netdev - RE: [PATCH v9] net/packet: support mergeable feature of virtio

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Date:   Wed, 19 Apr 2023 09:59:40 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     沈安琪(凛玥) <amy.saq@...group.com>,
        linux-kernel@...r.kernel.org
Cc:     谈鉴锋 <henry.tjf@...group.com>,
        沈安琪(凛玥) <amy.saq@...group.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Subject: RE: [PATCH v9] net/packet: support mergeable feature of virtio

沈安琪(凛玥) wrote:
> From: Jianfeng Tan <henry.tjf@...group.com>
> 
> Packet sockets, like tap, can be used as the backend for kernel vhost.
> In packet sockets, virtio net header size is currently hardcoded to be
> the size of struct virtio_net_hdr, which is 10 bytes; however, it is not
> always the case: some virtio features, such as mrg_rxbuf, need virtio
> net header to be 12-byte long.
> 
> Mergeable buffers, as a virtio feature, is worthy of supporting: packets
> that are larger than one-mbuf size will be dropped in vhost worker's
> handle_rx if mrg_rxbuf feature is not used, but large packets
> cannot be avoided and increasing mbuf's size is not economical.
> 
> With this virtio feature enabled by virtio-user, packet sockets with
> hardcoded 10-byte virtio net header will parse mac head incorrectly in
> packet_snd by taking the last two bytes of virtio net header as part of
> mac header.
> This incorrect mac header parsing will cause packet to be dropped due to
> invalid ether head checking in later under-layer device packet receiving.
> 
> By adding extra field vnet_hdr_sz with utilizing holes in struct
> packet_sock to record currently used virtio net header size and supporting
> extra sockopt PACKET_VNET_HDR_SZ to set specified vnet_hdr_sz, packet
> sockets can know the exact length of virtio net header that virtio user
> gives.
> In packet_snd, tpacket_snd and packet_recvmsg, instead of using
> hardcoded virtio net header size, it can get the exact vnet_hdr_sz from
> corresponding packet_sock, and parse mac header correctly based on this
> information to avoid the packets being mistakenly dropped.
> 
> Signed-off-by: Jianfeng Tan <henry.tjf@...group.com>
> Co-developed-by: Anqi Shen <amy.saq@...group.com>
> Signed-off-by: Anqi Shen <amy.saq@...group.com>

Reviewed-by: Willem de Bruijn <willemb@...gle.com>