netdev - Re: [PATCH rfc 1/3] virtio-net: support transmit hash report

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Date:   Mon, 28 Dec 2020 11:47:45 -0500
From:   Willem de Bruijn <willemdebruijn.kernel@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        virtualization@...ts.linux-foundation.org,
        Network Development <netdev@...r.kernel.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [PATCH rfc 1/3] virtio-net: support transmit hash report

On Mon, Dec 28, 2020 at 11:28 AM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Dec 28, 2020 at 11:22:31AM -0500, Willem de Bruijn wrote:
> > From: Willem de Bruijn <willemb@...gle.com>
> >
> > Virtio-net supports sharing the flow hash from host to guest on rx.
> > Do the same on transmit, to allow the host to infer connection state
> > for more robust routing and telemetry.
> >
> > Linux derives ipv6 flowlabel and ECMP multipath from sk->sk_txhash,
> > and updates these fields on error with sk_rethink_txhash. This feature
> > allows the host to make similar decisions.
> >
> > Besides the raw hash, optionally also convey connection state for
> > this hash. Specifically, the hash rotates on transmit timeout. To
> > avoid having to keep a stateful table in the host to detect flow
> > changes, explicitly notify when a hash changed due to timeout.
>
> I don't actually see code using VIRTIO_NET_HASH_STATE_TIMEOUT_BIT
> in this series. Want to split out that part to a separate patch?

Will do.

I wanted to make it clear that these bits must be reserved (i.e.,
zero) until a later patch specifies them.

The timeout notification feature requires additional plumbing between
the TCP protocol stack and device driver, probably an skb bit. I'd
like to leave that as follow-up for now.

Thanks for the fast feedback!