netdev - Re: virtio_net: can change MTU after installing program

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Date:   Wed, 26 Feb 2020 02:37:01 -0500 (EST)
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     David Ahern <dahern@...italocean.com>, netdev@...r.kernel.org
Subject: Re: virtio_net: can change MTU after installing program

----- Original Message -----
> On Tue, Feb 25, 2020 at 08:32:14PM -0700, David Ahern wrote:
> > Another issue is that virtio_net checks the MTU when a program is
> > installed, but does not restrict an MTU change after:
> > 
> > # ip li sh dev eth0
> > 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 xdp qdisc fq_codel
> > state UP mode DEFAULT group default qlen 1000
> >     link/ether 5a:39:e6:01:a5:36 brd ff:ff:ff:ff:ff:ff
> >     prog/xdp id 13 tag c5595e4590d58063 jited
> > 
> > # ip li set dev eth0 mtu 8192
> > 
> > # ip li sh dev eth0
> > 2: eth0: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 8192 xdp qdisc fq_codel
> > state UP mode DEFAULT group default qlen 1000
> 
> Well the reason XDP wants to limit MTU is this:
>     the MTU must be less than a page
>     size to avoid having to handle XDP across multiple pages
> 

But even if we limit MTU is guest there's no way to limit the packet
size on host. It looks to me we need to introduce new commands to
change the backend MTU (e.g TAP) accordingly.

Thanks