[PATCH v1] vdpa: Warn if MTU configured is too low

[Eli Cohen]

Following the recommendation in virio spec 1.1, a device offering
VIRTIO_NET_F_MTU should set the mtu to at least 1280 bytes.

Print a warning if this recommendation is not met.

Signed-off-by: Eli Cohen <elic@xxxxxxxxxx>
---
v0 -> v1:
  chage pr_warn to netlink warning to userspace

 drivers/vdpa/vdpa.c | 9 ++++++++-
 1 file changed, 8 insertions(+), 1 deletion(-)

diff --git a/drivers/vdpa/vdpa.c b/drivers/vdpa/vdpa.c
index 91f4c13c7c7c..0fb4a615f267 100644
--- a/drivers/vdpa/vdpa.c
+++ b/drivers/vdpa/vdpa.c
@@ -583,6 +583,9 @@ vdpa_nl_cmd_mgmtdev_get_dumpit(struct sk_buff *msg, struct netlink_callback *cb)
 				 BIT_ULL(VDPA_ATTR_DEV_NET_CFG_MTU)     | \
 				 BIT_ULL(VDPA_ATTR_DEV_NET_CFG_MAX_VQP))
 
+/* Recommended virtio spec 1.1 section 5.1.4.1 */
+#define VIRTIO_MIN_PREFERRED_MTU 1280
+
 static int vdpa_nl_cmd_dev_add_set_doit(struct sk_buff *skb, struct genl_info *info)
 {
 	struct vdpa_dev_set_config config = {};
@@ -634,6 +637,10 @@ static int vdpa_nl_cmd_dev_add_set_doit(struct sk_buff *skb, struct genl_info *i
 		err = PTR_ERR(mdev);
 		goto err;
 	}
+	if ((mdev->supported_features & BIT_ULL(VIRTIO_NET_F_MTU)) &&
+	    (config.mask & BIT_ULL(VDPA_ATTR_DEV_NET_CFG_MTU) &&
+	    config.net.mtu < VIRTIO_MIN_PREFERRED_MTU))
+		NL_SET_ERR_MSG_MOD(info->extack, "MTU is below recommended value\n");
 	if ((config.mask & mdev->config_attr_mask) != config.mask) {
 		NL_SET_ERR_MSG_MOD(info->extack,
 				   "All provided attributes are not supported");
@@ -1135,7 +1142,7 @@ static const struct nla_policy vdpa_nl_policy[VDPA_ATTR_MAX + 1] = {
 	[VDPA_ATTR_DEV_NAME] = { .type = NLA_STRING },
 	[VDPA_ATTR_DEV_NET_CFG_MACADDR] = NLA_POLICY_ETH_ADDR,
 	/* virtio spec 1.1 section 5.1.4.1 for valid MTU range */
-	[VDPA_ATTR_DEV_NET_CFG_MTU] = NLA_POLICY_MIN(NLA_U16, 68),
+	[VDPA_ATTR_DEV_NET_CFG_MTU] = NLA_POLICY_MIN(NLA_U16, ETH_MIN_MTU),
 };
 
 static const struct genl_ops vdpa_nl_ops[] = {
-- 
2.35.1