netdev - Re: [PATCH] SIW: iWARP Protocol headers

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Date:	Thu, 7 Oct 2010 16:55:03 +0200
From:	Bernard Metzler <BMT@...ich.ibm.com>
To:	David Dillow <dave@...dillows.org>
Cc:	Bart Van Assche <bvanassche@....org>,
	Jason Gunthorpe <jgunthorpe@...idianresearch.com>,
	linux-rdma@...r.kernel.org, linux-rdma-owner@...r.kernel.org,
	netdev@...r.kernel.org
Subject: Re: [PATCH] SIW: iWARP Protocol headers



David Dillow <dave@...dillows.org> wrote on 10/06/2010 09:31:44 PM:

> On Wed, 2010-10-06 at 12:37 -0600, Jason Gunthorpe wrote:
> > On Wed, Oct 06, 2010 at 02:22:55PM -0400, David Dillow wrote:
> > > On Wed, 2010-10-06 at 11:25 -0600, Jason Gunthorpe wrote:
> > > > On Wed, Oct 06, 2010 at 02:52:46PM +0200, Bernard Metzler wrote:
> > > > It is actually a little more complicated than just this. I assume
you
> > > > are casting the structures over packet payloads? In this case you
> > > > have to guarentee alignment (or used packed everywhere). Does iwarp
> > > > have provisions for alignment? If so you can construct your
bitfields
> > > > using the alignment type, ie if iWarp guarantees 4 bytes then the
> > > > biggest type you can use is u32 - then you can avoid using packed.
> > > >
> > > > Mind you, I'm not sure how to guarentee alignment when you consider
> > > > all the possible sources of unalignment in the stack: TCP, IP,L2
stack ?
> > >
> > > You don't have to. The TCP stack, IIRC, requires the architecture to
fix
> > > up misaligned accesses, or at least it used to. It will try to keep
> > > things aligned if possible -- see the comment above NET_IP_ALIGN in
> > > include/linux/skbuff.h.
> >
> > I was under the impression that this was just to align the IP
> > header.
>
> It is, and it assumes an ethernet header of 14 bytes. As you note, VLANs
> and everything else can mess with that, which is why the TCP stack
> requires the architecture to handle unaligned accesses. It's a bonus if
> iWarp can guarantee an alignment WRT the IP or TCP header, but not
> required.
>
> > > The structures in Bernard's header have all fields naturally
> > > aligned, so no need for the packed attribute and its baggage.
> >
> > No, they arent:
> >
> > > +struct iwarp_rdma_write {
> > > +   struct iwarp_ctrl   ctrl;
> > > +   __u32         sink_stag;
> > > +   __u64         sink_to;
> > > +} __attribute__((__packed__));
> >
> > For instance has sink_to unaligned if the maximum aligment of the
> > payload is only guarenteed to be 4. The proper thing to do in this
> > case is probably __attribute__((__packed__, __aligned__(4))) if that
> > works the way I think.. :)
>
> struct iwarp_ctrl consists of 2 u16s, so sink_stag is aligned to 4 bytes
> from the start of the struct iwarp_rdma_write, and sink_to is at 8 bytes
> -- so they are naturally aligned with respect to the structure. The
> compiler will not add padding, so no need for the packed attribute.
>
> Note I'm not talking about alignment in memory -- that'll depend on
> where the data starts, and as I mentioned, the stack doesn't make any
> guarantees about that, relying on the architecture to fixup any
> misaligned accesses.

agreed. all n-bytes are starting on a multipe of n-byte offset
relative to the start of the structure.

many thanks,
bernard.

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