[PATCH v2] net: Allow ethtool to set interface in loopback mode.

[Mahesh Bandewar <maheshb@google.com>]

This patch enables ethtool to set the loopback mode on a given interface.
By configuring the interface in loopback mode in conjunction with a policy
route / rule, a userland application can stress the egress / ingress path
exposing the flows of the change in progress and potentially help developer(s)
understand the impact of those changes without even sending a packet out
on the network.

Following set of commands illustrates one such example -
	a) ip -4 addr add 192.168.1.1/24 dev eth1
	b) ip -4 rule add from all iif eth1 lookup 250
	c) ip -4 route add local 0/0 dev lo proto kernel scope host table 250
	d) arp -Ds 192.168.1.100 eth1
	e) arp -Ds 192.168.1.200 eth1
	f) sysctl -w net.ipv4.ip_nonlocal_bind=1
	g) sysctl -w net.ipv4.conf.all.accept_local=1
	# Assuming that the machine has 8 cores
	h) taskset 000f netserver -L 192.168.1.200
	i) taskset 00f0 netperf -t TCP_CRR -L 192.168.1.100 -H 192.168.1.200 -l 30

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
Reviewed-by: Ben Hutchings <bhutchings@solarflare.com>
---
 include/linux/ethtool.h |   16 ++++++++++++++++
 net/core/ethtool.c      |   39 +++++++++++++++++++++++++++++++++++++++
 2 files changed, 55 insertions(+), 0 deletions(-)

diff --git a/include/linux/ethtool.h b/include/linux/ethtool.h
index 6628a50..c036347 100644
--- a/include/linux/ethtool.h
+++ b/include/linux/ethtool.h
@@ -616,6 +616,18 @@ void ethtool_ntuple_flush(struct net_device *dev);
  *	Should validate the magic field.  Don't need to check len for zero
  *	or wraparound.  Update len to the amount written.  Returns an error
  *	or zero.
+ *
+ * get_loopback:
+ * set_loopback:
+ *	These are the driver specific get / set methods to report / enable-
+ *	disable loopback mode. The idea is to stress test the ingress/egress
+ *	paths by enabling this mode. There are multiple places this could be
+ *	done and choice of place will most likely be affected by the device
+ *	capabilities. So as a guiding principle; select a place to implement 
+ *	loopback mode as close to the host as possible. This would maximize
+ *	the soft-path length and maintain parity in terms of comparison with
+ *	different set of drivers.
+ *
  */
 struct ethtool_ops {
 	int	(*get_settings)(struct net_device *, struct ethtool_cmd *);
@@ -678,6 +690,8 @@ struct ethtool_ops {
 				  struct ethtool_rxfh_indir *);
 	int	(*set_rxfh_indir)(struct net_device *,
 				  const struct ethtool_rxfh_indir *);
+	int	(*get_loopback)(struct net_device *, u32 *);
+	int	(*set_loopback)(struct net_device *, u32);
 };
 #endif /* __KERNEL__ */
 
@@ -741,6 +755,8 @@ struct ethtool_ops {
 #define ETHTOOL_GSSET_INFO	0x00000037 /* Get string set info */
 #define ETHTOOL_GRXFHINDIR	0x00000038 /* Get RX flow hash indir'n table */
 #define ETHTOOL_SRXFHINDIR	0x00000039 /* Set RX flow hash indir'n table */
+#define ETHTOOL_SLOOPBACK	0x0000003a /* Enable / Disable Loopback */
+#define ETHTOOL_GLOOPBACK	0x0000003b /* Get Loopback status */
 
 /* compatibility with older code */
 #define SPARC_ETH_GSET		ETHTOOL_GSET
diff --git a/net/core/ethtool.c b/net/core/ethtool.c
index 956a9f4..5c87c93 100644
--- a/net/core/ethtool.c
+++ b/net/core/ethtool.c
@@ -1434,6 +1434,39 @@ static noinline_for_stack int ethtool_flash_device(struct net_device *dev,
 	return dev->ethtool_ops->flash_device(dev, &efl);
 }
 
+static int ethtool_set_loopback(struct net_device *dev, void __user *useraddr)
+{
+	struct ethtool_value edata;
+	const struct ethtool_ops *ops = dev->ethtool_ops;
+
+	if (!ops || !ops->set_loopback)
+		return -EOPNOTSUPP;
+
+	if (copy_from_user(&edata, useraddr, sizeof(edata)))
+		return -EFAULT;
+
+	return ops->set_loopback(dev, edata.data);
+}
+
+static int ethtool_get_loopback(struct net_device *dev, void __user *useraddr)
+{
+	struct ethtool_value edata;
+	const struct ethtool_ops *ops = dev->ethtool_ops;
+	int err;
+
+	if (!ops || !ops->get_loopback)
+		return -EOPNOTSUPP;
+
+	err = ops->get_loopback(dev, &edata.data);	
+	if (err)
+		return (err);
+
+	if (copy_to_user(useraddr, &edata, sizeof(edata)))
+		return -EFAULT;
+
+	return 0;
+}
+
 /* The main entry point in this file.  Called from net/core/dev.c */
 
 int dev_ethtool(struct net *net, struct ifreq *ifr)
@@ -1678,6 +1711,12 @@ int dev_ethtool(struct net *net, struct ifreq *ifr)
 	case ETHTOOL_SRXFHINDIR:
 		rc = ethtool_set_rxfh_indir(dev, useraddr);
 		break;
+	case ETHTOOL_SLOOPBACK:
+		rc = ethtool_set_loopback(dev, useraddr);
+		break;
+	case ETHTOOL_GLOOPBACK:
+		rc = ethtool_get_loopback(dev, useraddr);
+		break;
 	default:
 		rc = -EOPNOTSUPP;
 	}
-- 
1.7.3.1