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Date:   Wed, 19 Apr 2023 09:59:40 -0400
From:   Willem de Bruijn <willemdebruijn.kernel@gmail.com>
To:     =?UTF-8?B?5rKI5a6J55CqKOWHm+eOpSk=?= <amy.saq@antgroup.com>,
        linux-kernel@vger.kernel.org
Cc:     =?UTF-8?B?6LCI6Ym06ZSL?= <henry.tjf@antgroup.com>,
        =?UTF-8?B?5rKI5a6J55CqKOWHm+eOpSk=?= <amy.saq@antgroup.com>,
        Willem de Bruijn <willemdebruijn.kernel@gmail.com>,
        "David S. Miller" <davem@davemloft.net>,
        Eric Dumazet <edumazet@google.com>,
        Jakub Kicinski <kuba@kernel.org>,
        Paolo Abeni <pabeni@redhat.com>, netdev@vger.kernel.org
Message-ID: <643ff3cca2eb3_38347529476@willemb.c.googlers.com.notmuch>
In-Reply-To: <20230419072420.315079-1-amy.saq@antgroup.com>
References: <20230419072420.315079-1-amy.saq@antgroup.com>
Subject: RE: [PATCH v9] net/packet: support mergeable feature of virtio
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=E6=B2=88=E5=AE=89=E7=90=AA(=E5=87=9B=E7=8E=A5) wrote:
> From: Jianfeng Tan <henry.tjf@antgroup.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 no=
t
> 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: packet=
s
> 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 t=
o
> invalid ether head checking in later under-layer device packet receivin=
g.
> =

> By adding extra field vnet_hdr_sz with utilizing holes in struct
> packet_sock to record currently used virtio net header size and support=
ing
> 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@antgroup.com>
> Co-developed-by: Anqi Shen <amy.saq@antgroup.com>
> Signed-off-by: Anqi Shen <amy.saq@antgroup.com>

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