> Lorenzo Bianconi <lorenzo@xxxxxxxxxx> writes:
>
> > Introduce veth_convert_xdp_buff_from_skb routine in order to
> > convert a non-linear skb into a xdp buffer. If the received skb
> > is cloned or shared, veth_convert_xdp_buff_from_skb will copy it
> > in a new skb composed by order-0 pages for the linear and the
> > fragmented area. Moreover veth_convert_xdp_buff_from_skb guarantees
> > we have enough headroom for xdp.
> > This is a preliminary patch to allow attaching xdp programs with frags
> > support on veth devices.
> >
> > Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
>
> It's cool that we can do this! A few comments below:
Hi Toke,
thx for the review :)
[...]
> > +static int veth_convert_xdp_buff_from_skb(struct veth_rq *rq,
> > + struct xdp_buff *xdp,
> > + struct sk_buff **pskb)
> > +{
>
> nit: It's not really "converting" and skb into an xdp_buff, since the
> xdp_buff lives on the stack; so maybe 'veth_init_xdp_buff_from_skb()'?
I kept the previous naming convention used for xdp_convert_frame_to_buff()
(my goal would be to move it in xdp.c and reuse this routine for the
generic-xdp use case) but I am fine with veth_init_xdp_buff_from_skb().
>
> > + struct sk_buff *skb = *pskb;
> > + u32 frame_sz;
> >
> > if (skb_shared(skb) || skb_head_is_locked(skb) ||
> > - skb_is_nonlinear(skb) || headroom < XDP_PACKET_HEADROOM) {
> > + skb_shinfo(skb)->nr_frags) {
>
> So this always clones the skb if it has frags? Is that really needed?
if we look at skb_cow_data(), paged area is always considered not writable
>
> Also, there's a lot of memory allocation and copying going on here; have
> you measured the performance?
even in the previous implementation we always reallocate the skb if the
conditions above are verified so I do not expect any difference in the single
buffer use-case but I will run some performance tests.
>
[...]
> > +
> > + if (xdp_buff_has_frags(&xdp))
> > + skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > + else
> > + skb->data_len = 0;
>
> We can remove entire frags using xdp_adjust_tail, right? Will that get
> propagated in the right way to the skb frags due to the dual use of
> skb_shared_info, or?
bpf_xdp_frags_shrink_tail() can remove entire frags and it will modify metadata
contained in the skb_shared_info (e.g. nr_frags or the frag size of
the given page). We should consider the data_len field in this case. Agree?
Regards,
Lorenzo
>
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