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References: <20220511084804.82482-1-elic@nvidia.com>
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From:   Jason Wang <jasowang@redhat.com>
Date:   Wed, 11 May 2022 17:34:25 +0800
Message-ID: <CACGkMEt9h=hGH9cGj=NNLrGtvg2PguS8-pe4trxNUQOxmTRsog@mail.gmail.com>
Subject: Re: [PATCH] vdpa: Warn if MTU configured is too low
To:     Eli Cohen <elic@nvidia.com>
Cc:     mst <mst@redhat.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Si-Wei Liu <si-wei.liu@oracle.com>
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On Wed, May 11, 2022 at 4:48 PM Eli Cohen <elic@nvidia.com> wrote:
>
> 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@nvidia.com>

I wonder why it's a must?

> ---
>  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 <= ?

Thanks

> +               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
>