netdev - Re: [PATCH v17 bpf-next 13/23] bpf: add multi-buffer support to xdp copy helpers

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Date:   Thu, 11 Nov 2021 08:50:46 +0100
From:   Eelco Chaudron <echaudro@...hat.com>
To:     Lorenzo Bianconi <lorenzo@...nel.org>
Cc:     Jakub Kicinski <kuba@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, lorenzo.bianconi@...hat.com,
        davem@...emloft.net, ast@...nel.org, daniel@...earbox.net,
        shayagr@...zon.com, john.fastabend@...il.com, dsahern@...nel.org,
        brouer@...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 v17 bpf-next 13/23] bpf: add multi-buffer support to xdp copy helpers



On 8 Nov 2021, at 14:59, Lorenzo Bianconi wrote:

>> On Thu,  4 Nov 2021 18:35:33 +0100 Lorenzo Bianconi wrote:
>>> -static unsigned long bpf_xdp_copy(void *dst_buff, const void *src_buff,
>>> +static unsigned long bpf_xdp_copy(void *dst_buff, const void *ctx,
>>>  				  unsigned long off, unsigned long len)
>>>  {
>>> -	memcpy(dst_buff, src_buff + off, len);
>>> +	unsigned long base_len, copy_len, frag_off_total;
>>> +	struct xdp_buff *xdp = (struct xdp_buff *)ctx;
>>> +	struct skb_shared_info *sinfo;
>>> +	int i;
>>> +
>>> +	if (likely(!xdp_buff_is_mb(xdp))) {
>>
>> Would it be better to do
>>
>> 	if (xdp->data_end - xdp->data >= off + len)
>>
>> ?
>
> Hi Jakub,
>
> I am fine with the patch (just a typo inline), thx :)
> I will let Eelco to comment since he wrote the original code.
> If there is no objections, I will integrate it in v18.

Sorry for the late response, but both suggestions look fine to me.

>>
>>> +		memcpy(dst_buff, xdp->data + off, len);
>>> +		return 0;
>>> +	}
>>> +
>>> +	base_len = xdp->data_end - xdp->data;
>>> +	frag_off_total = base_len;
>>> +	sinfo = xdp_get_shared_info_from_buff(xdp);
>>> +
>>> +	/* If we need to copy data from the base buffer do it */
>>> +	if (off < base_len) {
>>> +		copy_len = min(len, base_len - off);
>>> +		memcpy(dst_buff, xdp->data + off, copy_len);
>>> +
>>> +		off += copy_len;
>>> +		len -= copy_len;
>>> +		dst_buff += copy_len;
>>> +	}
>>> +
>>> +	/* Copy any remaining data from the fragments */
>>> +	for (i = 0; len && i < sinfo->nr_frags; i++) {
>>> +		skb_frag_t *frag = &sinfo->frags[i];
>>> +		unsigned long frag_len, frag_off;
>>> +
>>> +		frag_len = skb_frag_size(frag);
>>> +		frag_off = off - frag_off_total;
>>> +		if (frag_off < frag_len) {
>>> +			copy_len = min(len, frag_len - frag_off);
>>> +			memcpy(dst_buff,
>>> +			       skb_frag_address(frag) + frag_off, copy_len);
>>> +
>>> +			off += copy_len;
>>> +			len -= copy_len;
>>> +			dst_buff += copy_len;
>>> +		}
>>> +		frag_off_total += frag_len;
>>> +	}
>>> +
>>
>> nit: can't help but feel that you can merge base copy and frag copy:
>>
>> 	sinfo = xdp_get_shared_info_from_buff(xdp);
>> 	next_frag = &sinfo->frags[0];
>> 	end_frag = &sinfo->frags[sinfo->nr_frags];
>>
>> 	ptr_off = 0;
>> 	ptr_buf = xdp->data;
>> 	ptr_len = xdp->data_end - xdp->data;
>>
>> 	while (true) {
>> 		if (off < ptr_off + ptr_len) {
>> 			copy_off = ptr_off - off;
>
> I guess here should be:
> 			copy_off = off - ptr_off;
>
>> 			copy_len = min(len, ptr_len - copy_off);
>> 			memcpy(dst_buff, ptr_buf + copy_off, copy_len);
>>
>> 			off += copy_len;
>> 			len -= copy_len;
>> 			dst_buff += copy_len;
>> 		}
>>
>> 		if (!len || next_frag == end_frag)
>> 			break;
>> 	
>> 		ptr_off += ptr_len;
>> 		ptr_buf = skb_frag_address(next_frag);
>> 		ptr_len = skb_frag_size(next_frag);
>> 		next_frag++;
>> 	}
>>
>> Up to you.