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Subject: Re: [RFC PATCH net-next V2 6/6] virtio-net: support XDP rx handler
To:     Jesper Dangaard Brouer <jbrouer@redhat.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        ast@kernel.org, daniel@iogearbox.net, mst@redhat.com
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 <20180814112222.42177a24@redhat.com>
From:   Jason Wang <jasowang@redhat.com>
Message-ID: <e3e2fe66-c324-9bb2-a289-1cc8fe76147a@redhat.com>
Date:   Tue, 14 Aug 2018 21:01:10 +0800
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On 2018年08月14日 17:22, Jesper Dangaard Brouer wrote:
> On Mon, 13 Aug 2018 11:17:30 +0800
> Jason Wang<jasowang@redhat.com>  wrote:
>
>> This patch tries to add the support of XDP rx handler to
>> virtio-net. This is straight-forward, just call xdp_do_pass() and
>> behave depends on its return value.
>>
>> Test was done by using XDP_DROP (xdp1) for macvlan on top of
>> virtio-net. PPS of SKB mode was ~1.2Mpps while PPS of native XDP mode
>> was ~2.2Mpps. About 83% improvement was measured.
> I'm not convinced...
>
> Why are you not using XDP_REDIRECT, which is already implemented in
> receive_mergeable (which you modify below).
>
> The macvlan driver just need to implement ndo_xdp_xmit(), and then you
> can redirect (with XDP prog from physical driver into the guest).  It
> should be much faster...
>
>

Macvlan is different from macvtap. For host RX, macvtap deliver the 
packet to a pointer ring which could be accessed through a socket but 
macvlan deliver the packet to the normal networking stack. As an example 
of XDP rx handler, this series just try to make native XDP works for 
macvlan, macvtap path will still go for skb (but it's not hard to add it 
on top).

Consider the case of fast forwarding between host and guest. For TAP, 
XDP_REDIRECT works perfectly since from the host point of view, host RX 
is guest TX and host guest RX is host TX. But for macvtap which is based 
on macvlan, transmitting packet to macvtap/macvlan means transmitting 
packets to under layer device which is either a physical NIC or another 
macvlan device. That's why we can't use XDP_REDIRECT with ndo_xdp_xmit().

Thanks