netdev - Re: [net PATCH 1/4] virtio_net: disable XDP_REDIRECT in receive

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Message-ID: <71fc11a2-ea95-766c-0d7a-2433abd9dcbc@redhat.com>
Date:   Tue, 27 Feb 2018 10:28:58 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Daniel Borkmann <borkmann@...earbox.net>,
        "David S. Miller" <davem@...emloft.net>,
        Tariq Toukan <tariqt@...lanox.com>
Subject: Re: [net PATCH 1/4] virtio_net: disable XDP_REDIRECT in
 receive_mergeable() case



On 2018年02月27日 10:25, Jason Wang wrote:
>
>
> On 2018年02月27日 08:40, Michael S. Tsirkin wrote:
>>> IMHO we should consider NOT supporting XDP in receive_mergeable() at
>>> all, because the principles behind XDP are to gain speed by (1) code
>>> simplicity, (2) sacrificing memory and (3) where possible moving
>>> runtime checks to setup time.  These principles are clearly being
>>> violated in receive_mergeable(), that e.g. runtime track average
>>> buffer size to save memory consumption.
>>>
>>> In the longer run, we should consider introducing a separate receive
>>> function when attaching an XDP program, and also change the memory
>>> model to be compatible with XDP when attaching an XDP prog.
>> I agree with a separate function approach.
>>
>> So each buffer is tagged as xdp/non xdp, we check that
>> and handle appropriately - where non xdp could be handled
>> by the generic path.
>
> If we want to have separated function, should we do it for all XDP 
> capable drivers instead of virtio-net only?
>
> Thanks
>

What's more, since we don't stop device during XDP set. Even if a buffer 
is tagged as xdp/non xdp during refill, we still can't make sure whether 
or not XDP was used at receive function. So we can not handle them 
separately.

Thanks