Re: [PATCH v4] virtio-net: Ignore num_buffers when unused

On Mon, May 26, 2025 at 02:15:19PM +0900, Akihiko Odaki wrote:
> If VIRTIO_NET_F_MRG_RXBUF has not been negotiated, vhost_net of Linux
> prior to 6.14 and QEMU prior to 10.0.0 leave num_buffers uninitialized
> while the specification tells to set it to one.
> 
> Let drivers ignore this field to make them compatible with old
> vhost_net/QEMU, similar to the hdr_len field, which is also marked as
> unreliable due to implementation bugs.
> 
> Signed-off-by: Akihiko Odaki <[email protected]>
> Reviewed-by: Parav Pandit <[email protected]>
> ---
> Changes in v4:
> - Noted that the mentioned bugs are fixed with newer implementation
>   versions in the patch message.
> - Noted that the bugs are of devices. (Parav Pandit)
> - Link to v3: https://lore.kernel.org/r/[email protected]
> 
> Changes in v3:
> - s/MUST/SHOULD/ (Michael S. Tsirkin)
> - Link to v2: https://lore.kernel.org/r/[email protected]
> 
> Changes in v2:
> - Corrected the change description to tell vhost_net and QEMU leaves
>   num_buffers uninitialized where it used to tell they set it to zero.
> - Restored statements that require devices to set num_buffers to one.
> - Link to v1: https://lore.kernel.org/r/[email protected]
> ---
>  device-types/net/description.tex | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/device-types/net/description.tex b/device-types/net/description.tex
> index 76585b0..4a2df5a 100644
> --- a/device-types/net/description.tex
> +++ b/device-types/net/description.tex
> @@ -863,6 +863,12 @@ \subsubsection{Processing of Incoming Packets}\label{sec:Device Types / Network
>  This is due to various bugs in implementations.
>  \end{note}
>  
> +If VIRTIO_NET_F_MRG_RXBUF has not been negotiated, the driver SHOULD ignore
> +\field{num_buffers}.
> +\begin{note}
> +This is due to bugs in device implementations.
> +\end{note}
> +
>  If neither VIRTIO_NET_HDR_F_NEEDS_CSUM nor
>  VIRTIO_NET_HDR_F_DATA_VALID is set, the driver MUST NOT
>  rely on the packet checksum being correct.

Acked-by: Michael S. Tsirkin <[email protected]>


> ---
> base-commit: b495841a8e80d12c1130f8868f4128866291142d
> change-id: 20241226-reserved-d5960f173223
> 
> Best regards,
> -- 
> Akihiko Odaki <[email protected]>