Subject: Re: [PATCH 3/5] virtio: Don't expose legacy net features when VIRTIO_NET_NO_LEGACY defined.

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sun, 8 Feb 2015 11:59:08 +0100

On Fri, Feb 06, 2015 at 03:36:54PM +1030, Rusty Russell wrote:
> In particular, the virtio header always has the u16 num_buffers field.
> We define a new 'struct virtio_net_modern_hdr' for this (rather than
> simply calling it 'struct virtio_net_hdr', to avoid nasty type errors
> if some parts of a project define VIRTIO_NET_NO_LEGACY and some don't.
> 
> Signed-off-by: Rusty Russell <[email-masked]>
> ---
>  include/uapi/linux/virtio_net.h | 30 ++++++++++++++++++++++++++++--
>  1 file changed, 28 insertions(+), 2 deletions(-)
> 
> diff --git a/include/uapi/linux/virtio_net.h b/include/uapi/linux/virtio_net.h
> index b5f1677b291c..32754f3000e8 100644
> --- a/include/uapi/linux/virtio_net.h
> +++ b/include/uapi/linux/virtio_net.h
> @@ -35,7 +35,6 @@
>  #define VIRTIO_NET_F_CSUM	0	/* Host handles pkts w/ partial csum */
>  #define VIRTIO_NET_F_GUEST_CSUM	1	/* Guest handles pkts w/ partial csum */
>  #define VIRTIO_NET_F_MAC	5	/* Host has given MAC address. */
> -#define VIRTIO_NET_F_GSO	6	/* Host handles pkts w/ any GSO type */
>  #define VIRTIO_NET_F_GUEST_TSO4	7	/* Guest can handle TSOv4 in. */
>  #define VIRTIO_NET_F_GUEST_TSO6	8	/* Guest can handle TSOv6 in. */
>  #define VIRTIO_NET_F_GUEST_ECN	9	/* Guest can handle TSO[6] w/ ECN in. */
> @@ -56,6 +55,10 @@
>  					 * Steering */
>  #define VIRTIO_NET_F_CTRL_MAC_ADDR 23	/* Set MAC address */
>  
> +#ifndef VIRTIO_NET_NO_LEGACY
> +#define VIRTIO_NET_F_GSO	6	/* Host handles pkts w/ any GSO type */
> +#endif /* VIRTIO_NET_NO_LEGACY */
> +
>  #define VIRTIO_NET_S_LINK_UP	1	/* Link is up */
>  #define VIRTIO_NET_S_ANNOUNCE	2	/* Announcement is needed */
>  
> @@ -71,8 +74,9 @@ struct virtio_net_config {
>  	__u16 max_virtqueue_pairs;
>  } __attribute__((packed));
>  
> +#ifndef VIRTIO_NET_NO_LEGACY
>  /* This header comes first in the scatter-gather list.
> - * If VIRTIO_F_ANY_LAYOUT is not negotiated, it must
> + * For legacy virtio, if VIRTIO_F_ANY_LAYOUT is not negotiated, it must
>   * be the first element of the scatter-gather list.  If you don't
>   * specify GSO or CSUM features, you can simply ignore the header. */
>  struct virtio_net_hdr {
> @@ -97,6 +101,28 @@ struct virtio_net_hdr_mrg_rxbuf {
>  	struct virtio_net_hdr hdr;
>  	__virtio16 num_buffers;	/* Number of merged rx buffers */
>  };
> +#else /* ... VIRTIO_NET_NO_LEGACY */
> +/*
> + * This header comes first in the scatter-gather list.  If you don't
> + * specify GSO or CSUM features, you can simply ignore the header.
> + */
> +struct virtio_net_modern_hdr {
> +#define VIRTIO_NET_HDR_F_NEEDS_CSUM	1	/* Use csum_start, csum_offset */
> +#define VIRTIO_NET_HDR_F_DATA_VALID	2	/* Csum is valid */
> +	__u8 flags;
> +#define VIRTIO_NET_HDR_GSO_NONE		0	/* Not a GSO frame */
> +#define VIRTIO_NET_HDR_GSO_TCPV4	1	/* GSO frame, IPv4 TCP (TSO) */
> +#define VIRTIO_NET_HDR_GSO_UDP		3	/* GSO frame, IPv4 UDP (UFO) */
> +#define VIRTIO_NET_HDR_GSO_TCPV6	4	/* GSO frame, IPv6 TCP */
> +#define VIRTIO_NET_HDR_GSO_ECN		0x80	/* TCP has ECN set */
> +	__u8 gso_type;
> +	__virtio16 hdr_len;	/* Ethernet + IP + tcp/udp hdrs */
> +	__virtio16 gso_size;	/* Bytes to append to hdr_len per frame */
> +	__virtio16 csum_start;	/* Position to start checksumming from */
> +	__virtio16 csum_offset;	/* Offset after that to place checksum */
> +	__virtio16 num_buffers;	/* Number of merged rx buffers */
> +};
> +#endif /* ...VIRTIO_NET_NO_LEGACY */

This kind of masks the fact that it's the same as
virtio_net_hdr_mrg_rxbuf. So it's forcing people to duplicate
code for transitional devices.

How about
struct virtio_net_modern_hdr {
	struct virtio_net_hdr_mrg_rxbuf hdr;
}

This will also make it look nicer when we start
adding stuff in the header, the main header
is separated in a struct by its own, so it's
easy to apply operations such as sizeof.

>  /*
>   * Control virtqueue data structures
> -- 
> 2.1.0