Subject: Re: [net PATCH v5 6/6] virtio_net: XDP support for adjust_head

From: John Fastabend <[email-masked]>
Date: Thu, 19 Jan 2017 19:38:22 -0800

On 17-01-19 01:11 PM, Michael S. Tsirkin wrote:
> On Thu, Jan 19, 2017 at 11:05:40AM +0800, Jason Wang wrote:
>>
>>
>> On 2017年01月18日 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<[email-masked]>
>>> 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.
> 

Maybe I'm not following, is the suggestion to drop the packets after XDP
is setup for all outstanding buffers until we have done a reallocation of
all the buffers? In this case we can't just detach the buffers we have to
wait until the backend retires them by using them correct?

But when XDP setup call returns we need to guarantee that buffers and
driver are setup. Otherwise the next n packets get dropped in the future.
If there is no traffic currently this could be at some undetermined point
in the future. This will be very buggy.

Did I miss something?

Thanks,
John