netdev - Re: [PATCH net-next v4] xen networking: add basic XDP support for xen-netfront

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Message-ID: <CAOJe8K3+ddELP=nac+WRB1d5ccsDQu2UBVY4T2GiFFUfhk0jcQ@mail.gmail.com>
Date:   Mon, 30 Mar 2020 15:16:10 +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 3/23/20, 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

Thanks!

>
>
> Juergen
>