Re: vhost-net

[Jason Wang]

On Mon, Jan 16, 2023 at 4:59 PM Jonas Bonn <jonas@xxxxxxxxxxx> wrote:
>
> Hi,
>
> I have a question about using vhost-net with an IFF_TUN device.  I'm
> uncertain about the behaviour I'm seeing which is:
>
> i)  on RX, the descriptor contains no Ethernet header, which is what I
> was expecting
> ii)  on TX, the first 14 bytes of the transmitted _IP_ packet are lost;
> if I prepend an extra 14 bytes (zeroes) before the IP packet and extend
> the packet length accordingly, then things appear to work as expected.
>
> In vhost_net_build_xdp() it appears that the userspace packet data is
> copied verbatim to an XDP packet structure that assumes the presence of
> an ethernet header; as such, the IP header is copied into the ethernet
> header area.  I think this accounts for losing the first 14 bytes of the
> IP header...
>
> If I set SO_SNDBUF to something less than INT_MAX, then the XDP path is
> bypassed and transmission of IP packets works.  This means that knowing
> the value of SO_SNDBUF becomes important in the userspace application in
> order to know whether an extra 14 bytes needs to be prepended to the IP
> packet... which is awkward, at best.

It's a bug.

>
> For an IFF_TUN device, should vhost-net not be adding an implicit
> ethernet header in _build_xdp()?

Probably.

Actually, this makes me think that we should disable XDP for TUN?

> Can this be done without backward
> compatibility implications?
>

The path is used by vhost-net only, so I think we are fine.

Patch is more than welcomed.

Thanks

> Thanks,
> Jonas
>