[PATCH 2/3 Utopic LTS SRU v2] hv_netvsc: Add support to set MTU reservation from guest side

[tim.gardner at canonical.com]

From: Haiyang Zhang <haiyangz at microsoft.com>

BugLink: http://bugs.launchpad.net/bugs/1494431

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 at microsoft.com>
Reviewed-by: K. Y. Srinivasan <kys at microsoft.com>
Signed-off-by: David S. Miller <davem at davemloft.net>
(cherry picked from commit f9cbce34c34bcc05ea0dd78c8999bfe88b5b6b86)
Signed-off-by: Tim Gardner <tim.gardner at canonical.com>
---
 drivers/net/hyperv/hyperv_net.h   | 1 +
 drivers/net/hyperv/netvsc_drv.c   | 3 +--
 drivers/net/hyperv/rndis_filter.c | 2 +-
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/net/hyperv/hyperv_net.h b/drivers/net/hyperv/hyperv_net.h
index 6cc37c1..7a72bcb 100644
--- a/drivers/net/hyperv/hyperv_net.h
+++ b/drivers/net/hyperv/hyperv_net.h
@@ -581,6 +581,7 @@ struct nvsp_message {
 
 
 #define NETVSC_MTU 65536
+#define NETVSC_MTU_MIN 68
 
 #define NETVSC_RECEIVE_BUFFER_SIZE		(1024*1024*16)	/* 16MB */
 #define NETVSC_RECEIVE_BUFFER_SIZE_LEGACY	(1024*1024*15)  /* 15MB */
diff --git a/drivers/net/hyperv/netvsc_drv.c b/drivers/net/hyperv/netvsc_drv.c
index e88d694..a7a7fbd 100644
--- a/drivers/net/hyperv/netvsc_drv.c
+++ b/drivers/net/hyperv/netvsc_drv.c
@@ -692,8 +692,7 @@ 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 < NETVSC_MTU_MIN || mtu > limit)
 		return -EINVAL;
 
 	nvdev->start_remove = true;
diff --git a/drivers/net/hyperv/rndis_filter.c b/drivers/net/hyperv/rndis_filter.c
index d1b2e03..2455f86 100644
--- a/drivers/net/hyperv/rndis_filter.c
+++ b/drivers/net/hyperv/rndis_filter.c
@@ -1053,7 +1053,7 @@ int rndis_filter_device_add(struct hv_device *dev,
 	ret = rndis_filter_query_device(rndis_device,
 					RNDIS_OID_GEN_MAXIMUM_FRAME_SIZE,
 					&mtu, &size);
-	if (ret == 0 && size == sizeof(u32))
+	if (ret == 0 && size == sizeof(u32) && mtu < net_device->ndev->mtu)
 		net_device->ndev->mtu = mtu;
 
 	/* Get the mac address */
-- 
1.9.1