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Date: Mon, 19 Aug 2013 15:38:13 -0700
From: Peter Grehan <grehan@freebsd.org>
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To: Andre Oppermann <andre@freebsd.org>
Subject: M_NOFREE removal (was Re: svn commit: r254520 - in head/sys: kern sys)
References: <201308191116.r7JBGsc6065793@svn.freebsd.org>
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Cc: svn-src-head@freebsd.org, freebsd-net@freebsd.org,
 Navdeep Parhar <np@FreeBSD.org>
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Hi Andre,

  (moving to the more appropriate freebsd-net)

> I'm sorry for ambushing but this stuff has to be done.  I have provided
> an alternative way of handling it and I'm happy to help you with your
> use case to make it good for you and to prevent the mbuf system from
> getting bloated and hackish again.

  Sure. I'm not really upset since my code wasn't too far along, but 
with any API, you never know who consumers might be so it's always worth 
being proactive about announcing it's removal.

> Can you please describe your intended use of M_NOFREE to better understand
> the shortcomings of the current mbuf systems and the additional advantages
> of the M_NOFREE case?

  I was looking at something similar to Linux's vhost-net, where a 
guest's virtio ring would be processed in-kernel. An mbuf chain with 
external buffers would be used to pass guest tx buffer/len segments 
directly into FreeBSD drivers.

  The intent of M_NOFREE was to avoid small mbuf allocations/frees in 
what is a hot path. This code was intended to run at 10/40G.

  Note this code isn't really generic - it would require interfaces to 
be 'owned' by the guest, except that direct PCI-level pass-through 
wouldn't be needed.

  If there's an alternative to M_NOFREE, I'd be more than happy to use that.

later,

Peter.