Re: [PATCH v9 13/20] virtio-net: Return an error when vhost cannot enable RSS

[Yuri Benditovich <yuri.benditovich@daynix.com>]

On Tue, Apr 16, 2024 at 10:14 AM Jason Wang <jasowang@redhat.com> wrote:
>
> On Tue, Apr 16, 2024 at 1:43 PM Yuri Benditovich
> <yuri.benditovich@daynix.com> wrote:
> >
> > On Tue, Apr 16, 2024 at 7:00 AM Jason Wang <jasowang@redhat.com> wrote:
> > >
> > > On Mon, Apr 15, 2024 at 10:05 PM Yuri Benditovich
> > > <yuri.benditovich@daynix.com> wrote:
> > > >
> > > > On Wed, Apr 3, 2024 at 2:11 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
> > > > >
> > > > > vhost requires eBPF for RSS. When eBPF is not available, virtio-net
> > > > > implicitly disables RSS even if the user explicitly requests it. Return
> > > > > an error instead of implicitly disabling RSS if RSS is requested but not
> > > > > available.
> > > > >
> > > > > Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
> > > > > ---
> > > > >  hw/net/virtio-net.c | 97 ++++++++++++++++++++++++++---------------------------
> > > > >  1 file changed, 48 insertions(+), 49 deletions(-)
> > > > >
> > > > > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > > > > index 61b49e335dea..3d53eba88cfc 100644
> > > > > --- a/hw/net/virtio-net.c
> > > > > +++ b/hw/net/virtio-net.c
> > > > > @@ -793,9 +793,6 @@ static uint64_t virtio_net_get_features(VirtIODevice *vdev, uint64_t features,
> > > > >          return features;
> > > > >      }
> > > > >
> > > > > -    if (!ebpf_rss_is_loaded(&n->ebpf_rss)) {
> > > > > -        virtio_clear_feature(&features, VIRTIO_NET_F_RSS);
> > > > > -    }
> > > > >      features = vhost_net_get_features(get_vhost_net(nc->peer), features);
> > > > >      vdev->backend_features = features;
> > > > >
> > > > > @@ -3591,6 +3588,50 @@ static bool failover_hide_primary_device(DeviceListener *listener,
> > > > >      return qatomic_read(&n->failover_primary_hidden);
> > > > >  }
> > > > >
> > > > > +static void virtio_net_device_unrealize(DeviceState *dev)
> > > > > +{
> > > > > +    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > > +    VirtIONet *n = VIRTIO_NET(dev);
> > > > > +    int i, max_queue_pairs;
> > > > > +
> > > > > +    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
> > > > > +        virtio_net_unload_ebpf(n);
> > > > > +    }
> > > > > +
> > > > > +    /* This will stop vhost backend if appropriate. */
> > > > > +    virtio_net_set_status(vdev, 0);
> > > > > +
> > > > > +    g_free(n->netclient_name);
> > > > > +    n->netclient_name = NULL;
> > > > > +    g_free(n->netclient_type);
> > > > > +    n->netclient_type = NULL;
> > > > > +
> > > > > +    g_free(n->mac_table.macs);
> > > > > +    g_free(n->vlans);
> > > > > +
> > > > > +    if (n->failover) {
> > > > > +        qobject_unref(n->primary_opts);
> > > > > +        device_listener_unregister(&n->primary_listener);
> > > > > +        migration_remove_notifier(&n->migration_state);
> > > > > +    } else {
> > > > > +        assert(n->primary_opts == NULL);
> > > > > +    }
> > > > > +
> > > > > +    max_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
> > > > > +    for (i = 0; i < max_queue_pairs; i++) {
> > > > > +        virtio_net_del_queue(n, i);
> > > > > +    }
> > > > > +    /* delete also control vq */
> > > > > +    virtio_del_queue(vdev, max_queue_pairs * 2);
> > > > > +    qemu_announce_timer_del(&n->announce_timer, false);
> > > > > +    g_free(n->vqs);
> > > > > +    qemu_del_nic(n->nic);
> > > > > +    virtio_net_rsc_cleanup(n);
> > > > > +    g_free(n->rss_data.indirections_table);
> > > > > +    net_rx_pkt_uninit(n->rx_pkt);
> > > > > +    virtio_cleanup(vdev);
> > > > > +}
> > > > > +
> > > > >  static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > >  {
> > > > >      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > > @@ -3760,53 +3801,11 @@ static void virtio_net_device_realize(DeviceState *dev, Error **errp)
> > > > >
> > > > >      net_rx_pkt_init(&n->rx_pkt);
> > > > >
> > > > > -    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
> > > > > -        virtio_net_load_ebpf(n);
> > > > > -    }
> > > > > -}
> > > > > -
> > > > > -static void virtio_net_device_unrealize(DeviceState *dev)
> > > > > -{
> > > > > -    VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> > > > > -    VirtIONet *n = VIRTIO_NET(dev);
> > > > > -    int i, max_queue_pairs;
> > > > > -
> > > > > -    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS)) {
> > > > > -        virtio_net_unload_ebpf(n);
> > > > > +    if (virtio_has_feature(n->host_features, VIRTIO_NET_F_RSS) &&
> > > > > +        !virtio_net_load_ebpf(n) && get_vhost_net(nc->peer)) {
> > > > > +        virtio_net_device_unrealize(dev);
> > > > > +        error_setg(errp, "Can't load eBPF RSS for vhost");
> > > > >      }
> > > >
> > > > As I already mentioned, I think this is an extremely bad idea to
> > > > fail to run qemu due to such a reason as .absence of one feature.
> > > > What I suggest is:
> > > > 1. Redefine rss as tri-state (off|auto|on)
> > > > 2. Fail to run only if rss is on and not available via ebpf
> > > > 3. On auto - silently drop it
> > >
> > > "Auto" might be promatic for migration compatibility which is hard to
> > > be used by management layers like libvirt. The reason is that there's
> > > no way for libvirt to know if it is supported by device or not.
> >
> > In terms of migration every feature that somehow depends on the kernel
> > is problematic, not only RSS.
>
> True, but if we can avoid more, it would still be better.
>
> > Last time we added the USO feature - is
> > it different?
>
> I may miss something but we never define tristate for USO?
>
>     DEFINE_PROP_BIT64("guest_uso4", VirtIONet, host_features,
>                       VIRTIO_NET_F_GUEST_USO4, true),
>     DEFINE_PROP_BIT64("guest_uso6", VirtIONet, host_features,
>                       VIRTIO_NET_F_GUEST_USO6, true),
>     DEFINE_PROP_BIT64("host_uso", VirtIONet, host_features,
>                       VIRTIO_NET_F_HOST_USO, true),
>
When I've added USO feature I followed the existing approach of virtio-net.
On get_features - check what was "requested" including those that were "on"
by default and drop those that aren't supported (vhost by itself also
can drop some of features).

