Date:	Wed, 24 Feb 2016 09:40:57 +0100
From:	Jiri Pirko <jiri@...nulli.us>
To:	"Amir Vadai\"" <amir@...ai.me>
Cc:	John Fastabend <john.fastabend@...il.com>, daniel@...earbox.net,
	netdev@...r.kernel.org, alexei.starovoitov@...il.com,
	davem@...emloft.net, jhs@...atatu.com
Subject: Re: [net-next PATCH 3/4] net: sched: cls_u32 add bit to specify
 software only rules

Wed, Feb 24, 2016 at 09:04:40AM CET, amir@...ai.me wrote:
>On Tue, Feb 23, 2016 at 11:03:21AM -0800, John Fastabend wrote:
>> In the initial implementation the only way to stop a rule from being
>> inserted into the hardware table was via the device feature flag.
>> However this doesn't work well when working on an end host system
>> where packets are expect to hit both the hardware and software
>> datapaths.
>> 
>> For example we can imagine a rule that will match an IP address and
>> increment a field. If we install this rule in both hardware and
>> software we may increment the field twice. To date we have only
>> added support for the drop action so we have been able to ignore
>> these cases. But as we extend the action support we will hit this
>> example plus more such cases. Arguably these are not even corner
>> cases in many working systems these cases will be common.
>> 
>> To avoid forcing the driver to always abort (i.e. the above example)
>> this patch adds a flag to add a rule in software only. A careful
>> user can use this flag to build software and hardware datapaths
>> that work together. One example we have found particularly useful
>> is to use hardware resources to set the skb->mark on the skb when
>> the match may be expensive to run in software but a mark lookup
>> in a hash table is cheap. The idea here is hardware can do in one
>> lookup what the u32 classifier may need to traverse multiple lists
>> and hash tables to compute. The flag is only passed down on inserts
>> on deletion to avoid stale references in hardware we always try
>> to remove a rule if it exists.
>> 
>> Notice we do not add a hardware only case here. If you were to
>> add a hardware only case then you are stuck with the problem
>> of where to stick the software representation of that filter
>> rule. If its stuck on the same filter list as the software only and
>> software/hardware rules it then has to be walked over and ignored
>> in the classify path. The overhead is not huge but is measurable.
>> And with so much work being invested in speeding up rx/tx of
>> pkt processing this is unacceptable IMO. The other option is to
>> have a special hook just for hardware only resources. This is
>> implemented in the next patch.
>> 
>> Signed-off-by: John Fastabend <john.r.fastabend@...el.com>
>
>[...]
>
>>  
>> -static bool u32_should_offload(struct net_device *dev)
>> +static bool u32_should_offload(struct net_device *dev, u32 flags)
>>  {
>>  	if (!(dev->features & NETIF_F_HW_TC))
>>  		return false;
>>  
>> -	return dev->netdev_ops->ndo_setup_tc;
>> +	if (flags & TCA_U32_FLAGS_SOFTWARE)
>> +		return false;
>> +
>> +	if (!dev->netdev_ops->ndo_setup_tc)
>> +		return false;
>> +
>> +	return true;
>>  }
>This function and flag should be a generic filter attribute - not just
>u32.

I agree, this should be generic.

Regarding flags attr, we have the same situation as with other common
attrs:
TCA_U32_POLICE
TCA_FLOW_POLICE
TCA_CGROUP_POLICE
TCA_BPF_POLICE

TCA_U32_ACT
TCA_FLOW_ACT
TCA_CGROUP_ACT
TCA_BPF_ACT
TCA_FLOWER_ACT

I guess we have no other choice then to have
TCA_U32_FLAGS
TCA_FLOWER_FLAGS etc :(

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