Date:   Mon, 28 Sep 2020 19:44:18 -0700
From:   David Ahern <dsahern@...il.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>
Cc:     Toke Høiland-Jørgensen <toke@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        David Ahern <dsahern@...nel.org>,
        Network Development <netdev@...r.kernel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        Magnus Karlsson <magnus.karlsson@...el.com>
Subject: Re: [PATCH RFC net-next] virtio_net: Relax queue requirement for
 using XDP

On 9/28/20 7:25 AM, Magnus Karlsson wrote:
> On Mon, Sep 28, 2020 at 5:13 AM David Ahern <dsahern@...il.com> wrote:
>>
>> On 2/27/20 2:41 AM, Magnus Karlsson wrote:
>>> I will unfortunately be after Netdevconf due to other commitments. The
>>> plan is to send out the RFC to the co-authors of the Plumbers
>>> presentation first, just to check the sanity of it. And after that
>>> send it to the mailing list. Note that I have taken two shortcuts in
>>> the RFC to be able to make quicker progress. The first on is the
>>> driver implementation of the dynamic queue allocation and
>>> de-allocation. It just does this within a statically pre-allocated set
>>> of queues. The second is that the user space interface is just a
>>> setsockopt instead of a rtnetlink interface. Again, just to save some
>>> time in this initial phase. The information communicated in the
>>> interface is the same though. In the current code, the queue manager
>>> can handle the queues of the networking stack, the XDP_TX queues and
>>> queues allocated by user space and used for AF_XDP. Other uses from
>>> user space is not covered due to my setsockopt shortcut. Hopefully
>>> though, this should be enough for an initial assessment.
>>
>> Any updates on the RFC? I do not recall seeing a patch set on the
>> mailing list, but maybe I missed it.
> 
> No, you have unfortunately not missed anything. It has been lying on
> the shelf collecting dust for most of this time. The reason was that
> the driver changes needed to support dynamic queue allocation just
> became too complex as it would require major surgery to at least all
> Intel drivers, and probably a large number of other ones as well. Do
> not think any vendor would support such a high effort solution and I
> could not (at that time at least) find a way around it. So, gaining
> visibility into what queues have been allocated (by all entities in
> the kernel that uses queue) seems to be rather straightforward, but
> the dynamic allocation part seems to be anything but.

retrofitting a new idea is usually a high level of effort.

> 
> I also wonder how useful this queue manager proposal would be in light
> of Mellanox's subfunction proposal. If people just start to create
> many small netdevs (albeit at high cost which people may argue
> against) consisting of just an rx/tx queue pair, then the queue
> manager dynamic allocation proposal would not be as useful. We could
> just use one of these netdevs to bind to in the AF_XDP case and always
> just specify queue 0. But one can argue that queue management is
> needed even for the subfunction approach, but then it would be at a
> much lower level than what I proposed. What is your take on this?
> Still worth pursuing in some form or another? If yes, then we really
> need to come up with an easy way of supporting this in current
> drivers. It is not going to fly otherwise, IMHO.
> 

I need to find some time to take a deep dive on the subfunction idea. I
like the intent, but need to understand the details.

Thanks for the update.

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