netdev - Re: [PATCH net-next 0/2] virtio-net: re enable XDP

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Message-ID: <20180301101038.4253424d@redhat.com>
Date:   Thu, 1 Mar 2018 10:10:38 +0100
From:   Jesper Dangaard Brouer <brouer@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        john.fastabend@...il.com, brouer@...hat.com
Subject: Re: [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for
 mergeable buffer

On Thu,  1 Mar 2018 11:19:03 +0800
Jason Wang <jasowang@...hat.com> wrote:

> This series tries to re-enable XDP_REDIRECT for mergeable buffer which
> was removed since commit 7324f5399b06 ("virtio_net: disable
> XDP_REDIRECT in receive_mergeable() case"). Main concerns are:
> 
> - not enough tailroom was reserved which breaks cpumap

To address this at a more fundamental level, I would suggest that we/you
instead extend XDP to know it's buffers "frame" size/end.  (The
assumption use to be, xdp_buff->data_hard_start + PAGE_SIZE, but
ixgbe+virtio_net broke that assumption).

It should actually be fairly easy to implement:
 * Simply extend xdp_buff with a "data_hard_end" pointer.

Now cpumap is more safe... instead of crashing, it can now know/see when
it is safe to create an SKB using build_skb (could fallback to
dev_alloc_skb).

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Principal Kernel Engineer at Red Hat
  LinkedIn: http://www.linkedin.com/in/brouer