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

On Wed, May 11, 2022 at 6:02 PM Eli Cohen [off-list ref] wrote:

quoted

From: Jason Wang <redacted>
Sent: Wednesday, May 11, 2022 12:34 PM
To: Eli Cohen <redacted>
Cc: mst <mst@redhat.com>; virtualization <redacted>; linux-kernel <redacted>; Si-
Wei Liu [off-list ref]
Subject: Re: [PATCH] vdpa: Warn if MTU configured is too low

On Wed, May 11, 2022 at 4:48 PM Eli Cohen [off-list ref] wrote:

quoted

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 <redacted>

I wonder why it's a must?

It's definitely not a must but I thought if the spec says "should" it deserves a warning

Right.

but we can drop this if you think the warning is not in place.

I remember netlink has an extra log buffer, I wonder if it's better to
warn there?

quoted

quoted

---
 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..961168fe9094 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))

Should be <= ?

I don't think so. If it equals 1280 you don't want to warn.

Yes, you're right.

Thanks

quoted

Thanks

quoted

+               pr_warn("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

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