>
>
> On 14/12/2023 15.29, Lorenzo Bianconi wrote:
> > Similar to native xdp, do not always linearize the skb in
> > netif_receive_generic_xdp routine but create a non-linear xdp_buff to be
> > processed by the eBPF program. This allow to add multi-buffer support
> > for xdp running in generic mode.
> >
> > Signed-off-by: Lorenzo Bianconi <lorenzo@xxxxxxxxxx>
> > ---
> > net/core/dev.c | 153 +++++++++++++++++++++++++++++++++++++++++++------
> > 1 file changed, 134 insertions(+), 19 deletions(-)
> >
> > diff --git a/net/core/dev.c b/net/core/dev.c
> > index d7857de03dba..47164acc3268 100644
> > --- a/net/core/dev.c
> > +++ b/net/core/dev.c
> > @@ -4854,6 +4854,12 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> > xdp_init_buff(xdp, frame_sz, &rxqueue->xdp_rxq);
> > xdp_prepare_buff(xdp, hard_start, skb_headroom(skb) - mac_len,
> > skb_headlen(skb) + mac_len, true);
> > + if (skb_is_nonlinear(skb)) {
> > + skb_shinfo(skb)->xdp_frags_size = skb->data_len;
> > + xdp_buff_set_frags_flag(xdp);
> > + } else {
> > + xdp_buff_clear_frags_flag(xdp);
> > + }
> > orig_data_end = xdp->data_end;
> > orig_data = xdp->data;
> > @@ -4883,6 +4889,14 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> > skb->len += off; /* positive on grow, negative on shrink */
> > }
> > + /* XDP frag metadata (e.g. nr_frags) are updated in eBPF helpers
> > + * (e.g. bpf_xdp_adjust_tail), we need to update data_len here.
> > + */
> > + if (xdp_buff_has_frags(xdp))
> > + skb->data_len = skb_shinfo(skb)->xdp_frags_size;
> > + else
> > + skb->data_len = 0;
> > +
> > /* check if XDP changed eth hdr such SKB needs update */
> > eth = (struct ethhdr *)xdp->data;
> > if ((orig_eth_type != eth->h_proto) ||
> > @@ -4916,12 +4930,118 @@ u32 bpf_prog_run_generic_xdp(struct sk_buff *skb, struct xdp_buff *xdp,
> > return act;
> > }
> > +static int netif_skb_segment_for_xdp(struct sk_buff **pskb,
>
> This function "...segment_for_xdp" always reallocate SKB and copies all
> bits over.
> Should it have been named "skb_realloc_for_xdp" ?
Hi Jesper,
ack, naming is always hard :)
>
> I was really hopeing we can find a design to avoid doing this realloc.
>
> If the BPF-prog doesn't write into any of the fragments, then we can
> avoid this realloc (+copy) dance. We designed XDP multi-buff to have
> exactly the same layout+location as SKB in skb_shared_info, exactly to
> avoid having to reallocated.
I 100% agree with you, but we will need a similar copy fallback approach
anyway, right? It is just a matter to understand if we should implement it
with page_pool or page_frag_cache (or something different).
>
> More comments inline below...
>
> > + struct bpf_prog *prog)
> > +{
> > +#if IS_ENABLED(CONFIG_PAGE_POOL)
> > + struct softnet_data *sd = this_cpu_ptr(&softnet_data);
> > + u32 size, truesize, len, max_head_size, off;
> > + struct sk_buff *skb = *pskb, *nskb;
> > + int err, i, head_off;
> > + void *data;
> > +
> > + /* XDP does not support fraglist so we need to linearize
> > + * the skb.
> > + */
> > + if (skb_has_frag_list(skb) || !prog->aux->xdp_has_frags)
> > + return -EOPNOTSUPP;
> > +
> > + max_head_size = SKB_WITH_OVERHEAD(PAGE_SIZE - XDP_PACKET_HEADROOM);
> > + if (skb->len > max_head_size + MAX_SKB_FRAGS * PAGE_SIZE)
> > + return -ENOMEM;
> > +
> > + size = min_t(u32, skb->len, max_head_size);
> > + truesize = SKB_HEAD_ALIGN(size) + XDP_PACKET_HEADROOM;
> > + data = page_pool_dev_alloc_va(sd->page_pool, &truesize);
> > + if (!data)
> > + return -ENOMEM;
> > +
> > + nskb = napi_build_skb(data, truesize);
> > + if (!nskb) {
> > + page_pool_free_va(sd->page_pool, data, true);
> > + return -ENOMEM;
> > + }
> > +
> > + skb_reserve(nskb, XDP_PACKET_HEADROOM);
> > + skb_copy_header(nskb, skb);
> > + skb_mark_for_recycle(nskb);
> > +
> > + err = skb_copy_bits(skb, 0, nskb->data, size);
>
> This will likely copy part of the "frags" into the SKB "head" area.
>
> Especially for netstack generated TCP packets, this will change the
> segmentation layout significantly. I wonder what (performance) effects
> this will have on further handling of these SKBs.
Do you think it can be a problem? I think in this way we can reduce the
number of allocated page. Moreover, what about the case when skb head is
bigger than a single page? Do you think we should use bigger allocation order?
