Re: [PATCH] libibverbs: A possible solution for allowing arbitrary MTU values.

[Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org>]

On Wed, Jun 05, 2013 at 06:02:25PM +0000, Jeff Squyres (jsquyres) wrote:
> On Jun 5, 2013, at 10:19 AM, Jason Gunthorpe <jgunthorpe-ePGOBjL8dl3ta4EC/59zMFaTQe2KTcn/@public.gmane.org> wrote:
> 
> > The concept of a libibverbs 2.0 has been NAK's by pretty much everyone
> > involved. This is why we are suffering with the complex extension
> > mechanism.
> 
> Are you saying that libibverbs must always always always be
> backwards compatible, and there will never be an ABI break at any
> version in the future?

I won't say never, but this is what people want. Bumping the soname is
seen as too difficult now.

> > The mixed approach that was brought up, where values like 1500 were
> > passed as 1500, and values like 1024 were passed as 3 seemed doable to
> > me. Did you see a problem with it for your use?
> 
> It just seems overly complex in terms of implementation.

Right. Preserving the ABI really is complex..

> > Thoughts:
> > - 1024 and 3 both mean 1024, the library must accept both values,
> >   it should only ever return 3 though.
> 
> Why?  If the caller can pass in 1024, it seems like 1024 should be
> able to be passed out, too.

If the caller passes in 1024 then it is probably OK to return 1024,
but you have to keep track of that specially. That seems more complex
than just always returning 3. 3 is guarenteed compatible with all
users.

Old users will test directly against 3.
New users will call ibv_from_mtu which tests against 3 as well.

> > - 1500/etc means 1500, the libray can return that.
> > - Make a ibv_from/to_mtu inline function to translate from bytes to
> >   the encoded MTU value.
> > - Switch ibv_mtu from a enum to a typedef int ibv_mtu
> 
> That also breaks ABI, doesn't it?

No, the change from 'enum ibv_mtu' to int is ABI compatible, we have
done those changes in the past. The underlying type for 'enum ibv_mtu'
is well defined by the various ELF ABI documents.

Jason
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