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SubjectRe: [PATCH net-next] hv_netvsc: Add support to set MTU reservation from guest side
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From: Dan Carpenter <dan.carpenter@oracle.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@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.

Agreed.


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