netdev - Re: [PATCH v3] net/packet: support mergeable feature of virtio

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Date:   Thu, 09 Mar 2023 10:18:23 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     沈安琪(凛玥) <amy.saq@...group.com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        netdev@...r.kernel.org
Cc:     mst@...hat.com, davem@...emloft.net, jasowang@...hat.com,
        谈鉴锋 <henry.tjf@...group.com>
Subject: Re: [PATCH v3] net/packet: support mergeable feature of virtio

沈安琪(凛玥) wrote:
> 
> 在 2023/3/7 下午11:49, Willem de Bruijn 写道:
> > 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 mergeable 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.
> >>
> >> Besides, has_vnet_hdr field in struct packet_sock is removed since all
> >> the information it provides is covered by vnet_hdr_sz field: a packet
> >> socket has a vnet header if and only if its vnet_hdr_sz is not zero.
> >>
> >> 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>
> >> ---
> >> diff --git a/net/packet/internal.h b/net/packet/internal.h
> >> index 48af35b..9b52d93 100644
> >> --- a/net/packet/internal.h
> >> +++ b/net/packet/internal.h
> >> @@ -119,9 +119,9 @@ struct packet_sock {
> >>   	unsigned int		running;	/* bind_lock must be held */
> >>   	unsigned int		auxdata:1,	/* writer must hold sock lock */
> >>   				origdev:1,
> >> -				has_vnet_hdr:1,
> >>   				tp_loss:1,
> >> -				tp_tx_has_off:1;
> >> +				tp_tx_has_off:1,
> >> +				vnet_hdr_sz:8;
> > just a separate u8 variable , rather than 8 bits in a u32.
> >
> >>   	int			pressure;
> >>   	int			ifindex;	/* bound device		*/
> 
> 
> We plan to add
> 
> +	   u8	vnet_hdr_sz:8;
> 
> here.
> Is this a proper place to add this field to make sure the cacheline will not be broken?

When in doubt, use pahole (`pahole -C packet_sock net/packet/af_packet.o`).

There currently is a 27-bit hole before pressure. That would be a good spot. 
> 
> >>   	__be16			num;
> >> -- 
> >> 1.8.3.1
> >>