Re: MTU on a virtio-net device?

[Dor Laor <dor@redhat.com>]

Michael Tokarev wrote:
> Right now (2.6.27), there's no way to change MTU of a
> virtio-net interface, since the mtu-changing method is
> not provided.  Is there a simple way to add such a
> beast?
>
It should be a nice easy patch for mtu < 4k.
You can just implement a 'change_mtu' handler like:

static int virtio_change_mtu(struct net_device *netdev, int new_mtu)
{
        if(new_mtu < ETH_ZLEN || new_mtu > PAGE_SIZE)
                return -EINVAL;
        netdev->mtu = new_mtu;
        return 0;
}

> I'm asking because I'm not familiar with the internals,
> and because, I think, increasing MTU (so that the
> resulting skb still fits in a single page) will increase
> performance significantly, at least on a internal/virtual
> network -- currently there are just way too many context
> switches and the like while copying data from one guest
> to another or between guest and host.
>
> Thanks!
>
> /mjt
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