[PATCH] net-sysfs: export gso_max_size attribute

[Solio Sarabia]

The netdevice gso_max_size is exposed to allow users fine-control on
systems with multiple NICs with different GSO buffer sizes, and where
the virtual devices like bridge and veth, need to be aware of the GSO
size of the underlying devices.

In a virtualized environment, setting the right GSO sizes for physical
and virtual devices makes all TSO work to be on physical NIC, improving
throughput and reducing CPU util. If virtual devices send buffers
greater than what NIC supports, it forces host to do TSO for buffers
exceeding the limit, increasing CPU utilization in host.

Suggested-by: Shiny Sebastian <shiny.sebastian@xxxxxxxxx>
Signed-off-by: Solio Sarabia <solio.sarabia@xxxxxxxxx>
---
In one test scenario with Hyper-V host, Ubuntu 16.04 VM, with Docker
inside VM, and NTttcp sending 40 Gbps from one container, setting the
right gso_max_size values for all network devices in the chain, reduces
CPU overhead about 3x (for the sender), since all TSO work is done by
physical NIC.

 net/core/net-sysfs.c | 30 ++++++++++++++++++++++++++++++
 1 file changed, 30 insertions(+)

diff --git a/net/core/net-sysfs.c b/net/core/net-sysfs.c
index 799b752..7314bc8 100644
--- a/net/core/net-sysfs.c
+++ b/net/core/net-sysfs.c
@@ -376,6 +376,35 @@ static ssize_t gro_flush_timeout_store(struct device *dev,
 }
 NETDEVICE_SHOW_RW(gro_flush_timeout, fmt_ulong);
 
+static int change_gso_max_size(struct net_device *dev, unsigned long new_size)
+{
+	unsigned int orig_size = dev->gso_max_size;
+
+	if (new_size != (unsigned int)new_size)
+		return -ERANGE;
+
+	if (new_size == orig_size)
+		return 0;
+
+	if (new_size <= 0 || new_size > GSO_MAX_SIZE)
+		return -ERANGE;
+
+	dev->gso_max_size = new_size;
+	return 0;
+}
+
+static ssize_t gso_max_size_store(struct device *dev,
+				  struct device_attribute *attr,
+				  const char *buf, size_t len)
+{
+	if (!capable(CAP_NET_ADMIN))
+		return -EPERM;
+
+	return netdev_store(dev, attr, buf, len, change_gso_max_size);
+}
+
+NETDEVICE_SHOW_RW(gso_max_size, fmt_dec);
+
 static ssize_t ifalias_store(struct device *dev, struct device_attribute *attr,
 			     const char *buf, size_t len)
 {
@@ -543,6 +572,7 @@ static struct attribute *net_class_attrs[] __ro_after_init = {
 	&dev_attr_flags.attr,
 	&dev_attr_tx_queue_len.attr,
 	&dev_attr_gro_flush_timeout.attr,
+	&dev_attr_gso_max_size.attr,
 	&dev_attr_phys_port_id.attr,
 	&dev_attr_phys_port_name.attr,
 	&dev_attr_phys_switch_id.attr,
-- 
2.7.4