netdev - Re: [PATCH bpf-next 10/12] tun: run XDP program in tx path

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Date:   Fri, 24 Jan 2020 22:44:43 +0900
From:   Prashant Bhole <bholeprashant.oss@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        David Ahern <dsahern@...nel.org>
Cc:     netdev@...r.kernel.org, prashantbhole.linux@...il.com,
        jasowang@...hat.com, davem@...emloft.net,
        jakub.kicinski@...ronome.com, jbrouer@...hat.com, toke@...hat.com,
        toshiaki.makita1@...il.com, daniel@...earbox.net,
        john.fastabend@...il.com, ast@...nel.org, kafai@...com,
        songliubraving@...com, yhs@...com, andriin@...com,
        dsahern@...il.com
Subject: Re: [PATCH bpf-next 10/12] tun: run XDP program in tx path



On 1/23/2020 5:23 PM, Michael S. Tsirkin wrote:
> On Wed, Jan 22, 2020 at 06:42:08PM -0700, David Ahern wrote:
>> From: Prashant Bhole <prashantbhole.linux@...il.com>
>>
>> Run the XDP program as soon as packet is removed from the ptr
>> ring. Since this is XDP in tx path, the traditional handling of
>> XDP actions XDP_TX/REDIRECT isn't valid. For this reason we call
>> do_xdp_generic_core instead of do_xdp_generic. do_xdp_generic_core
>> just runs the program and leaves the action handling to us.
>>
>> Signed-off-by: Prashant Bhole <prashantbhole.linux@...il.com>
>> ---
>>   drivers/net/tun.c | 153 +++++++++++++++++++++++++++++++++++++++++++++-
>>   1 file changed, 150 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/net/tun.c b/drivers/net/tun.c

[...]

>> +static int tun_consume_packets(struct tun_file *tfile, void **ptr_array, int n)
>> +{
>> +	void **pkts = tfile->pkt_ptrs;
>> +	struct xdp_frame *frame;
>> +	struct tun_struct *tun;
>> +	int i, num_ptrs;
>> +	int pkt_cnt = 0;
>> +	void *ptr;
>> +	u32 act;
>> +	int batchsz;
>> +
>> +	if (unlikely(!tfile))
>> +		return 0;
>> +
>> +	rcu_read_lock();
>> +	tun = rcu_dereference(tfile->tun);
>> +	if (unlikely(!tun)) {
>> +		rcu_read_unlock();
>> +		return 0;
>> +	}
>> +
>> +	while (n) {
>> +		batchsz = (n > MAX_TAP_BATCH) ? MAX_TAP_BATCH : n;
>> +		n -= batchsz;
>> +		num_ptrs = ptr_ring_consume_batched(&tfile->tx_ring, pkts,
>> +						    batchsz);
>> +		if (!num_ptrs)
>> +			break;
> 
> Can't we avoid looping over the packets in the current case
> where there are no xdp programs at all?

That's doable. Thanks.

Prashant