Re: [PATCH v1] add mtu set in virtio

[Stephen Hemminger <stephen@networkplumber.org>]

On Fri, 26 Aug 2016 20:54:28 -0400
souvikdey33 <sodey@sonusnet.com> wrote:

> This functionality is required mostly in the cloud infrastructure.
> For example, if we use gre or vxlan network between compute and
> controller, then we should not use 1500 mtu in the guest as with
> encapsulation the sixe of the packet will be more and will get 
> dropped in the infrastructure. So, in that case we should honor
> the mtu size sent by the dhcp server and configure the same on 
> the virtual interfaces in the guest. This will also keep a 
> consistent mtu through out the infrastructure.
> 
> souvikdey33 (1):
>   Signed-off-by: Souvik Dey <sodey@sonusnet.com>
> 
>  drivers/net/virtio/virtio_ethdev.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 

Thanks for the patch, it is a good step forward but it looks
like more code is needed to do this safely. At a minimum,
need to check that MTU is not greater than VIRTIO_MAX_RX_PKTLEN.
And error return should be negative errno not -1.

Something like:
   if (mtu < VIRTIO_MIN_MTU || mtu > VIRTIO_MAX_RX_PKTLEN)
	return -EINVAL;

Looking at Linux driver, it allows MTU of up to 64K, yet DPDK
only allows 9728.  That should probably be fixed.