netdev - Re: [PATCH RFC-v5 bpf-next 09/12] dev: Support xdp in the Tx path for xdp

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Date:   Thu, 16 Apr 2020 17:50:58 -0600
From:   David Ahern <dsahern@...il.com>
To:     Toke Høiland-Jørgensen <toke@...hat.com>,
        David Ahern <dsahern@...nel.org>, netdev@...r.kernel.org
Cc:     davem@...emloft.net, kuba@...nel.org,
        prashantbhole.linux@...il.com, jasowang@...hat.com,
        brouer@...hat.com, toshiaki.makita1@...il.com,
        daniel@...earbox.net, john.fastabend@...il.com, ast@...nel.org,
        kafai@...com, songliubraving@...com, yhs@...com, andriin@...com,
        David Ahern <dahern@...italocean.com>
Subject: Re: [PATCH RFC-v5 bpf-next 09/12] dev: Support xdp in the Tx path for
 xdp_frames

On 4/16/20 8:02 AM, Toke Høiland-Jørgensen wrote:
>> diff --git a/kernel/bpf/devmap.c b/kernel/bpf/devmap.c
>> index 58bdca5d978a..bedecd07d898 100644
>> --- a/kernel/bpf/devmap.c
>> +++ b/kernel/bpf/devmap.c
>> @@ -322,24 +322,33 @@ static int bq_xmit_all(struct xdp_dev_bulk_queue *bq, u32 flags)
>>  {
>>  	struct net_device *dev = bq->dev;
>>  	int sent = 0, drops = 0, err = 0;
>> +	unsigned int count = bq->count;
>>  	int i;
>>  
>> -	if (unlikely(!bq->count))
>> +	if (unlikely(!count))
>>  		return 0;
>>  
>> -	for (i = 0; i < bq->count; i++) {
>> +	for (i = 0; i < count; i++) {
>>  		struct xdp_frame *xdpf = bq->q[i];
>>  
>>  		prefetch(xdpf);
>>  	}
>>  
>> -	sent = dev->netdev_ops->ndo_xdp_xmit(dev, bq->count, bq->q, flags);
>> +	if (static_branch_unlikely(&xdp_egress_needed_key)) {
>> +		count = do_xdp_egress_frame(dev, bq->q, &count);
> 
> nit: seems a bit odd to pass the point to count, then reassign it with
> the return value?

thanks for noticing that. leftover from the evolution of this. changed to
		count = do_xdp_egress_frame(dev, bq->q, count);


>> diff --git a/net/core/dev.c b/net/core/dev.c
>> index 1bbaeb8842ed..f23dc6043329 100644
>> --- a/net/core/dev.c
>> +++ b/net/core/dev.c
>> @@ -4720,6 +4720,76 @@ u32 do_xdp_egress_skb(struct net_device *dev, struct sk_buff *skb)
>>  }
>>  EXPORT_SYMBOL_GPL(do_xdp_egress_skb);
>>  
>> +static u32 __xdp_egress_frame(struct net_device *dev,
>> +			      struct bpf_prog *xdp_prog,
>> +			      struct xdp_frame *xdp_frame,
>> +			      struct xdp_txq_info *txq)
>> +{
>> +	struct xdp_buff xdp;
>> +	u32 act;
>> +
>> +	xdp.data_hard_start = xdp_frame->data - xdp_frame->headroom;
>> +	xdp.data = xdp_frame->data;
>> +	xdp.data_end = xdp.data + xdp_frame->len;
>> +	xdp_set_data_meta_invalid(&xdp);
> 
> Why invalidate the metadata? On the contrary we'd want metadata from the
> RX side to survive, wouldn't we?

right, replaced with:
	xdp.data_meta = xdp.data - metasize;

> 
>> +	xdp.txq = txq;
>> +
>> +	act = bpf_prog_run_xdp(xdp_prog, &xdp);
>> +	act = handle_xdp_egress_act(act, dev, xdp_prog);
>> +
>> +	/* if not dropping frame, readjust pointers in case
>> +	 * program made changes to the buffer
>> +	 */
>> +	if (act != XDP_DROP) {
>> +		int headroom = xdp.data - xdp.data_hard_start;
>> +		int metasize = xdp.data - xdp.data_meta;
>> +
>> +		metasize = metasize > 0 ? metasize : 0;
>> +		if (unlikely((headroom - metasize) < sizeof(*xdp_frame)))
>> +			return XDP_DROP;
>> +
>> +		xdp_frame = xdp.data_hard_start;
>> +		xdp_frame->data = xdp.data;
>> +		xdp_frame->len  = xdp.data_end - xdp.data;
>> +		xdp_frame->headroom = headroom - sizeof(*xdp_frame);
>> +		xdp_frame->metasize = metasize;
>> +		/* xdp_frame->mem is unchanged */
>> +	}
>> +
>> +	return act;
>> +}
>> +
>> +unsigned int do_xdp_egress_frame(struct net_device *dev,
>> +				 struct xdp_frame **frames,
>> +				 unsigned int *pcount)
>> +{
>> +	struct bpf_prog *xdp_prog;
>> +	unsigned int count = *pcount;
>> +
>> +	xdp_prog = rcu_dereference(dev->xdp_egress_prog);
>> +	if (xdp_prog) {
>> +		struct xdp_txq_info txq = { .dev = dev };
> 
> Do you have any thoughts on how to populate this for the redirect case?

not sure I understand. This is the redirect case. ie.., On rx a program
is run, XDP_REDIRECT is returned and the packet is queued. Once the
queue fills or flush is done, bq_xmit_all is called to send the frames.

> I guess using Magnus' HWQ abstraction when that lands? Or did you have
> something different in mind?
>