Re: [Xen-devel] [PATCH net-next v3 2/5] xen-netback: add support for IPv6 checksum offload from guest

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: xen-devel-bounces@xxxxxxxxxxxxx [mailto:xen-devel-
> bounces@xxxxxxxxxxxxx] On Behalf Of Paul Durrant
> Sent: 11 October 2013 12:10
> To: Wei Liu
> Cc: netdev@xxxxxxxxxxxxxxx; Ian Campbell; Wei Liu; David Vrabel; xen-
> devel@xxxxxxxxxxxxx
> Subject: Re: [Xen-devel] [PATCH net-next v3 2/5] xen-netback: add support
> for IPv6 checksum offload from guest
> 
> > -----Original Message-----
> > From: Wei Liu [mailto:wei.liu2@xxxxxxxxxx]
> > Sent: 11 October 2013 11:10
> > To: Paul Durrant
> > Cc: xen-devel@xxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Wei Liu; David
> Vrabel;
> > Ian Campbell
> > Subject: Re: [PATCH net-next v3 2/5] xen-netback: add support for IPv6
> > checksum offload from guest
> >
> > On Thu, Oct 10, 2013 at 04:25:30PM +0100, Paul Durrant wrote:
> > [...]
> > > -#define PKT_PROT_LEN    (ETH_HLEN + \
> > > -                  VLAN_HLEN + \
> > > -                  sizeof(struct iphdr) + MAX_IPOPTLEN + \
> > > -                  sizeof(struct tcphdr) + MAX_TCP_OPTION_SPACE)
> > > +#define PKT_PROT_LEN 128
> >
> > I'm not against changing this, but could you please add comment on why
> > 128 is chosen.
> >
> 
> Hmm. I did mod the comment, but that change seems to have got lost
> somewhere.
> 
> > >
> > [...]
> > > + while ((off <= sizeof(struct ipv6hdr) + ntohs(ipv6h->payload_len))
> > &&
> > > +        !done) {
> > > +         switch (nexthdr) {
> > > +         case IPPROTO_FRAGMENT: {
> > > +                 struct frag_hdr *hp = (void *)(skb->data + off);
> > > +
> > > +                 header_size = skb->network_header +
> > > +                         off +
> > > +                         sizeof(struct frag_hdr);
> > > +                 maybe_pull_tail(skb, header_size);
> > > +
> > > +                 nexthdr = hp->nexthdr;
> > > +                 off += 8;
> >
> > Can you use sizeof(frag_hdr) instead of 8?
> >
> > > +                 break;
> > > +         }
> > > +         case IPPROTO_DSTOPTS:
> > > +         case IPPROTO_HOPOPTS:
> > > +         case IPPROTO_ROUTING: {
> > > +                 struct ipv6_opt_hdr *hp = (void *)(skb->data + off);
> > > +
> > > +                 header_size = skb->network_header +
> > > +                         off +
> > > +                         sizeof(struct ipv6_opt_hdr);
> > > +                 maybe_pull_tail(skb, header_size);
> > > +
> > > +                 nexthdr = hp->nexthdr;
> > > +                 off += ipv6_optlen(hp);
> > > +                 break;
> > > +         }
> > > +         case IPPROTO_AH: {
> > > +                 struct ip_auth_hdr *hp = (void *)(skb->data + off);
> > > +
> > > +                 header_size = skb->network_header +
> > > +                         off +
> > > +                         sizeof(struct ip_auth_hdr);
> > > +                 maybe_pull_tail(skb, header_size);
> > > +
> > > +                 nexthdr = hp->nexthdr;
> > > +                 off += (hp->hdrlen+2)<<2;
> > > +                 break;
> > > +         }
> > > +         case IPPROTO_TCP:
> > > +         case IPPROTO_UDP:
> > > +                 found = true;
> > > +                 /* Fall through */
> > > +         default:
> > > +                 done = true;
> > > +                 break;
> >
> > The above 'switch' doesn't seem to cover all IPPROTO_*. Will it cause
> > the loop to exit too early? In other words, does any IPPROTO_* not
> > listed above marks the end of parsing?
> >
> 
> AFAIK, hitting anything not in that switch would mean we don't have a TCP or
> UDP packet. I think the original code structure made that clearer so I'm going
> to go back to that.
> 

Looking at this again I realize I should not parse the fragment header; there's 
no way anyone should be sending a fragment with partial checksum.

  Paul

> > [...]
> > >   unsigned long now = jiffies;
> > > @@ -1428,12 +1598,7 @@ static int xenvif_tx_submit(struct xenvif *vif,
> int
> > budget)
> > >
> > >           xenvif_fill_frags(vif, skb);
> > >
> > > -         /*
> > > -          * If the initial fragment was < PKT_PROT_LEN then
> > > -          * pull through some bytes from the other fragments to
> > > -          * increase the linear region to PKT_PROT_LEN bytes.
> > > -          */
> > > -         if (skb_headlen(skb) < PKT_PROT_LEN &&
> > skb_is_nonlinear(skb)) {
> > > +         if (skb_is_nonlinear(skb) && skb_headlen(skb) <
> > PKT_PROT_LEN) {
> >
> > This change is not necessary: the comment is useful, and swapping two
> > operands of && doesn't have any effect.
> 
> Sorry, that was too much cutting and pasting around. I'll just ditch that 
> hunk.
> 
>   Paul
> 
> >
> > Wei.
> 
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