netdev - Re: [PATCH RFC 0/3] kcm: Kernel Connection Multiplexor (KCM)

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Message-ID: <20150921212624.GD3861@oracle.com>
Date:	Mon, 21 Sep 2015 17:26:24 -0400
From:	Sowmini Varadhan <sowmini.varadhan@...cle.com>
To:	Tom Herbert <tom@...bertland.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Kernel Team <kernel-team@...com>
Subject: Re: [PATCH RFC 0/3] kcm: Kernel Connection Multiplexor (KCM)

On (09/21/15 10:33), Tom Herbert wrote:
> >
> > Some things that were not clear to me from the patch-set:
> >
> > The doc statses that we re-assemble packets the "stated length" -
> > but how will the receiver know the "stated length"?
> 
> BPF program returns the length of the next message. In my testing so
> far I've been using HTTP/2 which defines a frame format with first 3
> bytes being header length field . The BPF program (using LLVM/Clang--
> thanks Alexei!) is just:

Maybe I dont see something about the mux/demux here (I have to 
take a closer look at reserve_psock/unreserve_psock), but 
will every tcp segment have a 3 byte length in the payload?

Not every TCP segment in the RDS-TCP case will have a RDS header,
thus the comments before rds_send_xmit(), thus applying the bpf filter
to a TCP segment holding some "from-the-middle" piece of the RDS dgram
may not be possible 

> > the notes say one can "accept()" over a kcm socket- but "accept()"
> > is itself a connection-oriented concept- one does not accept() on
    :
> The accept method is overloaded on KCM sockets to do the socket
> cloning operation. This is unrelated to TCP semantics, connection
> management is performed on TCP sockets (i.e. before being attached to
> a KCM multiplexor).

If possible,it might be better to use some other
glibc-func/name/syscall/sockopt/whatever
for this, rather than overloading accept().. feels like that would
keep the semantics cleaner, and probably less likely to trip 
up on accept code in the kernel..

--Sowmini
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