>>> In that case,
>>> VIRTIO_NET_CTRL_GUEST_OFFLOADS command text to be updated to
>> indicate
>>> feature fits A to D map to VIRTIO_NET_F_CTRL_GUEST_OFFLOADS.offloads
>> bits A' to D'.
>>>
>>> I am fine with option #2 and #3.
>>> Doing #1 for sure is wrong.
>>> Wrong because it delays the problem of #1 from this to another feature [A]
>> who's voting already completed.
>>>
>>> [A]
>>> https://lore.kernel.org/virtio-
>> comment/DM4PR18MB4269F73B786E83EF68A70F
>>> 37DFB82@DM4PR18MB4269.namprd18.prod.outlook.com/T/#t
>>
>>
>> #3 seems more conservate.
> 
> Are you good with #3?

I'm sorry for the latency. Let me double check to avoid possible
misunderstanding; #3 means:

- 0 to 23, and 50 to 127 Feature bits for the specific device type
+ 0 to 23, and 45 to 127 Feature bits for the specific device type

using bits 46-39 for UDP tunnel offloads and likely bit 45 for
VIRTIO_NET_F_OUT_NET_HEADER.

The VIRTIO_NET_F_CTRL_GUEST_OFFLOADS mapping should be specified after
eventually a new offload feature will be defined using a bit >= 64.

Am I correct?

Thanks,

Paolo