RE: [PATCH bpf-next] xsk: build skb by page

* RE: [PATCH bpf-next] xsk: build skb by page
       [not found] <579fa463bba42ac71591540a1811dca41d725350.1610764948.git.xuanzhuo@linux.alibaba.com>
@ 2021-01-17 21:55 ` John Fastabend
  2021-01-18 11:57 ` Michael S. Tsirkin
  1 sibling, 0 replies; 2+ messages in thread
From: John Fastabend @ 2021-01-17 21:55 UTC (permalink / raw)
  To: Xuan Zhuo, netdev
  Cc: Miaohe Lin, Michal Kubecek, Song Liu, Michael S. Tsirkin,
	Alexander Lobakin, Alexei Starovoitov, virtualization,
	Meir Lichtinger, Andrew Lunn, Steffen Klassert, Xuan Zhuo,
	Florian Fainelli, Daniel Borkmann, Mauro Carvalho Chehab,
	John Fastabend, Andrii Nakryiko, Yonghong Song,
	Björn Töpel, Jesper Dangaard Brouer, KP Singh,
	Jakub Kicinski, Magnus Karlsson, Willem de Bruijn,
	Antoine Tenart, David S. Miller, Jonathan Lemon, bpf,
	Martin KaFai Lau

Xuan Zhuo wrote:
> This patch is used to construct skb based on page to save memory copy
> overhead.
> 
> This has one problem:
> 
> We construct the skb by fill the data page as a frag into the skb. In
> this way, the linear space is empty, and the header information is also
> in the frag, not in the linear space, which is not allowed for some
> network cards. For example, Mellanox Technologies MT27710 Family
> [ConnectX-4 Lx] will get the following error message:
> 
>     mlx5_core 0000:3b:00.1 eth1: Error cqe on cqn 0x817, ci 0x8, qn 0x1dbb, opcode 0xd, syndrome 0x1, vendor syndrome 0x68
>     00000000: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000020: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000030: 00 00 00 00 60 10 68 01 0a 00 1d bb 00 0f 9f d2
>     WQE DUMP: WQ size 1024 WQ cur size 0, WQE index 0xf, len: 64
>     00000000: 00 00 0f 0a 00 1d bb 03 00 00 00 08 00 00 00 00
>     00000010: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     00000020: 00 00 00 2b 00 08 00 00 00 00 00 05 9e e3 08 00
>     00000030: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>     mlx5_core 0000:3b:00.1 eth1: ERR CQE on SQ: 0x1dbb
> 
> I also tried to use build_skb to construct skb, but because of the
> existence of skb_shinfo, it must be behind the linear space, so this
> method is not working. We can't put skb_shinfo on desc->addr, it will be
> exposed to users, this is not safe.
> 
> Finally, I added a feature NETIF_F_SKB_NO_LINEAR to identify whether the
> network card supports the header information of the packet in the frag
> and not in the linear space.
> 
> ---------------- Performance Testing ------------
> 
> The test environment is Aliyun ECS server.
> Test cmd:
> ```
> xdpsock -i eth0 -t  -S -s <msg size>
> ```
> 
> Test result data:
> 
> size    64      512     1024    1500
> copy    1916747 1775988 1600203 1440054
> page    1974058 1953655 1945463 1904478
> percent 3.0%    10.0%   21.58%  32.3%

Looks like a good perf bump. Some easy suggestions below

> +static struct sk_buff *xsk_build_skb(struct xdp_sock *xs,
> +				     struct xdp_desc *desc, int *err)
> +{

Passing a 'int *err' here is ugly IMO use the ERR_PTR/PTR_ERR macros
and roll it into the return value.

or maybe use the out: pattern used in the kernel, but just doing direct
returns like now but with ERR_PTR() would also be fine.

> +	struct sk_buff *skb ;
        struct sk_buff *skb = NULL;
        err = -ENOMEM;
> +
> +	if (xs->dev->features & NETIF_F_SKB_NO_LINEAR) {
> +		skb = xsk_build_skb_zerocopy(xs, desc);
> +		if (unlikely(!skb)) {
			goto out

> +			*err = -ENOMEM;
> +			return NULL;
> +		}
> +	} else {
> +		char *buffer;
> +		u64 addr;
> +		u32 len;
> +		int err;
> +
> +		len = desc->len;
> +		skb = sock_alloc_send_skb(&xs->sk, len, 1, &err);
> +		if (unlikely(!skb)) {
			goto out;
> +			*err = -ENOMEM;
> +			return NULL;
> +		}
> +
> +		skb_put(skb, len);
> +		addr = desc->addr;
> +		buffer = xsk_buff_raw_get_data(xs->pool, desc->addr);
> +		err = skb_store_bits(skb, 0, buffer, len);
> +
> +		if (unlikely(err)) {
> +			kfree_skb(skb);

			err = -EINVAL;
			goto out

> +			*err = -EINVAL;
> +			return NULL;
> +		}
> +	}
> +
> +	skb->dev = xs->dev;
> +	skb->priority = xs->sk.sk_priority;
> +	skb->mark = xs->sk.sk_mark;
> +	skb_shinfo(skb)->destructor_arg = (void *)(long)desc->addr;
> +	skb->destructor = xsk_destruct_skb;
> +
> +	return skb;

out:
	kfree_skb(skb)
	return ERR_PTR(err);

> +}
> +

Otherwise looks good thanks.
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