netdev - RE: [PATCH v20 bpf-next 00/23] mvneta: introduce XDP multi-buffer support

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Message-ID: <61b3edf34d399_2c40320815@john.notmuch>
Date:   Fri, 10 Dec 2021 16:16:51 -0800
From:   John Fastabend <john.fastabend@...il.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org
Cc:     lorenzo.bianconi@...hat.com, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, daniel@...earbox.net, shayagr@...zon.com,
        john.fastabend@...il.com, dsahern@...nel.org, brouer@...hat.com,
        echaudro@...hat.com, jasowang@...hat.com,
        alexander.duyck@...il.com, saeed@...nel.org,
        maciej.fijalkowski@...el.com, magnus.karlsson@...el.com,
        tirthendu.sarkar@...el.com, toke@...hat.com
Subject: RE: [PATCH v20 bpf-next 00/23] mvneta: introduce XDP multi-buffer
 support

Lorenzo Bianconi wrote:
> This series introduce XDP multi-buffer support. The mvneta driver is
> the first to support these new "non-linear" xdp_{buff,frame}. Reviewers
> please focus on how these new types of xdp_{buff,frame} packets
> traverse the different layers and the layout design. It is on purpose
> that BPF-helpers are kept simple, as we don't want to expose the
> internal layout to allow later changes.
> 
> The main idea for the new multi-buffer layout is to reuse the same
> structure used for non-linear SKB. This rely on the "skb_shared_info"
> struct at the end of the first buffer to link together subsequent
> buffers. Keeping the layout compatible with SKBs is also done to ease
> and speedup creating a SKB from an xdp_{buff,frame}.
> Converting xdp_frame to SKB and deliver it to the network stack is shown
> in patch 05/18 (e.g. cpumaps).
> 
> A multi-buffer bit (mb) has been introduced in the flags field of xdp_{buff,frame}
> structure to notify the bpf/network layer if this is a xdp multi-buffer frame
> (mb = 1) or not (mb = 0).
> The mb bit will be set by a xdp multi-buffer capable driver only for
> non-linear frames maintaining the capability to receive linear frames
> without any extra cost since the skb_shared_info structure at the end
> of the first buffer will be initialized only if mb is set.
> Moreover the flags field in xdp_{buff,frame} will be reused even for
> xdp rx csum offloading in future series.
> 
> Typical use cases for this series are:
> - Jumbo-frames
> - Packet header split (please see Google���s use-case @ NetDevConf 0x14, [0])
> - TSO/GRO for XDP_REDIRECT
> 
> The three following ebpf helpers (and related selftests) has been introduced:
> - bpf_xdp_load_bytes:
>   This helper is provided as an easy way to load data from a xdp buffer. It
>   can be used to load len bytes from offset from the frame associated to
>   xdp_md, into the buffer pointed by buf.
> - bpf_xdp_store_bytes:
>   Store len bytes from buffer buf into the frame associated to xdp_md, at
>   offset.
> - bpf_xdp_get_buff_len:
>   Return the total frame size (linear + paged parts)
> 
> bpf_xdp_adjust_tail and bpf_xdp_copy helpers have been modified to take into
> account xdp multi-buff frames.
> Moreover, similar to skb_header_pointer, we introduced bpf_xdp_pointer utility
> routine to return a pointer to a given position in the xdp_buff if the
> requested area (offset + len) is contained in a contiguous memory area
> otherwise it must be copied in a bounce buffer provided by the caller running
> bpf_xdp_copy_buf().
> 
> BPF_F_XDP_MB flag for bpf_attr has been introduced to notify the kernel the
> eBPF program fully support xdp multi-buffer.
> SEC("xdp_mb/"), SEC_DEF("xdp_devmap_mb/") and SEC_DEF("xdp_cpumap_mb/" have been
> introduced to declare xdp multi-buffer support.
> The NIC driver is expected to reject an eBPF program if it is running in XDP
> multi-buffer mode and the program does not support XDP multi-buffer.
> In the same way it is not possible to mix xdp multi-buffer and xdp legacy
> programs in a CPUMAP/DEVMAP or tailcall a xdp multi-buffer/legacy program from
> a legacy/multi-buff one.
> 
> More info about the main idea behind this approach can be found here [1][2].

Thanks for sticking with this.

OK for the series, I really want to see this on some other hardware though,
preferably 40Gbps or more ASAP...

Acked-by: John Fastabend <john.fastabend@...il.com>