netdev - RE: [net PATCH v5 6/6] virtio_net: XDP support for adjust

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Message-ID: <063D6719AE5E284EB5DD2968C1650D6DB026921C@AcuExch.aculab.com>
Date:   Fri, 20 Jan 2017 16:59:11 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Michael S. Tsirkin'" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>
CC:     John Fastabend <john.fastabend@...il.com>,
        "john.r.fastabend@...el.com" <john.r.fastabend@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "alexei.starovoitov@...il.com" <alexei.starovoitov@...il.com>,
        "daniel@...earbox.net" <daniel@...earbox.net>
Subject: RE: [net PATCH v5 6/6] virtio_net: XDP support for adjust_head

From: Michael S. Tsirkin
> Sent: 19 January 2017 21:12
> > On 2017?01?18? 23:15, Michael S. Tsirkin wrote:
> > > On Tue, Jan 17, 2017 at 02:22:59PM -0800, John Fastabend wrote:
> > > > Add support for XDP adjust head by allocating a 256B header region
> > > > that XDP programs can grow into. This is only enabled when a XDP
> > > > program is loaded.
> > > >
> > > > In order to ensure that we do not have to unwind queue headroom push
> > > > queue setup below bpf_prog_add. It reads better to do a prog ref
> > > > unwind vs another queue setup call.
> > > >
> > > > At the moment this code must do a full reset to ensure old buffers
> > > > without headroom on program add or with headroom on program removal
> > > > are not used incorrectly in the datapath. Ideally we would only
> > > > have to disable/enable the RX queues being updated but there is no
> > > > API to do this at the moment in virtio so use the big hammer. In
> > > > practice it is likely not that big of a problem as this will only
> > > > happen when XDP is enabled/disabled changing programs does not
> > > > require the reset. There is some risk that the driver may either
> > > > have an allocation failure or for some reason fail to correctly
> > > > negotiate with the underlying backend in this case the driver will
> > > > be left uninitialized. I have not seen this ever happen on my test
> > > > systems and for what its worth this same failure case can occur
> > > > from probe and other contexts in virtio framework.
> > > >
> > > > Signed-off-by: John Fastabend<john.r.fastabend@...el.com>
> > > I've been thinking about it - can't we drop
> > > old buffers without the head room which were posted before
> > > xdp attached?
> > >
> > > Avoiding the reset would be much nicer.
> > >
> > > Thoughts?
> > >
> >
> > As been discussed before, device may use them in the same time so it's not
> > safe. Or do you mean detect them after xdp were set and drop the buffer
> > without head room, this looks sub-optimal.
> >
> > Thanks
> 
> Yes, this is what I mean.  Why is this suboptimal? It's a single branch
> in code. Yes we might lose some packets but the big hammer of device
> reset will likely lose more.

Why not leave let the hardware receive into the 'small' buffer (without
headroom) and do a copy when a frame is received.
Replace the buffers with 'big' ones for the next receive.
A data copy on a ring full of buffers won't really be noticed.

	David