Re: [PATCH] virtio-net: clarify guest offloads <> features mapping

On Mon, May 19, 2025 at 6:12 PM Paolo Abeni <[email protected]> wrote:
>
> The guest offloads, as specified by 'Setting Offloads State,' use a
> 64-bit wide space, while some of the corresponding virtio net features
> are in the upper 64 bits of a 128-bit wide space.
>
> Clarify the relevant mapping as somewhat implied by the current text.
> Also rename the offload definitions to avoid referring to different
> numeric values with the same name in the features table and in the
> offload table.
>
> Signed-off-by: Paolo Abeni <[email protected]>
> ---
>  device-types/net/description.tex | 7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
>
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index 1b6b54d..8c501bc 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -2186,8 +2186,8 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  #define VIRTIO_NET_F_GUEST_TSO6       8
>  #define VIRTIO_NET_F_GUEST_ECN        9
>  #define VIRTIO_NET_F_GUEST_UFO        10
> -#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO  46
> -#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM 47
> +#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_MAPPED  46
> +#define VIRTIO_NET_F_GUEST_UDP_TUNNEL_GSO_CSUM_MAPPED 47

As the introduction of more offload features in the future, I wonder
if it's better to just avoid reusing feature names here.

For example, rename all here to something like

VIRTIO_NET_CTRL_GUEST_OFFLOAD_TSO6
...

Or introduce a new command to be as wide as the feature bits.

>  #define VIRTIO_NET_F_GUEST_USO4       54
>  #define VIRTIO_NET_F_GUEST_USO6       55
>
> @@ -2205,6 +2205,9 @@ \subsubsection{Control Virtqueue}\label{sec:Device Types / Network Device / Devi
>  negotiation corresponding offload gets enabled to preserve backward
>  compatibility.
>
> +Note that device features bits in the [65 to 68] range correspond to
> +offload bits in the [46 to 49] range.
> +
>  \drivernormative{\subparagraph}{Setting Offloads State}{Device Types / Network Device / Device Operation / Control Virtqueue / Offloads State Configuration / Setting Offloads State}
>
>  A driver MUST NOT enable an offload for which the appropriate feature
> --
> 2.49.0
>

Thanks