>
>
>
> > + if (err) {
> > + consume_skb(nskb);
> > + return err;
> > + }
> > + skb_put(nskb, size);
> > +
> > + head_off = skb_headroom(nskb) - skb_headroom(skb);
> > + skb_headers_offset_update(nskb, head_off);
> > +
> > + off = size;
> > + len = skb->len - off;
> > + for (i = 0; i < MAX_SKB_FRAGS && off < skb->len; i++) {
> > + struct page *page;
> > + u32 page_off;
> > +
> > + size = min_t(u32, len, PAGE_SIZE);
> > + truesize = size;
> > +
> > + page = page_pool_dev_alloc(sd->page_pool, &page_off,
> > + &truesize);
> > + if (!data) {
> > + consume_skb(nskb);
> > + return -ENOMEM;
> > + }
> > +
> > + skb_add_rx_frag(nskb, i, page, page_off, size, truesize);
> > + err = skb_copy_bits(skb, off, page_address(page) + page_off,
> > + size);
>
> I think it is correct, but we can easily endup with the new SKB (nskb)
> having a different nskb->nr_frags.
see above
>
>
> > + if (err) {
> > + consume_skb(nskb);
> > + return err;
> > + }
> > +
> > + len -= size;
> > + off += size;
> > + }
> > +
> > + consume_skb(skb);
> > + *pskb = nskb;
> > +
> > + return 0;
> > +#else
> > + return -EOPNOTSUPP;
> > +#endif
> > +}
> > +
> > +static int netif_skb_check_for_xdp(struct sk_buff **pskb,
> > + struct bpf_prog *prog)
> > +{
> > + struct sk_buff *skb = *pskb;
> > + int err, hroom, troom;
> > +
> > + if (!netif_skb_segment_for_xdp(pskb, prog))
> > + return 0;
>
> IMHO the code call logic, does not make it easy to add cases where we
> can avoid the realloc. With this patch, it feels like the realloc+copy
> code path is the "main" code path for XDP-generic.
ack, I think it would depend about the logic to avoid realloc+copy. We could
revaluate it when we have this logic in place. What do you think?
>
> Our goal should be to avoid realloc.
>
> My goal for XDP multi-buff was/is that it can co-exist with GSO/GRO
> packets. This patchset is a step in the direction of enabling GRO on
> devices with XDP (generic) loaded. And I was really excited about this,
> but the overhead is going to be massive compared to normal GRO (without
> realloc+copy) that XDP end-users are going to be disappointed.
>
>
> > +
> > + /* In case we have to go down the path and also linearize,
> > + * then lets do the pskb_expand_head() work just once here.
> > + */
> > + hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
> > + troom = skb->tail + skb->data_len - skb->end;
> > + err = pskb_expand_head(skb,
> > + hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
> > + troom > 0 ? troom + 128 : 0, GFP_ATOMIC);
> > + if (err)
> > + return err;
> > +
> > + return skb_linearize(skb);
> > +}
> > +
> > static u32 netif_receive_generic_xdp(struct sk_buff **pskb,
> > struct xdp_buff *xdp,
> > struct bpf_prog *xdp_prog)
> > {
> > struct sk_buff *skb = *pskb;
> > - u32 act = XDP_DROP;
> > + u32 mac_len, act = XDP_DROP;
> > /* Reinjected packets coming from act_mirred or similar should
> > * not get XDP generic processing.
> > @@ -4929,41 +5049,36 @@ static u32 netif_receive_generic_xdp(struct sk_buff **pskb,
> > if (skb_is_redirected(skb))
> > return XDP_PASS;
> > - /* XDP packets must be linear and must have sufficient headroom
> > - * of XDP_PACKET_HEADROOM bytes. This is the guarantee that also
> > - * native XDP provides, thus we need to do it here as well.
> > + /* XDP packets must have sufficient headroom of XDP_PACKET_HEADROOM
> > + * bytes. This is the guarantee that also native XDP provides,
> > + * thus we need to do it here as well.
>
> Some "native" XDP provider only have 192 bytes as HEADROOM and XDP code
> can this not being static (256 bytes). So, perhaps it is time to allow
> XDP generic to only require 192 bytes?
ack, agree. I think this can be added a separated patch.
Regards,
Lorenzo
>
> > */
> > + mac_len = skb->data - skb_mac_header(skb);
> > + __skb_push(skb, mac_len);
> > +
> > if (skb_cloned(skb) || skb_is_nonlinear(skb) ||
> > skb_headroom(skb) < XDP_PACKET_HEADROOM) {
> > - int hroom = XDP_PACKET_HEADROOM - skb_headroom(skb);
> > - int troom = skb->tail + skb->data_len - skb->end;
> > -
> > - /* In case we have to go down the path and also linearize,
> > - * then lets do the pskb_expand_head() work just once here.
> > - */
> > - if (pskb_expand_head(skb,
> > - hroom > 0 ? ALIGN(hroom, NET_SKB_PAD) : 0,
> > - troom > 0 ? troom + 128 : 0, GFP_ATOMIC))
> > - goto do_drop;
> > - if (skb_linearize(skb))
> > + if (netif_skb_check_for_xdp(pskb, xdp_prog))
> > goto do_drop;
> > }
> > - act = bpf_prog_run_generic_xdp(skb, xdp, xdp_prog);
> > + __skb_pull(*pskb, mac_len);
> > +
> > + act = bpf_prog_run_generic_xdp(*pskb, xdp, xdp_prog);
> > switch (act) {
> > case XDP_REDIRECT:
> > case XDP_TX:
> > case XDP_PASS:
> > break;
> > default:
> > - bpf_warn_invalid_xdp_action(skb->dev, xdp_prog, act);
> > + bpf_warn_invalid_xdp_action((*pskb)->dev, xdp_prog, act);
> > fallthrough;
> > case XDP_ABORTED:
> > - trace_xdp_exception(skb->dev, xdp_prog, act);
> > + trace_xdp_exception((*pskb)->dev, xdp_prog, act);
> > fallthrough;
> > case XDP_DROP:
> > do_drop:
> > - kfree_skb(skb);
> > + kfree_skb(*pskb);
> > break;
> > }
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