netdev - Re: [PATCH net-next] hv_netvsc: Add support to set MTU reservation from guest side

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Date:	Fri, 03 Jul 2015 09:17:29 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	dan.carpenter@...cle.com
Cc:	haiyangz@...rosoft.com, netdev@...r.kernel.org, olaf@...fle.de,
	jasowang@...hat.com, driverdev-devel@...uxdriverproject.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next] hv_netvsc: Add support to set MTU reservation
 from guest side

From: Dan Carpenter <dan.carpenter@...cle.com>
Date: Fri, 3 Jul 2015 14:28:47 +0300

> 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@...rosoft.com>
>> Reviewed-by: K. Y. Srinivasan <kys@...rosoft.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.

Agreed.
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