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

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Date:   Thu, 27 Feb 2020 14:37:25 -0700
From:   David Ahern <dahern@...italocean.com>
To:     Michael Chan <michael.chan@...adcom.com>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Andy Gospodarek <andy@...yhouse.net>
Subject: Re: virtio_net: can change MTU after installing program

On 2/27/20 12:26 PM, Michael Chan wrote:
> 
> The Broadcom bnxt_en driver should not allow the MTU to be changed to
> an invalid value after an XDP program is attached.  We set the
> netdev->max_mtu to a smaller value and dev_validate_mtu() should
> reject MTUs that are not supported in XDP mode.
> 

That's what I proposed for virtio_net. Simplest option and since max mtu
is now dumped with device details very easy for a user to determine why
a particular MTU fails.