netdev - Re: [PATCH v3 bpf-next 2/2] net: xdp: introduce xdp_prepare

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Date:   Tue, 15 Dec 2020 15:51:07 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Lorenzo Bianconi <lorenzo.bianconi@...hat.com>,
        Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Cc:     Lorenzo Bianconi <lorenzo@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org,
        ast@...nel.org, brouer@...hat.com, alexander.duyck@...il.com,
        saeed@...nel.org
Subject: Re: [PATCH v3 bpf-next 2/2] net: xdp: introduce xdp_prepare_buff
 utility routine

On 12/15/20 2:47 PM, Lorenzo Bianconi wrote:
[...]
>>> diff --git a/drivers/net/xen-netfront.c b/drivers/net/xen-netfront.c
>>> index 329397c60d84..61d3f5f8b7f3 100644
>>> --- a/drivers/net/xen-netfront.c
>>> +++ b/drivers/net/xen-netfront.c
>>> @@ -866,10 +866,8 @@ static u32 xennet_run_xdp(struct netfront_queue *queue, struct page *pdata,
>>>   
>>>   	xdp_init_buff(xdp, XEN_PAGE_SIZE - XDP_PACKET_HEADROOM,
>>>   		      &queue->xdp_rxq);
>>> -	xdp->data_hard_start = page_address(pdata);
>>> -	xdp->data = xdp->data_hard_start + XDP_PACKET_HEADROOM;
>>> +	xdp_prepare_buff(xdp, page_address(pdata), XDP_PACKET_HEADROOM, len);
>>>   	xdp_set_data_meta_invalid(xdp);
>>> -	xdp->data_end = xdp->data + len;
>>>   
>>>   	act = bpf_prog_run_xdp(prog, xdp);
>>>   	switch (act) {
>>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>>> index 3fb3a9aa1b71..66d8a4b317a3 100644
>>> --- a/include/net/xdp.h
>>> +++ b/include/net/xdp.h
>>> @@ -83,6 +83,18 @@ xdp_init_buff(struct xdp_buff *xdp, u32 frame_sz, struct xdp_rxq_info *rxq)
>>>   	xdp->rxq = rxq;
>>>   }
>>>   
>>> +static inline void

nit: maybe __always_inline

>>> +xdp_prepare_buff(struct xdp_buff *xdp, unsigned char *hard_start,
>>> +		 int headroom, int data_len)
>>> +{
>>> +	unsigned char *data = hard_start + headroom;
>>> +
>>> +	xdp->data_hard_start = hard_start;
>>> +	xdp->data = data;
>>> +	xdp->data_end = data + data_len;
>>> +	xdp->data_meta = data;
>>> +}
>>> +
>>>   /* Reserve memory area at end-of data area.
>>>    *

For the drivers with xdp_set_data_meta_invalid(), we're basically setting xdp->data_meta
twice unless compiler is smart enough to optimize the first one away (did you double check?).
Given this is supposed to be a cleanup, why not integrate this logic as well so the
xdp_set_data_meta_invalid() doesn't get extra treatment?

Thanks,
Daniel