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

[Jason Wang]

On Wed, May 11, 2022 at 6:02 PM Eli Cohen <elic@xxxxxxxxxx> wrote:
>
> > From: Jason Wang <jasowang@xxxxxxxxxx>
> > Sent: Wednesday, May 11, 2022 12:34 PM
> > To: Eli Cohen <elic@xxxxxxxxxx>
> > Cc: mst <mst@xxxxxxxxxx>; virtualization <virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx>; linux-kernel <linux-kernel@xxxxxxxxxxxxxxx>; Si-
> > Wei Liu <si-wei.liu@xxxxxxxxxx>
> > Subject: Re: [PATCH] vdpa: Warn if MTU configured is too low
> >
> > On Wed, May 11, 2022 at 4:48 PM Eli Cohen <elic@xxxxxxxxxx> 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@xxxxxxxxxx>
> >
> > 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?

>
> >
> > > ---
> > >  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

>
> >
> > 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
> > >
>