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Date:   Tue, 19 Feb 2019 17:18:30 +0100
From:   Daniel Borkmann <daniel@...earbox.net>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, Daniel Borkmann <borkmann@...earbox.net>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH bpf-next] bpf: add skb->queue_mapping write access from tc
 clsact

On 02/19/2019 03:52 PM, Jesper Dangaard Brouer wrote:
> On Tue, 19 Feb 2019 12:46:57 +0100
> Daniel Borkmann <daniel@...earbox.net> wrote:
> 
>> On 02/19/2019 11:24 AM, Jesper Dangaard Brouer wrote:
>>> The skb->queue_mapping already have read access, via __sk_buff->queue_mapping.
>>>
>>> This patch allow BPF tc qdisc clsact write access to the queue_mapping via
>>> tc_cls_act_is_valid_access.
>>>
>>> It is already possible to change this via TC filter action skbedit
>>> tc-skbedit(8).  Due to the lack of TC examples, lets show one:
>>>
>>>  # tc qdisc  add  dev ixgbe1 handle ffff: ingress
>>>  # tc filter add  dev ixgbe1 parent ffff: matchall action skbedit queue_mapping 5
>>>  # tc filter list dev ixgbe1 parent ffff:  
>>
>> Using handles was in the old days, if we add examples, then lets do
>> something more user friendly ;)
>>
>>   # tc qdisc  add     dev ixgbe1 clsact
>>   # tc filter replace dev ixgbe1 ingress matchall action skbedit queue_mapping 5
>>   # tc filter list    dev ixgbe1 ingress
>>
>>> The most common mistake is that XPS (Transmit Packet Steering) takes
>>> precedence over setting skb->queue_mapping. XPS is configured per DEVICE
>>> via /sys/class/net/DEVICE/queues/tx-*/xps_cpus via a CPU hex mask. To
>>> disable set mask=00.
>>>
>>> The purpose of changing skb->queue_mapping is to influence the selection of
>>> the net_device "txq" (struct netdev_queue), which influence selection of
>>> the qdisc "root_lock" (via txq->qdisc->q.lock) and txq->_xmit_lock. When
>>> using the MQ qdisc the txq->qdisc points to different qdiscs and associated
>>> locks, and HARD_TX_LOCK (txq->_xmit_lock), allowing for CPU scalability.
>>>
>>> Due to lack of TC examples, lets show howto attach clsact BPF programs:
>>>
>>>  # tc qdisc  add     dev ixgbe2 clsact
>>>  # tc filter replace dev ixgbe2 egress bpf da obj XXX_kern.o sec tc_qmap2cpu
>>>  # tc filter list    dev ixgbe2 egress
>>>
>>> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
>>> ---
>>>  net/core/filter.c |   14 +++++++++++---
>>>  1 file changed, 11 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/net/core/filter.c b/net/core/filter.c
>>> index 353735575204..d05ae8d05397 100644
>>> --- a/net/core/filter.c
>>> +++ b/net/core/filter.c
>>> @@ -6238,6 +6238,7 @@ static bool tc_cls_act_is_valid_access(int off, int size,
>>>  		case bpf_ctx_range(struct __sk_buff, tc_classid):
>>>  		case bpf_ctx_range_till(struct __sk_buff, cb[0], cb[4]):
>>>  		case bpf_ctx_range(struct __sk_buff, tstamp):
>>> +		case bpf_ctx_range(struct __sk_buff, queue_mapping):
>>>  			break;
>>>  		default:
>>>  			return false;
>>> @@ -6642,9 +6643,16 @@ static u32 bpf_convert_ctx_access(enum bpf_access_type type,
>>>  		break;
>>>  
>>>  	case offsetof(struct __sk_buff, queue_mapping):
>>> -		*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
>>> -				      bpf_target_off(struct sk_buff, queue_mapping, 2,
>>> -						     target_size));
>>> +		if (type == BPF_WRITE)
>>> +			*insn++ = BPF_STX_MEM(BPF_H, si->dst_reg, si->src_reg,
>>> +					      bpf_target_off(struct sk_buff,
>>> +							     queue_mapping,
>>> +							     2, target_size));
>>> +		else
>>> +			*insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
>>> +					      bpf_target_off(struct sk_buff,
>>> +							     queue_mapping,
>>> +							     2, target_size));  
>>
>> One thing we should avoid would be to allow user to write NO_QUEUE_MAPPING
>> into skb->queue_mapping so we don't hit the warn in sk_tx_queue_set(), I'd
>> add this into the ctx rewrite here.
> 
> Makes sense. I would really appreciate if you could help me out writing
> the needed BPF instructions, as I'm not an expert here.

Untested / uncompiled, but should be:

        case offsetof(struct __sk_buff, queue_mapping):
                if (type == BPF_WRITE) {
                        *insn++ = BPF_JMP_IMM(BPF_JGE, si->src_reg, NO_QUEUE_MAPPING, 1);
                        *insn++ = BPF_STX_MEM(BPF_H, si->dst_reg, si->src_reg,
                                              bpf_target_off(struct sk_buff, queue_mapping, 2,
                                                             target_size));
                } else {
                        *insn++ = BPF_LDX_MEM(BPF_H, si->dst_reg, si->src_reg,
                                              bpf_target_off(struct sk_buff, queue_mapping, 2,
                                                             target_size));
                }
                break;

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