netdev - Re: [PATCH net v2] switchdev: don't abort hardware ipv4 fib offload on failure to program fib entry in hardware

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Message-ID: <20150528094244.GA19629@nanopsycho.orion>
Date:	Thu, 28 May 2015 11:42:44 +0200
From:	Jiri Pirko <jiri@...nulli.us>
To:	David Miller <davem@...emloft.net>
Cc:	roopa@...ulusnetworks.com, sfeldma@...il.com,
	john.fastabend@...il.com, netdev@...r.kernel.org,
	andy@...yhouse.net
Subject: Re: [PATCH net v2] switchdev: don't abort hardware ipv4 fib offload
 on failure to program fib entry in hardware

Mon, May 18, 2015 at 10:19:16PM CEST, davem@...emloft.net wrote:
>From: Roopa Prabhu <roopa@...ulusnetworks.com>
>Date: Sun, 17 May 2015 16:42:05 -0700
>
>> On most systems where you can offload routes to hardware,
>> doing routing in software is not an option (the cpu limitations
>> make routing impossible in software).
>
>You absolutely do not get to determine this policy, none of us
>do.
>
>What matters is that by default the damn switch device being there
>is %100 transparent to the user.
>
>And the way to achieve that default is to do software routes as
>a fallback.
>
>I am not going to entertain changes of this nature which fail
>route loading by default just because we've exceeded a device's
>HW capacity to offload.
>
>I thought I was _really_ clear about this at netdev 0.1

I certainly agree that by default, transparency 1:1 sw:hw mapping is
what we need for fib. The current code is a good start!

I see couple of issues regarding switchdev_fib_ipv4_abort:
1) If user adds and entry, switchdev_fib_ipv4_add fails, abort is
   executed -> and, error returned. I would expect that route entry should
   be added in this case. The next attempt of adding the same entry will
   be successful.
   The current behaviour breaks the transparency you are reffering to.
2) When switchdev_fib_ipv4_abort happens to be executed, the offload is
   disabled for good (until reboot). That is certainly not nice, alhough
   I understand that is the easiest solution for now.

I believe that we all agree that the 1:1 transparency, although it is a
default, may not be optimal for real-life usage. HW resources are
limited and user does not know them. The danger of hitting _abort and
screwing-up the whole system is huge, unacceptable.

So here, there are couple of more or less simple things that I suggest to
do in order to move a little bit forward:
1) Introduce system-wide option to switch _abort to just plain fail.
   When HW does not have capacity, do not flush and fallback to sw, but
   rather just fail to add the entry. This would not break anything.
   Userspace has to be prepared that entry add could fail.
2) Introduce a way to propagate resources to userspace. Driver knows about
   resources used/available/potentially_available. Switchdev infra could
   be extended in order to propagate the info to the user.
3) Introduce couple of flags for entry add that would alter the default
   behaviour. Something like:
   	NLM_F_SKIP_KERNEL
   	NLM_F_SKIP_OFFLOAD
   Again, this does not break the current users. On the other hand, this
   gives new users a leverage to instruct kernel where the entry should
   be added to (or not added to).

Any thoughts? Objections?

Thanks!

Jiri
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