Subject: Re: [PATCH v3] virtio-net: Fix probe of virtio-net on kvmtool

From: "Michael S. Tsirkin" <[email-masked]>
Date: Thu, 2 Mar 2023 04:48:38 -0500

On Thu, Mar 02, 2023 at 04:10:20PM +0800, Jason Wang wrote:
> On Wed, Mar 1, 2023 at 10:44 PM Michael S. Tsirkin <[email-masked]> wrote:
> >
> > On Wed, Mar 01, 2023 at 01:59:52PM +0000, Rob Bradford via B4 Relay wrote:
> > > From: Rob Bradford <[email-masked]>
> > >
> > > Since the following commit virtio-net on kvmtool has printed a warning
> > > during the probe:
> > >
> > > commit dbcf24d153884439dad30484a0e3f02350692e4c
> > > Author: Jason Wang <[email-masked]>
> > > Date:   Tue Aug 17 16:06:59 2021 +0800
> > >
> > >     virtio-net: use NETIF_F_GRO_HW instead of NETIF_F_LRO
> > >
> > > [    1.865992] net eth0: Fail to set guest offload.
> > > [    1.872491] virtio_net virtio2 eth0: set_features() failed (-22); wanted 0x0000000000134829, left 0x0080000000134829
> > >
> > > This is because during the probing the underlying netdev device has
> > > identified that the netdev features on the device has changed and
> > > attempts to update the virtio-net offloads through the virtio-net
> > > control queue. kvmtool however does not have a control queue that supports
> > > offload changing (VIRTIO_NET_F_CTRL_GUEST_OFFLOADS is not advertised)
> > >
> > > The netdev features have changed due to validation checks in
> > > netdev_fix_features():
> > >
> > > if (!(features & NETIF_F_RXCSUM)) {
> > >       /* NETIF_F_GRO_HW implies doing RXCSUM since every packet
> > >        * successfully merged by hardware must also have the
> > >        * checksum verified by hardware.  If the user does not
> > >        * want to enable RXCSUM, logically, we should disable GRO_HW.
> > >        */
> > >       if (features & NETIF_F_GRO_HW) {
> > >               netdev_dbg(dev, "Dropping NETIF_F_GRO_HW since no RXCSUM feature.\n");
> > >               features &= ~NETIF_F_GRO_HW;
> > >       }
> > > }
> > >
> > > Since kvmtool does not advertise the VIRTIO_NET_F_GUEST_CSUM feature the
> > > NETIF_F_RXCSUM bit is not present and so the NETIF_F_GRO_HW bit is
> > > cleared. This results in the netdev features changing, which triggers
> > > the attempt to reprogram the virtio-net offloads which then fails.
> > >
> > > This commit prevents that set of netdev features from changing by
> > > preemptively applying the same validation and only setting
> > > NETIF_F_GRO_HW if NETIF_F_RXCSUM is set because the device supports both
> > > VIRTIO_NET_F_GUEST_CSUM and VIRTIO_NET_F_GUEST_TSO{4,6}
> > >
> > > Signed-off-by: Rob Bradford <[email-masked]>
> > > ---
> > > Changes in v3:
> > > - Identified root-cause of feature bit changing and updated conditions
> > >   check
> > > - Link to v2: https://lore.kernel.org/r/[email-masked]
> > >
> > > Changes in v2:
> > > - Use parentheses to group logical OR of features
> > > - Link to v1:
> > >   https://lore.kernel.org/r/[email-masked]
> > > ---
> > >  drivers/net/virtio_net.c | 10 ++++++----
> > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/drivers/net/virtio_net.c b/drivers/net/virtio_net.c
> > > index 61e33e4dd0cd..2e7705142ca5 100644
> > > --- a/drivers/net/virtio_net.c
> > > +++ b/drivers/net/virtio_net.c
> > > @@ -3778,11 +3778,13 @@ static int virtnet_probe(struct virtio_device *vdev)
> > >                       dev->features |= dev->hw_features & NETIF_F_ALL_TSO;
> > >               /* (!csum && gso) case will be fixed by register_netdev() */
> > >       }
> > > -     if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM))
> > > +     if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_CSUM)) {
> > >               dev->features |= NETIF_F_RXCSUM;
> > > -     if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
> > > -         virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6))
> > > -             dev->features |= NETIF_F_GRO_HW;
> > > +             /* This dependency is enforced by netdev_fix_features */
> > > +             if (virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO4) ||
> > > +                 virtio_has_feature(vdev, VIRTIO_NET_F_GUEST_TSO6))
> > > +                     dev->features |= NETIF_F_GRO_HW;
> > > +     }
> > >       if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> > >               dev->hw_features |= NETIF_F_GRO_HW;
> 
> Should we move this also under the check of RXCSUM, otherwise we may
> end up the following case:
> 
> when CSUM is not negotiated but GUEST_OFFLOADS, can still try to
> enable-or-disable guest offloads? Or do we need to fail the probe in
> the case via virtnet_validate_features()?
> 
> > >
> >
> > I see. It is annoying that we are duplicating the logic from
> > netdev_fix_features here though :(
> > Maybe we should call netdev_update_features, in the callback check
> > the flags and decide what to set and what to clear?
> > Or export netdev_fix_features to modules?
> 
> There's a ndo_fix_features() that might be used here.
> 
> >
> >
> >
> > Also re-reading Documentation/networking/netdev-features.rst -
> >
> >  1. netdev->hw_features set contains features whose state may possibly
> >     be changed (enabled or disabled) for a particular device by user's
> >     request.  This set should be initialized in ndo_init callback and not
> >     changed later.
> >
> >  2. netdev->features set contains features which are currently enabled
> >     for a device.  This should be changed only by network core or in
> >     error paths of ndo_set_features callback.
> >
> >
> > is it then wrong that virtio sets NETIF_F_RXCSUM and NETIF_F_GRO_HW in
> > dev->features and not in dev->hw_features?
> 
> Looks not the core can try to enable and disable features according to
> the diff between features and hw_features
> 
> static inline netdev_features_t netdev_get_wanted_features(
>         struct net_device *dev)
> {
>         return (dev->features & ~dev->hw_features) | dev->wanted_features;
> }
> 
> Thanks

yes what we do work according to code.  So the documentation is wrong then?

> > We set it there because
> > without ctrl guest offload these can not be changed.
> > I suspect this is just a minor documentation bug yes? Maybe devices
> > where features can't be cleared are uncommon.
> >
> > Also:
> >         if (virtio_has_feature(vdev, VIRTIO_NET_F_CTRL_GUEST_OFFLOADS))
> >                 dev->hw_features |= NETIF_F_GRO_HW;
> >
> > but should we not set NETIF_F_RXCSUM there too?
> >
> >
> >
> > > ---
> > > base-commit: c39cea6f38eefe356d64d0bc1e1f2267e282cdd3
> > > change-id: 20230223-virtio-net-kvmtool-87f37515be22
> > >
> > > Best regards,
> > > --
> > > Rob Bradford <[email-masked]>
> >