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: Sat, 14 Feb 2015 21:43:26 +0100

On Sun, Feb 08, 2015 at 11:59:08AM +0100, Michael S. Tsirkin wrote:
> 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.

Ping.
Would you like a patch on top that does this?

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