netdev - Re: [PATCH] IB/ipoib: move back the IB LL address into the hard header

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Date:   Tue, 11 Oct 2016 20:10:07 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:     Doug Ledford <dledford@...hat.com>, linux-rdma@...r.kernel.org,
        Sean Hefty <sean.hefty@...el.com>,
        Hal Rosenstock <hal.rosenstock@...il.com>,
        netdev@...r.kernel.org
Subject: Re: [PATCH] IB/ipoib: move back the IB LL address into the hard
 header

On Tue, 2016-10-11 at 12:01 -0600, Jason Gunthorpe wrote:
> > AFAICS the max mtu is already underlying h/w dependent, how does such
> > differences are currently coped by ? (I'm sorry I lack some/a lot of IB
> > back-ground)
> 
> It isn't h/w dependent. In CM mode the MTU is 65520 because that is
> what is hard coded into the ipoib driver. We tell everyone to use that
> number. Eg see RH's docs on the subject:
> 
> https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/7/html/Networking_Guide/sec-Configuring_IPoIB.html
> 
> AFAIK, today everyone just wires that number into their scripts, so we
> have to mass change everything to the smaller number. That sounds
> really hard, IMHO if there is any way to avoid it we should, even if
> it is a little costly.

Thank you for the details!

The first s/g fragment (the head buffer) is not allocated with the page
allocator, so perhaps there is some not too difficult/costly way out of
this.