From mboxrd@z Thu Jan  1 00:00:00 1970
From: Dan Carpenter <dan.carpenter@oracle.com>
Subject: Re: [PATCH net-next] hv_netvsc: Add support to set MTU reservation
 from guest side
Date: Fri, 3 Jul 2015 14:28:47 +0300
Message-ID: <20150703112847.GA28762@mwanda>
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Cc: olaf@aepfle.de, netdev@vger.kernel.org, jasowang@redhat.com,
 driverdev-devel@linuxdriverproject.org, linux-kernel@vger.kernel.org,
 davem@davemloft.net
To: Haiyang Zhang <haiyangz@microsoft.com>
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On Thu, Jul 02, 2015 at 01:17:35PM -0700, Haiyang Zhang wrote:
> When packet encapsulation is in use, the MTU needs to be reduced for
> headroom reservation.
> The existing code takes the updated MTU value only from the host side.
> But vSwitch extensions, such as Open vSwitch, require the flexibility
> to change the MTU to different values from within a guest during the
> lifecycle of a vNIC, when the encapsulation protocol is changed. The
> patch supports this kind of MTU changes.
> 
> Signed-off-by: Haiyang Zhang <haiyangz@microsoft.com>
> Reviewed-by: K. Y. Srinivasan <kys@microsoft.com>
> ---
>  drivers/net/hyperv/netvsc_drv.c   |    3 +--
>  drivers/net/hyperv/rndis_filter.c |    2 +-
>  2 files changed, 2 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
> index 358475e..68e7ece 100644
> --- a/drivers/net/hyperv/netvsc_drv.c
> +++ b/drivers/net/hyperv/netvsc_drv.c
> @@ -743,8 +743,7 @@ static int netvsc_change_mtu(struct net_device *ndev, int mtu)
>  	if (nvdev->nvsp_version >= NVSP_PROTOCOL_VERSION_2)
>  		limit = NETVSC_MTU - ETH_HLEN;
>  
> -	/* Hyper-V hosts don't support MTU < ETH_DATA_LEN (1500) */
> -	if (mtu < ETH_DATA_LEN || mtu > limit)
> +	if (mtu < 68 || mtu > limit)

How did you calculate 68?  Avoid magic numbers like this, make it a
define.

regards,
dan carpenter