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

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Date:   Sat, 21 Jan 2017 20:14:19 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Jason Wang <jasowang@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        David Laight <David.Laight@...LAB.COM>
Cc:     "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

On 17-01-21 06:51 PM, Jason Wang wrote:
> 
> 
> On 2017年01月21日 01:48, Michael S. Tsirkin wrote:
>> On Fri, Jan 20, 2017 at 04:59:11PM +0000, David Laight wrote:
>>> 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
>>>
>> I like that. John?
>>
> 
> This works, I prefer this only if it uses simpler code (but I suspect) than reset.
> 
> Thanks

Before the reset path I looked at doing this but it seems to require tracking
if a buffer had headroom on a per buffer basis. I don't see a good spot to
put a bit like this? It could go in the inbuf 'ctx' added by virtqueue_add_inbuf
but I would need to change the current usage of ctx which in the mergeable case
at least is just a simple pointer today. I don't like this because it
complicates the normal path and the XDP hotpath.

Otherwise we could somehow mark the ring at the point where XDP is enabled so
that it can learn when a full iteration around the ring. But I can't see a
simple way to make this work either.

I don't know the reset look straight forward to me and although not ideal is
fairly common on hardware based drivers during configuration changes. I'm open
to any ideas on where to put the metadata to track headroom though.

Thanks,
John