netdev - Re: (Lack of) specification for RX n-tuple filtering

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Date:	Wed, 08 Dec 2010 17:29:24 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	David Miller <davem@...emloft.net>
Cc:	vladz@...adcom.com, dm@...lsio.com,
	peter.p.waskiewicz.jr@...el.com, netdev@...r.kernel.org
Subject: Re: (Lack of) specification for RX n-tuple filtering

On Wed, 2010-12-08 at 08:39 -0800, David Miller wrote:
> From: "Vladislav Zolotarov" <vladz@...adcom.com>
> Date: Wed, 8 Dec 2010 18:24:03 +0200
> 
> > I also agree with Dimitris: what we have here is an offload of some
> > Netfilter functionality to HW. Regardless the HW implementation (TCAM or
> > not) if it's allowed to configure more than one rule for the same
> > protocol the ordering of filtering rules is important: for instance if u
> > change the order of applying the rules in the example below the result
> > of the filtering for the traffic with both VLAN 4 and destination port
> > 3000 will be different.
> 
> It's not the same, this whole ordering thing you expect in netfilter
> land is simply not present in these hardware implementations.
> 
> The hardware does a parallel TCAM match lookup, and whatever matches
> is used.

I think the match with the lowest index wins, which is why it's possible
to specify the rule's index (location) with ETHTOOL_SRXCLSRLINS and why
Peter defined new commands without that for use with the ixgbe driver.

> Some hardware does link-level protocol lookups first, then L3/L4 later
> in the RX path right before computing the hash and selecting an RX
> queue.
>
> There really is no ordering available, so let's not pretend it can be
> used "just like" netfilter rules.
> 
> As per the difference between the various ethtool facilities, this
> just represents the fact that whats available to offload differs
> per device.  The best we can do is encapsulate commonality as best
> as we can, but each interface essentially represents what one
> major chipset provides.

I think the interfaces are actually somewhat more flexible than any of
the current implementations.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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