Re: [PATCH net] veth: xdp: use head instead of hard_start

[maowenan <maowenan@huawei.com>]

On 2020/3/31 7:35, Toshiaki Makita wrote:
> Hi Mao & Jesper
> (Resending with plain text...)
> 
> On 2020/03/30 20:34, Jesper Dangaard Brouer wrote:
>> On Mon, 30 Mar 2020 18:26:31 +0800
>> Mao Wenan <maowenan@huawei.com> wrote:
>>
>>> xdp.data_hard_start is mapped to the first
>>> address of xdp_frame, but the pointer hard_start
>>> is the offset(sizeof(struct xdp_frame)) of xdp_frame,
>>> it should use head instead of hard_start to
>>> set xdp.data_hard_start. Otherwise, if BPF program
>>> calls helper_function such as bpf_xdp_adjust_head, it
>>> will be confused for xdp_frame_end.
>>
>> I have noticed this[1] and have a patch in my current patchset for
>> fixing this.  IMHO is is not so important fix right now, as the effect
>> is that you currently only lose 32 bytes of headroom.
>>
I consider that it is needed because bpf_xdp_adjust_head() just a common helper function,
veth as one driver application should keep the same as 32 bytes of headroom as other driver.
And convert_to_xdp_frame set() also store info in top of packet, and set:
	xdp_frame = xdp->data_hard_start;

>> [1] https://lore.kernel.org/netdev/158446621887.702578.17234304084556809684.stgit@firesoul/
> 
> You are right, the subtraction is not necessary here.
I guess you mean that previous subtraction is not necessary ? this line : void *head = hard_start - sizeof(struct xdp_frame); ?
But in the veth_xdp_rcv_one,below line will use head pointer,
case XDP_TX:
                        orig_frame = *frame;
                        xdp.data_hard_start = head;


> Thank you for working on this.
> 
> Toshiaki Makita
> 
> .