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Date:   Wed, 11 May 2022 05:58:51 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     Eli Cohen <elic@nvidia.com>,
        virtualization <virtualization@lists.linux-foundation.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Si-Wei Liu <si-wei.liu@oracle.com>
Subject: Re: [PATCH] vdpa: Warn if MTU configured is too low
Message-ID: <20220511055517-mutt-send-email-mst@kernel.org>
References: <20220511084804.82482-1-elic@nvidia.com>
 <CACGkMEt9h=hGH9cGj=NNLrGtvg2PguS8-pe4trxNUQOxmTRsog@mail.gmail.com>
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On Wed, May 11, 2022 at 05:34:25PM +0800, Jason Wang wrote:
> 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?


It's a SHOULD in the spec.  I guess 1280 is to allow IPv6.

> > ---
> >  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
> >