Message-ID: <CAOJe8K2X39rEN+Rjvoc3_TYfKwf0h117U=-pbRPAcKZvMPWTmA@mail.gmail.com>
Date:   Mon, 30 Mar 2020 16:09:49 +0300
From:   Denis Kirjanov <kda@...ux-powerpc.org>
To:     Jürgen Groß <jgross@...e.com>
Cc:     netdev@...r.kernel.org, ilias.apalodimas@...aro.org,
        wei.liu@...nel.org, paul@....org
Subject: Re: [PATCH net-next v4] xen networking: add basic XDP support for xen-netfront

On 3/30/20, Jürgen Groß <jgross@...e.com> wrote:
> On 30.03.20 14:16, Denis Kirjanov wrote:
>> On 3/23/20, Jürgen Groß <jgross@...e.com> wrote:
>>> On 23.03.20 11:49, Denis Kirjanov wrote:
>>>> On 3/23/20, Jürgen Groß <jgross@...e.com> wrote:
>>>>> On 23.03.20 11:15, Denis Kirjanov wrote:
>>>>>> On 3/18/20, Jürgen Groß <jgross@...e.com> wrote:
>>>>>>> On 18.03.20 13:50, Denis Kirjanov wrote:
>>>>>>>> On 3/18/20, Jürgen Groß <jgross@...e.com> wrote:
>>>>>>>>> On 16.03.20 14:09, Denis Kirjanov wrote:
>>>>>>>>>> The patch adds a basic XDP processing to xen-netfront driver.
>>>>>>>>>>
>>>>>>>>>> We ran an XDP program for an RX response received from netback
>>>>>>>>>> driver. Also we request xen-netback to adjust data offset for
>>>>>>>>>> bpf_xdp_adjust_head() header space for custom headers.
>>>>>>>>>
>>>>>>>>> This is in no way a "verbose patch descriprion".
>>>>>>>>>
>>>>>>>>> I'm missing:
>>>>>>>>>
>>>>>>>>> - Why are you doing this. "Add XDP support" is not enough, for
>>>>>>>>> such
>>>>>>>>>        a change I'd like to see some performance numbers to get an
>>>>>>>>> idea
>>>>>>>>>        of the improvement to expect, or which additional
>>>>>>>>> functionality
>>>>>>>>>        for the user is available.
>>>>>>>> Ok, I'll try to measure  some numbers.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> - A short description for me as a Xen maintainer with only basic
>>>>>>>>>        networking know-how, what XDP programs are about (a link to
>>>>>>>>> some
>>>>>>>>>        more detailed doc is enough, of course) and how the
>>>>>>>>> interface
>>>>>>>>>        is working (especially for switching between XDP mode and
>>>>>>>>> normal
>>>>>>>>>        SKB processing).
>>>>>>>>
>>>>>>>> You can search for the "A practical introduction to XDP" tutorial.
>>>>>>>> Actually there is a lot of information available regarding XDP, you
>>>>>>>> can easily find it.
>>>>>>>>
>>>>>>>>>
>>>>>>>>> - A proper description of the netfront/netback communication when
>>>>>>>>>        enabling or disabling XDP mode (who is doing what, is
>>>>>>>>> silencing
>>>>>>>>>        of the virtual adapter required, ...).
>>>>>>>> Currently we need only a header offset from netback driver so that
>>>>>>>> we
>>>>>>>> can
>>>>>>>> put
>>>>>>>> custom encapsulation header if required and that's done using xen
>>>>>>>> bus
>>>>>>>> state switching,
>>>>>>>> so that:
>>>>>>>> - netback tells that it can adjust the header offset
>>>>>>>> - netfront part reads it
>>>>>>>
>>>>>>> Yes, but how is this synchronized with currently running network
>>>>>>> load?
>>>>>>> Assume you are starting without XDP being active and then you are
>>>>>>> activating it. How is the synchronization done from which request on
>>>>>>> the XDP headroom is available?
>>>>>>
>>>>>> Hi Jurgen,
>>>>>>
>>>>>> basically XDP is activated when you've assigned an xdp program to the
>>>>>> networking device.
>>>>>> Assigning an xdp program means that we have to adjust a pointer which
>>>>>> is RCU protected.
>>>>>
>>>>> This doesn't answer my question.
>>>>>
>>>>> You have basically two communication channels: the state of the
>>>>> frontend
>>>>> and backend for activation/deactivation of XDP, and the ring pages
>>>>> with
>>>>> the rx and tx requests and responses. How is the synchronization
>>>>> between
>>>>> those two channels done? So how does the other side know which of the
>>>>> packets in flight will then have XDP on or off?
>>>>
>>>> Right,
>>>> that's done in xen-netback using xenbus state:
>>>> - in xennet_xdp_set() we call xenbus_switch_state to tell xen-netback
>>>> to
>>>> adjust offset for an RX response.
>>>> -xen-netback reads the value from xenstore and adjusts the offset for
>>>> xen-netback
>>>> in xenvif_rx_data_slot() using vif->xdp_enabled flag.
>>>
>>> And before that all in-flight requests in the ring pages are being
>>> processed and no new requests are guaranteed to be enqueued?
>>
>> Actually I don't see the need to sync these requests since that all we
>> have to do is to copy
>> data with specified offset:
>> with xdp->enabled=1: copy with the offset XDP_PACKET_HEADROOM
>> with xdd->enabled=0: copy without the offset
>
> Isn't that racy?
>
> In xennet_xdp_set() you set queue->xdp_prog and then you change the
> state to Reconfiguring. From the time queue->xdp_prog is set you'll
> do the Xdp processing in xennet_get_responses(), even if the response
> you are working on doesn't have the headroom you need, as the backend
> didn't create it with headroom (it needs some time until it has seen
> the new state and could react on it by sending _new_ responses with
> headroom).

Ah, I see. You mean that we have to wait until XenbusStateReconfigured
is set in
xen-netfront and only after that it's safe to process packets.

>
> Or am I missing something about Xdp programs?
>
>
> Juergen
>

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