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SubjectRe: [PATCH 3/5] virtio: Don't expose legacy net features when VIRTIO_NET_NO_LEGACY defined.
>  	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.
> + *
> + * This is bitwise-equivalent to the legacy struct virtio_net_hdr_mrg_rxbuf.

I would add "but with all fields squashed into the main structure".

> + */
> +struct virtio_net_hdr_v1 {
> +#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 */

I note that this way host code can't be structured like this:


struct virtio_net_hdr_v1 modern;
/* handle modern guests */
....

#ifndef VIRTIO_NET_NO_LEGACY
struct virtio_net_hdr_mrg_rxbuf mrg;
/* handle legacy guests */

#endif


Define virtio_net_hdr_v1 unconditionally?










> /*
> * Control virtqueue data structures


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