netdev - Re: [PATCH net-next 5/8] net: implement virtio helpers to handle UDP GSO tunneling.

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Message-ID: <ca169183-55af-43d2-b78d-db82c33c6643@redhat.com>
Date: Fri, 23 May 2025 16:00:30 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Willem de Bruijn <willemdebruijn.kernel@...il.com>, netdev@...r.kernel.org
Cc: Jason Wang <jasowang@...hat.com>, Andrew Lunn <andrew+netdev@...n.ch>,
 "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>,
 Jakub Kicinski <kuba@...nel.org>, "Michael S. Tsirkin" <mst@...hat.com>,
 Xuan Zhuo <xuanzhuo@...ux.alibaba.com>, Eugenio Pérez
 <eperezma@...hat.com>
Subject: Re: [PATCH net-next 5/8] net: implement virtio helpers to handle UDP
 GSO tunneling.

On 5/23/25 3:42 PM, Willem de Bruijn wrote:
> Paolo Abeni wrote:
>> On 5/23/25 12:29 AM, Willem de Bruijn wrote:
>>> Paolo Abeni wrote:
>>>> +#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_MAPPED	46
>>>> +#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM_MAPPED	47
>>>
>>> I don't quite follow this. These are not real virtio bits?
>>
>> This comes directly from the recent follow-up on the virtio
>> specification. While the features space has been extended to 128 bit,
>> the 'guest offload' space is still 64bit. The 'guest offload' are
>> used/defined by the specification for the
>> VIRTIO_NET_CTRL_GUEST_OFFLOADS_SET command, which allows the guest do
>> dynamically enable/disable H/W GRO at runtime.
>>
>> Up to ~now each offload bit corresponded to the feature bit with the
>> same value and vice versa.
>>
>> Due to the limited 'guest offload' space, relevant features in the high
>> 64 bits are 'mapped' to free bits in the lower range. That is simpler
>> than defining a new command (and associated features) to exchange an
>> extended guest offloads set.
>>
>> It's also not a problem from a 'guest offload' space exhaustion PoV
>> because there are a lot of features in the lower 64 bits range that are
>> _not_ guest offloads and could be reused for mapping - among them the
>> 'reserved features' that started this somewhat problematic features
>> space expansion.
> 
> That's a great explanation thanks. Can you add it either in the commit
> message or as a comment at these definitions?

Sure, I'll add it to the commit message.

Thanks,

Paolo