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

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Message-ID: <ec1185ac-a2a1-e9d9-c116-ab42483c3b85@digitalocean.com>
Date:   Wed, 26 Feb 2020 09:58:00 -0700
From:   David Ahern <dahern@...italocean.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: virtio_net: can change MTU after installing program

On 2/26/20 9:55 AM, Michael S. Tsirkin wrote:
> 
> OK that seems to indicate an ndo callback as a reasonable way
> to handle this. Right? The only problem is this might break
> guests if they happen to reverse the order of
> operations:
> 	1. set mtu
> 	2. detach xdp prog
> would previously work fine, and would now give an error.

That order should not work at all. You should not be allowed to change
the MTU size that exceeds XDP limits while an XDP program is attached.

> 
> If we want to make it transparent for userspace,
> I guess we can defer the actual update until xdp prog is detached.
> Sound ugly and might still confuse some userspace ... worth it?
>