Still if we have on source machine kernel that supports USO (visible
on TAP flags)
and on dest we have older kernel without such support, the migration
will probably fail.

The available solution today is to reduce machine generation in
libvirt profile (as an example),
aligning the generation over all the machines that are expected to
participate in migration.

IMO we should think on some _generic_ solution, for example feature
negotiation between
machines before the migration - if the driver receives notification
from the device it
can negotiate the change of hardware features to OS (at least for most of them).
Not trivial, but IMO better than just failing the execution.

> ?
> > And in terms of migration "rss=on" is problematic the same way as "rss=auto".
>
> Failing early when launching Qemu is better than failing silently as a
> guest after a migration.

Do I understand correctly - you mean fail qemu initialization on the
destination machine?

>
> > Can you please show one scenario of migration where they will behave
> > differently?
>
> If you mean the problem of "auto", here's one:
>
> Assuming auto is used in both src and dst. On source, rss is enabled
> but not destination. RSS failed to work after migration.

I think in this case the migration will fail when set_feature is
called on destination.
The same way as with "on". Am I mistaken?

>
> > And in terms of regular experience there is a big advantage.
>
> Similarly, silent clearing a feature is also not good:
>
>     if (!peer_has_vnet_hdr(n)) {
>         virtio_clear_feature(&features, VIRTIO_NET_F_CSUM);
>         virtio_clear_feature(&features, VIRTIO_NET_F_HOST_TSO4);
>         virtio_clear_feature(&features, VIRTIO_NET_F_HOST_TSO6);
>         virtio_clear_feature(&features, VIRTIO_NET_F_HOST_ECN);
>
>         virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_CSUM);
>         virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_TSO4);
>         virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_TSO6);
>         virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_ECN);
>
>         virtio_clear_feature(&features, VIRTIO_NET_F_HOST_USO);
>         virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_USO4);
>         virtio_clear_feature(&features, VIRTIO_NET_F_GUEST_USO6);
>
>         virtio_clear_feature(&features, VIRTIO_NET_F_HASH_REPORT);
>     }
>
> The reason we never see complaints is probably because vhost/TAP are
> the only backend that supports migration where vnet support there has
> been more than a decade.

I think we never see complaints because we did not add new features
for a long time.

>
> Thanks
>
>
> >
> >
> > >
> > > Thanks
> > >
> > > > 4. The same with 'hash' option - it is not compatible with vhost (at
> > > > least at the moment)
> > > > 5. Reformat the patch as it is hard to review it due to replacing
> > > > entire procedures, i.e. one patch with replacing without changes,
> > > > another one - with real changes.
> > > > If this is hard to review only for me - please ignore that.
> > > >
> > > > > -
> > > > > -    /* This will stop vhost backend if appropriate. */
> > > > > -    virtio_net_set_status(vdev, 0);
> > > > > -
> > > > > -    g_free(n->netclient_name);
> > > > > -    n->netclient_name = NULL;
> > > > > -    g_free(n->netclient_type);
> > > > > -    n->netclient_type = NULL;
> > > > > -
> > > > > -    g_free(n->mac_table.macs);
> > > > > -    g_free(n->vlans);
> > > > > -
> > > > > -    if (n->failover) {
> > > > > -        qobject_unref(n->primary_opts);
> > > > > -        device_listener_unregister(&n->primary_listener);
> > > > > -        migration_remove_notifier(&n->migration_state);
> > > > > -    } else {
> > > > > -        assert(n->primary_opts == NULL);
> > > > > -    }
> > > > > -
> > > > > -    max_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
> > > > > -    for (i = 0; i < max_queue_pairs; i++) {
> > > > > -        virtio_net_del_queue(n, i);
> > > > > -    }
> > > > > -    /* delete also control vq */
> > > > > -    virtio_del_queue(vdev, max_queue_pairs * 2);
> > > > > -    qemu_announce_timer_del(&n->announce_timer, false);
> > > > > -    g_free(n->vqs);
> > > > > -    qemu_del_nic(n->nic);
> > > > > -    virtio_net_rsc_cleanup(n);
> > > > > -    g_free(n->rss_data.indirections_table);
> > > > > -    net_rx_pkt_uninit(n->rx_pkt);
> > > > > -    virtio_cleanup(vdev);
> > > > >  }
> > > > >
> > > > >  static void virtio_net_reset(VirtIODevice *vdev)
> > > > >
> > > > > --
> > > > > 2.44.0
> > > > >
> > > >
> > >
> >
>