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

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Date:   Mon, 28 Dec 2020 20:19:58 -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 12:28 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Dec 28, 2020 at 11:47:45AM -0500, Willem de Bruijn wrote:
> > 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.
>
> Already the case for the padding field I think ...

Good. Is this something that the device should enforce? Meaning, drop
requests with reserved bits set.

Once a bit is defined and device updated to accept it, it cannot
distinguish between a new driver aware of the bit and an old one that
wrote to padding. More problematic, a well behaved new driver will
gets packets dropped by an old device.

The proper way is to negotiate this is through features, of course.
But I don't think we want to add one for each new bit. That's why I
squished the bit definition in here, even in absence of an
implementation.