[xdp-hints] Re: [PATCH RESEND bpf-next 14/15] net, xdp: allow metadata > 32

[Daniel Borkmann <daniel@iogearbox.net>]

On 5/22/23 5:28 PM, Alexander Lobakin wrote:
> From: Jesper Dangaard Brouer <jbrouer@redhat.com>
> Date: Mon, 22 May 2023 13:41:43 +0200
>> On 19/05/2023 18.35, Alexander Lobakin wrote:
>>> From: Jesper Dangaard Brouer <jbrouer@redhat.com>
>>> Date: Tue, 16 May 2023 17:35:27 +0200
> 
> [...]
> 
>> Not talking about your changes (in this patch).
>>
>> I'm realizing that SKBs using metadata area will have a performance hit
>> due to accessing another cacheline (the meta_len in skb_shared_info).
>>
>> IIRC Daniel complained about this performance hit (in the past), I guess
>> this explains it.  IIRC Cilium changed to use percpu variables/datastore
>> to workaround this.
> 
> Why should we compare metadata of skbs on GRO anyway? I was disabling it
> the old hints series (conditionally, if driver asks), moreover...
> ...if metadata contains full checksum, GRO will be broken completely due
> to this comparison (or any other frame-unique fields. VLAN tags and
> hashes are okay).

This is when BPF prog on XDP populates metadata with custom data when it
wants to transfer information from XDP to skb aka tc BPF prog side. percpu
data store may not work here as it is not guaranteed that skb might end up
on same CPU.

>>> The whole xdp_metalen_invalid() gets expanded into:
>>>
>>>      return (metalen % 4) || metalen > 255;
>>>
>>> at compile-time. All those typeof shenanigans are only to not open-code
>>> meta_len's type/size/max.
>>>
>>>>
>>>> But only use for SKBs that gets created from xdp with metadata, right?
>>>>
>>
>> Normal netstack processing actually access this skb_shinfo->meta_len in
>> gro_list_prepare().  As the caller dev_gro_receive() later access other
>> memory in skb_shared_info, then the GRO code path already takes this hit
>> to begin with.
> 
> You access skb_shinfo() often even before running XDP program, for
> example, when a frame is multi-buffer. Plus HW timestamps are also
> there, and so on.