Date:   Mon, 23 Mar 2020 12:00:09 +0100
From:   Jürgen Groß <jgross@...e.com>
To:     Denis Kirjanov <kda@...ux-powerpc.org>
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 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?


Juergen

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