netdev - Re: [net-next PATCH v3 2/3] e1000: add initial XDP support

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Message-ID: <CALx6S36u+tQUEVhu0BaLZ6-_0=GSMwxd4m89-H+brVZ-rr-U2g@mail.gmail.com>
Date:   Tue, 13 Sep 2016 09:21:47 -0700
From:   Tom Herbert <tom@...bertland.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Eric Dumazet <eric.dumazet@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Brenden Blanco <bblanco@...mgrid.com>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>,
        Jesper Dangaard Brouer <brouer@...hat.com>,
        "David S. Miller" <davem@...emloft.net>,
        Cong Wang <xiyou.wangcong@...il.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>,
        William Tu <u9012063@...il.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [net-next PATCH v3 2/3] e1000: add initial XDP support

On Mon, Sep 12, 2016 at 6:28 PM, Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
> On Mon, Sep 12, 2016 at 05:03:25PM -0700, Tom Herbert wrote:
>> On Mon, Sep 12, 2016 at 4:46 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> > On Mon, 2016-09-12 at 16:07 -0700, Alexei Starovoitov wrote:
>> >
>> >> yep. there are various ways to shoot yourself in the foot with xdp.
>> >> The simplest program that drops all the packets will make the box unpingable.
>> >
>> > Well, my comment was about XDP_TX only, not about XDP_DROP or driving a
>> > scooter on 101 highway ;)
>> >
>> > This XDP_TX thing was one of the XDP marketing stuff, but there is
>> > absolutely no documentation on it, warning users about possible
>> > limitations/outcomes.
>> >
>> > BTW, I am not sure mlx4 implementation even works, vs BQL :
>> >
>> > mlx4_en_xmit_frame() does not call netdev_tx_sent_queue(),
>> > but tx completion will call netdev_tx_completed_queue() -> crash
>> >
>> > Do we have one test to validate that a XDP_TX implementation is actually
>> > correct ?
>> >
>> Obviously not for e1000 :-(. We really need some real test and
>> performance results and analysis on the interaction between the stack
>> data path and XDP data path.
>
> no. we don't need it for e1k and we cannot really do it.
> <broken record mode on> this patch is for debugging of xdp programs only.
>
You can say this "only for a debugging" a thousand times and that
still won't justify putting bad code into the kernel. Material issues
have been raised with these patches, I have proposed a fix for one
core issue, and we have requested a lot more testing. So, please, if
you really want to move these patches forward start addressing the
concerns being raised by reviewers.

Tom

>> The fact that these changes are being
>> passed of as something only needed for KCM is irrelevant, e1000 is a
>> well deployed a NIC and there's no restriction that I see that would
>> prevent any users from enabling this feature on real devices.
>
> e1k is not even manufactured any more. Probably the only place
> where it can be found is computer history museum.
> e1000e fairs slightly better, but it's a different nic and this
> patch is not about it.
>