netdev - Re: [RFC PATCH bpf-next 05/12] xdp: add MEM_TYPE_ZERO

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Date:   Thu, 17 May 2018 09:08:15 +0200
From:   Björn Töpel <bjorn.topel@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Magnus Karlsson <magnus.karlsson@...il.com>,
        "Karlsson, Magnus" <magnus.karlsson@...el.com>,
        "Duyck, Alexander H" <alexander.h.duyck@...el.com>,
        Alexander Duyck <alexander.duyck@...il.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <ast@...com>,
        Willem de Bruijn <willemdebruijn.kernel@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Netdev <netdev@...r.kernel.org>,
        Björn Töpel <bjorn.topel@...el.com>,
        michael.lundkvist@...csson.com,
        "Brandeburg, Jesse" <jesse.brandeburg@...el.com>,
        "Singhai, Anjali" <anjali.singhai@...el.com>,
        "Zhang, Qi Z" <qi.z.zhang@...el.com>,
        intel-wired-lan <intel-wired-lan@...ts.osuosl.org>
Subject: Re: [RFC PATCH bpf-next 05/12] xdp: add MEM_TYPE_ZERO_COPY

2018-05-17 7:57 GMT+02:00 Jesper Dangaard Brouer <brouer@...hat.com>:
> On Tue, 15 May 2018 21:06:08 +0200
> Björn Töpel <bjorn.topel@...il.com> wrote:
>
>> @@ -82,6 +88,10 @@ struct xdp_frame *convert_to_xdp_frame(struct xdp_buff *xdp)
>>       int metasize;
>>       int headroom;
>>
>> +     // XXX implement clone, copy, use "native" MEM_TYPE
>> +     if (xdp->rxq->mem.type == MEM_TYPE_ZERO_COPY)
>> +             return NULL;
>> +
>
> There is going to be significant tradeoffs between AF_XDP zero-copy and
> copy-variant.  The copy-variant, still have very attractive
> RX-performance, and other benefits like no exposing unrelated packets
> to userspace (but limit these to the XDP filter).
>
> Thus, as a user I would like to choose between AF_XDP zero-copy and
> copy-variant. Even if my NIC support zero-copy, I can be interested in
> only enabling the copy-variant. This patchset doesn't let me choose.
>
> How do we expose this to userspace?
> (Maybe as simple as an sockaddr_xdp->sxdp_flags flag?)
>

We planned to add these flags later, but I think you're right that
it's better to do that right away.

If we try to follow the behavior of the XDP netlink interface: Pick
the "the best mode" when there are no flags. A user would like to
"force" a mode -- meaning that you select, say copy, and getting an
error if that's not supported. Four new flags?

diff --git a/include/uapi/linux/if_xdp.h b/include/uapi/linux/if_xdp.h
index 77b88c4efe98..ce1f710847b7 100644
--- a/include/uapi/linux/if_xdp.h
+++ b/include/uapi/linux/if_xdp.h
@@ -22,7 +22,11 @@
 #include <linux/types.h>

 /* Options for the sxdp_flags field */
-#define XDP_SHARED_UMEM 1
+#define XDP_SHARED_UMEM        (1U << 0)
+#define XDP_COPY_TX_UMEM    (1U << 1)
+#define XDP_ZEROCOPY_TX_UMEM    (1U << 2)
+#define XDP_COPY_RX_UMEM    (1U << 3)
+#define XDP_ZEROCOPY_RX_UMEM    (1U << 4)

 struct sockaddr_xdp {
     __u16 sxdp_family;

A better way?




> --
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer