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Date:   Sun, 20 Feb 2022 08:17:30 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Willem de Bruijn <willemdebruijn.kernel@...il.com>
Cc:     Andrew Melnichenko <andrew@...nix.com>,
        Network Development <netdev@...r.kernel.org>,
        virtualization <virtualization@...ts.linux-foundation.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jason Wang <jasowang@...hat.com>,
        Yan Vugenfirer <yan@...nix.com>,
        Yuri Benditovich <yuri.benditovich@...nix.com>
Subject: Re: [PATCH v3 2/4] drivers/net/virtio_net: Added basic RSS support.

On Fri, Feb 18, 2022 at 08:43:20AM -0700, Willem de Bruijn wrote:
> On Thu, Feb 17, 2022 at 12:05 PM Andrew Melnichenko <andrew@...nix.com> wrote:
> >
> > Hi all,
> >
> > On Mon, Feb 14, 2022 at 12:09 AM Willem de Bruijn
> > <willemdebruijn.kernel@...il.com> wrote:
> > >
> > > > > > @@ -3113,13 +3270,14 @@ static int virtnet_probe(struct virtio_device *vdev)
> > > > > >         u16 max_queue_pairs;
> > > > > >         int mtu;
> > > > > >
> > > > > > -       /* Find if host supports multiqueue virtio_net device */
> > > > > > -       err = virtio_cread_feature(vdev, VIRTIO_NET_F_MQ,
> > > > > > -                                  struct virtio_net_config,
> > > > > > -                                  max_virtqueue_pairs, &max_queue_pairs);
> > > > > > +       /* Find if host supports multiqueue/rss virtio_net device */
> > > > > > +       max_queue_pairs = 1;
> > > > > > +       if (virtio_has_feature(vdev, VIRTIO_NET_F_MQ) || virtio_has_feature(vdev, VIRTIO_NET_F_RSS))
> > > > > > +               max_queue_pairs =
> > > > > > +                    virtio_cread16(vdev, offsetof(struct virtio_net_config, max_virtqueue_pairs));
> > > > >
> > > > > Instead of testing either feature and treating them as somewhat equal,
> > > > > shouldn't RSS be dependent on MQ?
> > > >
> > > > No, RSS is dependent on CTRL_VQ. Technically RSS and MQ are similar features.
> > >
> > > RSS depends on having multiple queues.
> > >
> > > What would enabling VIRTIO_NET_F_RSS without VIRTIO_NET_F_MQ do?
> >
> > RSS would work.
> 
> What does that mean, exactly? RSS is load balancing, does that not
> require multi-queue?

It does, but VIRTIO_NET_F_MQ is a misnomer.
\item[VIRTIO_NET_F_MQ(22)] Device supports multiqueue with automatic
    receive steering.

VIRTIO_NET_F_RSS implies multi queue and does not depend on VIRTIO_NET_F_MQ.

-- 
MST

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