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

[Haiyang Zhang]

> -----Original Message-----
> From: David Miller [mailto:davem@xxxxxxxxxxxxx]
> Sent: Friday, July 3, 2015 12:17 PM
> To: dan.carpenter@xxxxxxxxxx
> Cc: Haiyang Zhang; netdev@xxxxxxxxxxxxxxx; olaf@xxxxxxxxx;
> jasowang@xxxxxxxxxx; driverdev-devel@xxxxxxxxxxxxxxxxxxxxxx; linux-
> kernel@xxxxxxxxxxxxxxx
> Subject: Re: [PATCH net-next] hv_netvsc: Add support to set MTU
> reservation from guest side
> 
> From: Dan Carpenter <dan.carpenter@xxxxxxxxxx>
> 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@xxxxxxxxxxxxx>
> >> Reviewed-by: K. Y. Srinivasan <kys@xxxxxxxxxxxxx>
> >> ---
> >>  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.

The number 68 is same as the common code in eth_change_mtu(). I think it
came from the protocol requirement:
https://en.wikipedia.org/wiki/Maximum_transmission_unit

Sure, I will put the number in a define, and resubmit the patch.

Thanks,
- Haiyang


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