netdev - Re: [net PATCH] net: virtio: cap mtu when XDP programs are running

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Date:   Tue, 10 Jan 2017 01:05:57 +0200
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     John Fastabend <john.fastabend@...il.com>,
        john.r.fastabend@...el.com, netdev@...r.kernel.org,
        alexei.starovoitov@...il.com, daniel@...earbox.net
Subject: Re: [net PATCH] net: virtio: cap mtu when XDP programs are running

On Thu, Jan 05, 2017 at 11:09:14AM +0800, Jason Wang wrote:
> 
> 
> On 2017年01月05日 02:57, John Fastabend wrote:
> > [...]
> > 
> > > On 2017年01月04日 00:48, John Fastabend wrote:
> > > > On 17-01-02 10:14 PM, Jason Wang wrote:
> > > > > On 2017年01月03日 06:30, John Fastabend wrote:
> > > > > > XDP programs can not consume multiple pages so we cap the MTU to
> > > > > > avoid this case. Virtio-net however only checks the MTU at XDP
> > > > > > program load and does not block MTU changes after the program
> > > > > > has loaded.
> > > > > > 
> > > > > > This patch sets/clears the max_mtu value at XDP load/unload time.
> > > > > > 
> > > > > > Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
> > > > > > ---
> > [...]
> > 
> > > > OK so this logic is a bit too simply. When it resets the max_mtu I guess it
> > > > needs to read the mtu via
> > > > 
> > > >      virtio_cread16(vdev, ...)
> > > > 
> > > > or we may break the negotiated mtu.
> > > Yes, this is a problem (even use ETH_MAX_MTU). We may need a method to notify
> > > the device about the mtu in this case which is not supported by virtio now.
> > Note this is not really a XDP specific problem. The guest can change the MTU
> > after init time even without XDP which I assume should ideally result in a
> > notification if the MTU is negotiated.
> 
> Yes, Michael, do you think we need add some mechanism to notify host about
> MTU change in this case?
> 
> Thanks

Why does host care?