Subject: Re: [PATCH 2/2] net/packet: send and receive pkt with given vnet_hdr_sz

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 22 Feb 2023 06:37:59 -0500

On Wed, Feb 22, 2023 at 04:04:34PM +0800, 沈安琪(凛玥) wrote:
> 
> 在 2023/2/21 下午11:03, Willem de Bruijn 写道:
> > 沈安琪(凛玥) wrote:
> > > 在 2023/2/14 下午10:28, Willem de Bruijn 写道:
> > > > 沈安琪(凛玥) wrote:
> > > > > 在 2023/2/10 下午11:39, Willem de Bruijn 写道:
> > > > > > Michael S. Tsirkin wrote:
> > > > > > > On Fri, Feb 10, 2023 at 12:01:03PM +0800, 沈安琪(凛玥) wrote:
> > > > > > > > 在 2023/2/9 下午9:07, Michael S. Tsirkin 写道:
> > > > > > > > > On Thu, Feb 09, 2023 at 08:43:15PM +0800, 沈安琪(凛玥) wrote:
> > > > > > > > > > From: "Jianfeng Tan" <[email-masked]>
> > > > > > > > > > 
> > > > > > > > > > When raw socket is used as the backend for kernel vhost, currently it
> > > > > > > > > > will regard the virtio net header as 10-byte, which is not always the
> > > > > > > > > > case since some virtio features need virtio net header other than
> > > > > > > > > > 10-byte, such as mrg_rxbuf and VERSION_1 that both need 12-byte virtio
> > > > > > > > > > net header.
> > > > > > > > > > 
> > > > > > > > > > Instead of hardcoding virtio net header length to 10 bytes, tpacket_snd,
> > > > > > > > > > tpacket_rcv, packet_snd and packet_recvmsg now get the virtio net header
> > > > > > > > > > size that is recorded in packet_sock to indicate the exact virtio net
> > > > > > > > > > header size that virtio user actually prepares in the packets. By doing
> > > > > > > > > > so, it can fix the issue of incorrect mac header parsing when these
> > > > > > > > > > virtio features that need virtio net header other than 10-byte are
> > > > > > > > > > enable.
> > > > > > > > > > 
> > > > > > > > > > Signed-off-by: Jianfeng Tan <[email-masked]>
> > > > > > > > > > Co-developed-by: Anqi Shen <[email-masked]>
> > > > > > > > > > Signed-off-by: Anqi Shen <[email-masked]>
> > > > > > > > > Does it handle VERSION_1 though? That one is also LE.
> > > > > > > > > Would it be better to pass a features bitmap instead?
> > > > > > > > Thanks for quick reply!
> > > > > > > > 
> > > > > > > > I am a little confused abot what "LE" presents here?
> > > > > > > LE == little_endian.
> > > > > > > Little endian format.
> > > > > > > 
> > > > > > > > For passing a features bitmap to af_packet here, our consideration is
> > > > > > > > whether it will be too complicated for af_packet to understand the virtio
> > > > > > > > features bitmap in order to get the vnet header size. For now, all the
> > > > > > > > virtio features stuff is handled by vhost worker and af_packet actually does
> > > > > > > > not need to know much about virtio features. Would it be better if we keep
> > > > > > > > the virtio feature stuff in user-level and let user-level tell af_packet how
> > > > > > > > much space it should reserve?
> > > > > > > Presumably, we'd add an API in include/linux/virtio_net.h ?
> > > > > > Better leave this opaque to packet sockets if they won't act on this
> > > > > > type info.
> > > > > > This patch series probably should be a single patch btw. As else the
> > > > > > socket option introduced in the first is broken at that commit, since
> > > > > > the behavior is only introduced in patch 2.
> > > > > Good point, will merge this patch series into one patch.
> > > > > 
> > > > > 
> > > > > Thanks for Michael's enlightening advice, we plan to modify current UAPI
> > > > > change of adding an extra socketopt from only setting vnet header size
> > > > > only to setting a bit-map of virtio features, and implement another
> > > > > helper function in include/linux/virtio_net.h to parse the feature
> > > > > bit-map. In this case, packet sockets have no need to understand the
> > > > > feature bit-map but only pass this bit-map to virtio_net helper and get
> > > > > back the information, such as vnet header size, it needs.
> > > > > 
> > > > > This change will make the new UAPI more general and avoid further
> > > > > modification if there are more virtio features to support in the future.
> > > > > 
> > > > Please also comment how these UAPI extension are intended to be used.
> > > > As that use is not included in this initial patch series.
> > > > 
> > > > If the only intended user is vhost-net, we can consider not exposing
> > > > outside the kernel at all. That makes it easier to iterate if
> > > > necessary (no stable ABI) and avoids accidentally opening up new
> > > > avenues for bugs and exploits (syzkaller has a history with
> > > > virtio_net_header options).
> > > 
> > > Our concern is, it seems there is no other solution than uapi to let
> > > packet sockets know the vnet header size they should use.
> > > 
> > > Receiving packets in vhost driver, implemented in drivers/vhost/net.c:
> > > 1109 handle_rx(), will abstract the backend device it uses and directly
> > > invoke the corresponding socket ops with no extra information indicating
> > > it is invoked by vhost worker. Vhost worker actually does not know the
> > > type of backend device it is using; only virito-user knows what type of
> > > backend device it uses. Therefore, it seems impossible to let vhost set
> > > the vnet header information to the target backend device.
> > > 
> > > Tap, another kind of backend device vhost may use, lets virtio-user set
> > > whether it needs vnet header and how long the vnet header is through
> > > ioctl. (implemented in drivers/net/tap.c:1066)
> > > 
> > > In this case, we wonder whether we should align with what tap does and
> > > set vnet hdr size through setsockopt for packet_sockets.
> > > 
> > > We really appreciate suggestions on if any, potential approachs to pass
> > > this vnet header size information from virtio-user to packet-socket.
> > You're right. This is configured from userspace before the FD is passed
> > to vhost-net, so indeed this will require packet socket UAPI support.
> 
> 
> Thanks for quick reply. We will go with adding an extra UAPI here then.
> 
> 
> Another discussion for designing this UAPI is, whether it will be better to
> support setting only vnet header size, just like what TAP does in its ioctl,
> or to support setting a virtio feature bit-map.
> 
> 
> UAPI setting only vnet header size
> 
> Pros:
> 
> 1. It aligns with how other virito backend devices communicate with
> virtio-user
> 
> 2. We can use the holes in struct packet_socket (net/packet/internal.h:120)
> to record the extra information since the size info only takes 8 bits.
> 
> Cons:
> 
> 1. It may have more information that virtio-user needs to communicate with
> packet socket in the future and needs to add more UAPI supports here.
> 
> To Michael: Is there any other information that backend device needs and
> will be given from virtio-user?

Yes e.g. I already mentioned virtio 1.0 wrt LE versus native endian
format.

> 
> UAPI setting a virtio feature bit-map
> 
> Pros:
> 
> 1. It is more general and may reduce future UAPI changes.
> 
> Cons:
> 
> 1. A virtio feature bit-map needs 64 bits, which needs to add an extra field
> in packet_sock struct
> 
> 2. Virtio-user needs to aware that using packet socket as backend supports
> different approach to negotiate the vnet header size.
> 
> 
> We really appreciate any suggestion or discussion on this design choice of
> UAPI.

In the end it's ok with just size too, you just probably shouldn't say
you support VERSION_1 if you are not passing that bit.

-- 
MST