Date:   Fri, 2 Mar 2018 12:17:44 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     mst@...hat.com, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        john.fastabend@...il.com,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH net-next 0/2] virtio-net: re enable XDP_REDIRECT for
 mergeable buffer



On 2018年03月01日 22:16, Jesper Dangaard Brouer wrote:
> On Thu, 1 Mar 2018 21:15:36 +0800
> Jason Wang <jasowang@...hat.com> wrote:
>
>> On 2018年03月01日 18:35, Jesper Dangaard Brouer wrote:
>>> On Thu, 1 Mar 2018 17:23:37 +0800
>>> Jason Wang <jasowang@...hat.com> wrote:
>>>   
>>>> On 2018年03月01日 17:10, Jesper Dangaard Brouer wrote:
>>>>> 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.
>>>> Right, and then cpumap can warn and drop packets with insufficient
>>>> tailroom.
>>>>
>>>> But it should be a patch on top of this I think.
>>> Hmmm, not really.  If we/you instead fix the issue of XDP doesn't know
>>> the end/size of the frame, then we don't need this mixed XDP
>>> generic/native code path mixing.
>> I know this but I'm still a little bit confused. According to the commit
>> log of 7324f5399b06 ("virtio_net: disable XDP_REDIRECT in
>> receive_mergeable() case"), you said:
>>
>> """
>>       The longer explaination is that receive_mergeable() tries to
>>       work-around and satisfy these XDP requiresments e.g. by having a
>>       function xdp_linearize_page() that allocates and memcpy RX buffers
>>       around (in case packet is scattered across multiple rx buffers).  This
>>       does currently satisfy XDP_PASS, XDP_DROP and XDP_TX (but only because
>>       we have not implemented bpf_xdp_adjust_tail yet).
>> """
>>
>> So I consider the tailroom is a must for the (future) tail adjustment.
> That is true, BUT implementing the "data_hard_end" extension is a
> pre-requisite.  It will also be to catch the issue of too little
> tail-room if/when implementing bpf_xdp_adjust_tail().
>
> It is of-cause a "nice-to-have", to fix this virtio_net driver's
> receive_mergeable() call to have enough tail-room, but I don't see it
> as a solution to the fundamental problem.
>
>
>>> You could re-enable native redirect, and push the responsibility to
>>> cpumap for detecting this too-small frame "missing tailroom" (and avoid
>>> crashing...). (If we really want to support this, cpumap could fallback
>>> to dev_alloc_skb, and handle it gracefully).
>>>   
>> Right but it will be slower than build_skb().
> True, but bad argument in this context, as you are already using a
> similar function call napi_alloc_skb().  And it will be even slower to
> call generic-XDP code path.
>

Well, there's no generic skb implementation for cpumap redirection so I 
think we're talking about native XDP for cpumap, In this case, we won't 
even use napi_alloc_skb().

Thanks

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